THE CORN FIELD SPIDER COMMUNITY:
COMPOSITION, STRUCTURE, DEVELOPMENT AND FUNCTION

By

MICHAEL JOSEPH PLAGENS

A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL

OF THE UNIVERSITY OF FLORIDA IN

PARTIAL FULFILLMENT OF THE REQUIREMENTS

FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA
1985

Dedicated to my Parents

Joseph R. Plagens

and
Frances L. Plagens

ACKNOWLEDGMENTS

During a three year research project, such as this,
many people along the way lend suggestions and aid towards
its successful completion. Foremost among these has been my
wife, Paula, who has supported me emotionally, logistically
and financially every step of the way.

Of course, my major professor. Dr. Willard H. Whitcomb,
was central in helping develop my goals and basic
understanding of crop spider ecology. One of his greatest
contributions to my success has been his ability to
encourage a sense of the importance and significance in the
time consuming research.

Learning the taxonomy of spiders was a long process; I
owe much to the individual tutoring provided especially by
Dr. G. B. Edwards, and also by Dr. David B. Richman.
Dr. Charles Dondale was indispensable for help in
identifying the linyphiid spiders.

I am thoroughly indebted to Mr. Jack Simmons, Mr. Ralph
Brown and the Haufler Brothers who kindly granted me access
to their crop fields. Dr. Edward V. Komarek was helpful in

allowing me to study overwintering spiders at the Tall
Timbers Research Station. Dr. Matthew Greenstone lent
valuable advise and encouraged me to pursue the aerial
dispersal aspect of spiders.

Among the others who have made my job easier include
Helen Huseman who illustrated the corn plant and freely
provided much illustrative advise, T. Dave Gowan who
suggested computer hardware and software that greatly aided
the sorting of information, and finally my parents who were
always ready to provide encouragement and motivation.

TABLE OF CONTENTS

Page

ACKNOWLEDGMENTS iii

LIST OF TABLES vii

LIST OF FIGURES ix

ABSTRACT xiii

CHAPTERS

I INTRODUCTION 1

II THE SPIDER FAUNA OF CORN FIELDS IN ALACHUA

COUNTY, FLORIDA 3

Introduction 3

Site Descriptions 7

Sampling Methods 12

Results and Discussion 15

Checklist and Description of the Species 15

Community Structure 61

Spider Density 91

Within Field Distribution 92

The Corn Field as a Habitat for Spiders 96

Trophic Relationships 103

Conclusion 118

III AERIAL DISPERSAL BY SPIDERS IN THE CORN FIELD

ECOSYSTEM 120

Introduction 120

Site Descriptions 121

Methods and Materials 123

Results 126

Discussion 138

IV WEED ASSOCIATIONS OF SPIDERS IN THE CORN FIELD

ECOSYSTEM 146

Introduction I45

Site Descriptions 148

Methods and Materials I49

Results 151

Discussion 162

V CORN STUBBLE AS AN OVERWINTERING SITE FOR

SPIDERS IN FLORIDA 171

I nt rodu ct ion 171

Site Descriptions 172

Methods and Materials 172

Results 173

Discussion I75

VI CONCLUSION 182

LITERATURE CITED 190

BIOGRAPHICAL SKETCH 207

LIST OF TABLES

Table Page

2-1. List of weeds growing in sunny borders of

Archer, 1983 corn field 10

2-2. List of families and genera included in

each of the six defined spider guilds 84

2-3. Variance to mean ratios (measure of
aggregation) for samples of spiders from
corn fields in Alachua County, Florida 93

2-4. Spider predation on pests and occasional
pests as observed in corn fields in Alachua
County, Florida 106

2-5. Spider predation on predaceous and accessory
insects as observed in corn fields in
Alachua County, Florida 108

3-1. Ballooning spider trap locations and number

of trap days accumulated at each location 125

3-2. List of ballooning spiders caught on sticky

wire traps in Alachua County, Florida 131

3-3 Relative ballooning frequencies for spiders

at three sites in Alachua County, Florida 142

4-1. Association of spiders with weeds growing in
Alachua County, Florida corn fields at
maturity and postharvest 152

4-2. Association of spiders with perennial weeds
and grasses growing in sunny borders,
pastures and fallow fields adjacent to corn
fields in Alachua County, Florida 153

Table

Page

4-3. Association of spiders with trees growing in
fence rows and areas adjacent to corn fields
in Al achua County , Florida 154

4-4. Association of spiders with weeds, shrubs
and saplings growing in shaded areas
adjacent to corn fields in Alachua County,
Florida 155

5-1. Overwintering spiders collected from corn
stubble in Alachua and Leon Counties,
Florida 176

5-2. Overwintering predaceous insects collected
from corn stubble in Alachua and Leon
Counties, Florida 178

LIST OF FIGURES

Figure Pa^e

2-1. Distribution and typical web structures of
spiders occurring on the corn plant:
Theridiidae and Linyphiidae 19

2-2. Stratification of the web building spiders
(Theridiidae, Linyphiidae and Araneidae) on
the corn plant 3X

2-3. Stratification of the horizontal plane orb

webs, Araneidae 33

2-4. Stratification of the tangled space webs,

Theridiidae 35

2-5. Vertical stratification of Te t r a^natha
laboriosa on the corn plant as a function of
body length 43

2-6. Principal species and percent rank abundance
of spiders in field corn at Archer, Florida,
1982, Field A 62

2-7. Principal species and percent rank abundance
of spiders in field corn at Archer, Florida,
1982, Field B 63

2-8. Principal species and percent rank abundance
of spiders in field corn at Archer, Florida,
1983 64

2-9. Principal species and percent rank abundance
of spiders in field corn at Orange Heights,
Florida, 1983 65

2-10. Principal species and percent rank abundance
of spiders in field corn at Archer, Florida,
1984 66

2-11. Principal species and percent rank abundance
of spiders in field corn at Gainesville,
Florida, 1984 67

2-12. The major families of spiders collected in
field corn at Archer, Florida during 1982;
Field A and Field B 69

2-13. The major families of spiders collected in
field corn during 1983 at Archer and Orange
Heights, Florida 70

2-14. The major families of spiders collected in
field corn during 1984 at Archer and
Gainesville, Florida 71

2-15. Species diversity of spiders as measured by
H' in six corn fields in Alachua County,
Florida 72

2-16. Stratification of spiders in the corn field
canopy during 1982 at Archer, Florida, Field
A and Field B 75

2-17. Stratification of spiders in the corn field
canopy during 1983 at Archer and Orange
Heights , Florida 76

2-18. Stratification of spiders in the corn field
canopy during 1984 at Archer and
Gainesville, Florida 77

2-19. Age structure of spiders in corn fields
during 1982 at Archer, Florida, Field A and
Field B 80

2-20. Age structure of spiders in corn fields
during 1983 at Archer and Orange Heights,
Florida 81

2-21. Age structure of spiders in corn fields
during 1984 at Archer and Gainesville,
Florida 82

2-22. Guild structure of corn field spiders as a
function of phenological period during 1982
at Archer, Florida, Field A and Field B 86

2-23. Guild structure of corn field spiders as a
function of phenological period during 1983
at Archer, Florida 87

2-24. Guild structure of corn field spiders as a ■
function of phenological period during 1983
at Orange Heights, Florida 88

2-25. Guild structure of corn field spiders as a
function of phenological period during 1984
at Archer, Florida 89

2-26. Guild structure of corn field spiders as a
function of phenological period during 1984
at Gainesville, Florida 90

3-1. Trap used for studying aerial dispersal in

spiders 124

3-2. Family proportions of ballooning spiders
captured from January, 1983 through July,
1984 in Alachua County, Florida 127

3-3. Ballooning intensity of spiders by month at

Archer, Florida during 1983 and 1984 128

3-4. Ballooning intensity of spiders by month in
Alachua County, Florida at Orange Heights,
during 1983 and Gainesville during 1984 129

3-5. Size classes of ballooning spiders captured
on sticky wire traps in Alachua County,
Florida from January, 1983 through July,
1984 137

4-1. Temporal association of Peucetia viridans
with plants in and near the field corn
ecosystem in Alachua County, Florida 157

4-2. Temporal association of Met a£hi^d_i££us^
galathea with plants in and near the field
corn ecosystem in Alachua County, Florida 157

ZlaH^e Page

4-3. Temporal association of Hentzia palmerum
with plants in and near the field corn
ecosystem in Alachua County, Florida 159

4-4. Temporal association of Misumenops celer
with plants in and near the field corn
ecosystem in Alachua County, Florida 159

4-5. Temporal association of Ch_ir i^cant hium
inclusum with plants in and near the field
corn ecosystem in Alachua County, Florida 161

4-6. Temporal association of Aysha velox with
plants in and near the field corn ecosystem
in Alachua County, Florida 161

5-1. Cross section of corn stalk and leaf sheath

showing the protected cavity between them 174

5-2. Cross section of corn cob with enclosing

husks showing the protected layers of air 174

Abstract of a Dissertation Presented to the Graduate

School of the University of Florida in Partial Fulfillment

of the Requirements for the Degree of Doctor of Philosophy

THE CORN FIELD SPIDER COMMUNITY:
COMPOSITION, STRUCTURE, DEVELOPMENT AND FUNCTION

By

MICHAEL JOSEPH PLAGENS

August 1985

Chairman: Dr. Willard H. Whitcomb

Major Department: Entomology and Hematology

An agroecosys tern approach was taken in analyzing the
the spider communities in Alachua County, Florida,
cornfields .

Visual examination of the corn plants detected 140
species and details of 40 species' microhabi tat , prey and
reproduction. The most common were Pardosa inilvi^na,
^£ll^^^££Il£3. £J^obosa, M_i s^umeno£S^ ££JlS r , Tetra£natha
laboriosa, Eperiqone banksi , Me taphidippus qalathea and
Uloborus qlomosus.

Community structure and stratification are presented.
Guild dynamics was similar between fields and years.
Maximum species diversity was 61 species and H'=4.08. The
density of spiders ranged from 2.2 - 9.5/m'^.

Prey is recorded for 29 spider species. Solenopsis
ants are major prey of Achaearanea globosa and Eperigone
banksi. Parasitic hymenoptera are major prey for Araneidae.
Insects feeding on the copious pollen are chief prey for
spiders and divert predation away from major pests.

Aerial dispersal of spiders in the ecosystem was
studied using sticky wires which trap mostly spiders. The
seasonal phenology of the 15 families and 53 species trapped
revealed five dispersal strategies; spiders with widely
overlapping generations and several instars ballooning
throughout the year are dominant components of the aeronaut,
crop field, and weed faunas.

The principal spiders and their seasonal phenology on
the weeds of the ecosystem are presented. Most major
species are shared with crop fields and frequent movement
between plant species is characteristic. Oak trees in fence
rows are a major source of crop and weed spiders.

The overwintering ecology of spiders and predaceous
insects in the cornfield was examined; 24 species of spiders
and 25 predaceous insects were found in the cavities formed
by the leaf sheath and/or the imbricate bracts of the
shelled corn cob.

The determinates of crop field spider communities are
divided into those which affect the diversity of the pool of
dispersing colonists and the biological and ecological
factors which favor or exclude species which arrive in the

field. Cornfields are a tremendous sources of spiders,
parasitoids, and pests, and because corn is a major
worldwide crop, management of the agroecosystem and thus the
spider communities in cornfields is essential.

CHAPTER I
INTRODUCTION

Since the resurgence of research in biological control
in the early 1960's, and the pioneering work of Whitcomb,
Exline and Hunter (1963) the volume of literature on spiders
in crop fields has steadily increased. Luczac (1979) and
Riechert and Lockley (1984) provide useful reviews of this
material .

Most of the published studies list the species found
and their relative abundance on a particular crop in a
particular area, such as on cotton in Arkansas (Whitcomb et
al. 1963), Texas (Kagan 1943, Dean et al, 1982), California
(Leigh and Hunter 1969) and in Peru (Aguilar 1968); on
alfalfa in New York (Wheeler 1973), Virginia (Howell and
Pienkowski 1971) and California (Yeargan and Dondale 1974).

A few studies consider the seasonal dynamics of a
single spider (Plagens 1983) or groups of spiders (Lockley
et al. 1979). Culin and Yeargan (1983a, b) in a unique way
consider the dynamics and compare the spider communities
.found in soybean versus alfalfa. A central theme of
Luczac's (1979) paper was a contrast of the communities
occurring in different crops. Riechert and Lockley (1984)

focused on a theme concerned primarily with potential preda-
tor/prey relationships and their impact upon the effective-
ness of spiders as biological control agents.

However, the spider fauna of many of our most important
crops, particularly row crops, have yet to be considered.
In addition, basic questions concerning the mechanisms that
structure the communities and the potential for spiders to
affect control of pests or potential pests have received
scant attention. That habitat structure, microclimate,
habitat stability, prey availability and diversity can all
influence the composition and dynamics of a spider community
is generally accepted. But the relative importance of these
factors and their interactions are highly debatable given
the complexity of the systems and the dearth of information.

The following report explores as many aspects of the
spider community in a major crop, field corn, with the
intent of discerning the mechanisms which produce the
observed community structure. This knowledge is vital if we
are to manipulate the predator complexes of crop fields to
our benefit.

CHAPTER II

THE SPIDER FAUNA

OF CORN FIELDS IN ALACHUA COUNTY, FLORIDA

Introduction

Corn, Zea mays , is grown on some 30 X 10 hectares in
the United States and 163 X 10^ ha. world wide (Corn
Refiners Assoc. 1982). Yet little information has been
published on the predaceous arthropods found in this major
crop. This is surprising, especially since pest management
in corn is so dependent on naturally occurring biological
control because of its low cash value and its importance in
underdeveloped areas of the world. Everley (1938a, 1938b,
1939a, 1939b, 1940) in a series of papers listed the
arthropods, including the spiders, that he had collected in
a rather casual manner from a small field of sweet corn in
Ohio. Dritschilo and Erwin (1982) studied the effect of
cultivation techniques on the abundance and diversity of
carabidae in an Iowa corn field, while Isenhour and
Yeargan (1981) studied the phenology of Orius in field

Luczac (1975, 1979) in her extensive research into the
spiders of crop fields mentioned the spiders of maize in
Poland, but this crop was a minor component in her study.
Work on spiders in similar crops includes that of Baily and
Chada (1968) in Oklahoma who conducted a thorough study of
spiders occurring in sorghum which is botanically related to
corn. A few studies of spiders occurring in cereal crops
have been conducted in Canada (Doane and Dondale 1979, Fox
and Dondale 1972) and Europe (Nyffeler 1982, Luczac 1979).

Altieri (1979) has made major contributions to the
understanding of how diversifying cropping systems can
influence the diversity and abundance of the natural ene-
mies. In field corn, he has shown that predaceous arthro-
pods were more abundant in plots intercropped with selected
weeds than in plots kept free of weeds. In addition, levels
of infestation by Spodoptera f rugiperda were lower in the
weedy plots than in the weed free plots.

In Florida, corn is grown on approximately 130,000
hectares mostly in relatively small fields of 20 to 100
hectares (Komarek 1951). Most of the state's production is
concentrated in the panhandle region. A large percentage of
the crop is kept by the grower as feed for his own livestock
and is an integrated part of a farming system which normally
includes other summer crops such as sorghum, peanuts, to-
bacco, watermelons or soybeans, as well as livestock. Many
fields are double cropped with rye, winter wheat or winter

vegetables. And because we cannot consider a crop field to
be a closed system, but rather part of the total agro-
ecosystem, we must consider the role of the corn field both
as a source and as a sink for both pests and beneficial
insects .

The first step in understanding the role that spiders
play in the corn field ecosystem, as in any crop (Whitcomb
1980, Dean et al. 1982) is to determine the composition of
the community. In addition to this central goal, I hoped to
help answer several basic questions concerning the ecology
of crop spider communities:

(1) How are the various species distributed upon the
corn plant? Whitcomb et al. (1963) and LeSar and Unzicker
(1978) showed clearly that different strata in the crop
canopy are inhabited by different species. Do these strati-
fications relate to structural constraints, microclimate,
location of prey or a combination of these?

(2) How are the spiders distributed within the field?
That is, are there significant border effects on the density
and diversity of the spider community? Are some species
regularly or gregariously distributed in the field rather
than randomly? Pieters and Sterling (1974) found that
spiders were the least aggregated of the predaceous arthro-
pods in a Texas cotton field, while Bishop (1981) found just
the opposite in Australian cotton. Altieri and Todd (1981)
found that predaceous arthropods were more abundant in

soybean rows adjacent to weedy borders than in the field
center.

(3) By direct observation of predation in the corn
field, what species or classes of prey are taken by the
different spiders?

(4) How do the structure and size of the spider
community change during the season and how is this related
to the phenology of the corn crop?

(5) How does the nature of the surrounding vegetation
and land management influence the structure and development
of the spider community? Is there consistency in the struc-
ture of the spider communities from season to season even in
the same general area? Culin and Yeargan (1983a, 1983b)
found that the more perennially stable alfalfa tended
towards a more similar community structure from year to year
than did an adjacent soybean field which apparently started
with a new set of spider colonists of slightly different
species composition each year. They also suggested that the
nearby alfalfa field was contributing to the annual flux of
spiders into the new crop field. Dondale et al. (1979)
found consistency in the spider faunas of apple orchards
through the northeast United States and Canada.

(6) Which spiders in the corn field actually reproduce
there? LeSar and Unzicker (1978) stated that few if any
spiders can reproduce in Illinois soybean fields.

(7) And, finally, what factors, both biotic and abi-
otic, tend to limit or benefit the populations of spider
species and/or the community as a whole? Specifically, are
predation on spiders or competition with other predaceous
arthropods of importance in regulating spider populations?

Site Descriptions

A total of six corn fields were intensively studied
during the period of April 1982 through July 1984. Four of
these fields were located within a mile of each other near
Archer, situated in southwestern Alachua County. A fifth
field was located near Orange Heights at the eastern edge of
the county. The sixth field, just northwest of Gainesville,
was located near the county center. In the selection of the
fields, every effort was made to include the widest possible
range of adjacent habitats.

The Archer fields were part of the same farmers' opera-
tion where peanuts, watermelons, forage grasses and cattle
pasture were rotated with corn. The soil was a deep, well
drained and leached Candler Fine Sand (USDA 1980). Although
the nearby uncultivated habitat was primarily xeric hammock
of Quercus virginiana/Q. laurif olia (Live Oak/Laurel oak),
many years of cultivation had oxidized the organic matter in
the crop fields so that a sandhill habitat was resembled.
Burrows of the gopher tortoise were found in the fields, a

resident typical of sandhills in Florida. True sandhill
scrub of Quercus laevis (Turkey Oak)/ Pinus clausa (Sand
Pine) grow naturally within 8 kilometers of these fields.
Sprinkler irrigation is necessary, and during the hottest,
driest periods the pumps run continuously to keep the corn
adequately watered. Two adjacent fields were sampled during
1982 (AR82-A and AR82-B), and one each during 1983 (AR83),
and 1984 (AR84).

Field AR82-A had on its east border a fence row of
trees (Quercus spp. and Celt is laevigata ) beyond which was
a 50 ha. fallow field which was periodically grazed by
cattle. The vegetation was composed primarily of herbaceous
plants that cattle avoid grazing: Sida rhombif olia, Cheno-
podium ambrosioides , Ambrosia artemisii folia, Oenothera sp.
and Cassia spp. The north border had a 15 ha. enclosure
where hogs were kept; here there were patches of bare soil
and large areas of tall Amaranthus , Chenopodium , and Sida
rhombif ol ia. To the west and south were borders of trees
(again Quercus and Cel tis ) which divided the field from
adjacent corn fields.

This corn field to the south was AR82-B. The areas

east, south and west of this field were improved cattle

.pastures of Bahia Grass (Paspalum notatum) . The only weeds

in these pastures were scattered clumps of Asimina spp.

(Paw-Paw). The field to the east also contained a

residence, and a border of trees. -r ^ ^^a-^- ^u ^- -, ^

In addition, the field

also had two large laurel oaks growing within it, under

which were a variety of weeds, similar in species

composition to the weeds growing under the tree borders.

These were seedlings of trees such as Celtis and Quercus,

and herbs such as Ambrosia artemisiif olia, Callicarpa ameri-

cana, Solanum americana and Phytolacca americana.

In 1983 and 1984 the Archer fields were again very dry

and sandy, but the vegetation and adjacent habitat supported

different plant communities due to varying management and

use. The layout of AR8 3 was a large (50 ha.) continuous

field of corn; sampling was concentrated in the southwestern

corner of the field. Along the south border of the field

was a band, 20 meters in width, which supported a diverse

community of herbaceous and woody weeds. The most abundant

of these are listed in Table 2-1. Beyond this was an over

grazed horse pasture which was heavily infested with Eupato-

rium and Asimina. To the west was a border of large oak

trees and then a plantation of slash pine with a dense

understory of oak saplings. Several mature slash pines

were left standing within the corn field and were surrounded

by weed patches of Rubus and Ambrosia. The area north of

the field consisted of dense herbaceous weeds (Desmodium,

Indigof era , Cassia) surrounding a cattle pond. Beyond the

east end of the field was improved Bahia Grass pasture.

The 1984 Archer corn field (AR84) had a weedy (Rubus

and Asimina) bahia grass pasture to the south, a plantation

10

Table 2-1. Weeds growing in the south border of the Archer,
Florida corn field during May of 1983.

ASTERACEA
Heterotheca subaxillaris Lactuca graminif olia
Eupatorium capil lifolium Ambrosia artemisiif olia
Gnaphalium obtusif olium Hieracium gronovii

ONAGRACACEA

Gaura angustif olia

POLYGONACEA
Rumex sp.

PHYTOLACCACEAE

Phytolacca americana

MALVACEAE

Sida rombifolia

ROSACEAE
Rubus sp.

FABACEA

Arachis hypogaea

BRASS ICACEAE

Lepidium virginicum

POLEMONIACEAE

Phlox drummondii

SOLANACEAE

Solanum americanum

RUBIACEA

Richardia scabra

AMARANTHACEAE

Amaranthus hybridus

SCROPHULARIACEAE
Linaria canadensis

APIACEAE

Ptilimnium capil laceum

CYPERACEAE
Cyperus sp.

CAPRI FOLIACEAE
Triodanis perf olia Wahlenbergia marginata

VITACEAE

Vitis tomentosa

Parthenocissus quinquef olia

11

of slash pine with dense oak undergrowth to the southwest
and north, peanuts planted in May to the west, a wide border
of xeric hammock (Quercus spp. ) to the northeast and a
several hectare field of weeds on the southeast. This field
was also quite large (60 ha.) and sampling was concentrated
in the south end.

The field sampled in 1983, located near Orange Heights,
presented a radically different habitat. The soil,
classified as a Mulat Sand, was water logged for extended
periods and so a considerable quantity of black organic muck
was incorporated with the sand. Portions of the field were
too wet for farm equipment to be operated and planting was
delayed until April due to flooding. A pond at the center
of the field contained the remnants of a cypress dome:
Taxodium distichum (Pond Cypress), Nyssa aquatica (Tupelo)
and sedges. Completely surrounding the 40 ha. field were
slash pine plantations with an understory of Myrica ceri-
fera (Wax Myrtle) and Ilex glabra (Gall Berry). Cattle were
allowed to graze the rich flora of grasses and herbs under-
neath. This herbaceous flora included Eryngium prostratum,
Hypericum spp.. Lobelia nuttallii, Polyqala incarnata, Xyris
spp., Rhexia mariana and Sarracenia sp. Field corn was
rotated with soybeans, and vegetables such as cabbage, pole
beans, strawberries and sweet corn.

The last field, Gainesville 1984 (GN84), about 30 ha.
in size, was cut from terrain that had supported the diverse

12

southern mixed hardwood forest (Monk 1965) on Norfolk Loamy
Fine Sand. Relatively undisturbed sections of this vegeta-
tion type were located to the west, southeast, and east of
the field. Paved highways separated the field on the north
from a 15 ha. fallow field, and from the forest on the east.
The borders around the field were mostly quite wide (10 to
20m) and had scattered trees such as Pinus taeda (loblolly
pine), Quercus laurif olia, Q. nigra (water oak), Prunus ser-
rotina (black cherry), Liquidambar styracif lua (sweet gum),
Myr ica cer if era and Prunus anqustif olia (Chicksaw Plum).
Beneath these trees grew thickets of shrubs and tall herbs,
especially Rubus , Desmodium and saplings of trees. Beyond
the border on the southwest was another corn field of 20ha.

Sampling Methods

A sample, as defined in this study, consisted of a set
of continuous corn plants in one planted row. During the
1982 season this consisted of either 5 or 9 plants, while
all samples included 10 plants in 1983 and 1984. Spiders
were collected from the sample unit by careful visual
examination. First, the fast running Pardosa wolf spiders
were noted, and, if their identification was not certain,
they were captured in individually numbered snap cap vials
for later identification. Next, the soil was examined
closely for the delicate webs of Linyphiidae and Hahniidae

13

whose webs were often made visible by droplets of dew
(Whitcomb 1980) or pollen dust. These small, easily damaged
spiders were collected by aspirator. A small hand trowel
was used to uncover soil dwelling hunters from their burrows
in the soil .

The base of the corn plant warranted close attention.
Many spiders find a niche among the prop roots and lowest
leaves. Tapping of the leaves with a pencil was used to
dislodge spiders from the lower surface and pulling the
leaves away from the stem frequently revealed the hiding
spot for cursorial species. Moving up the plant, each leaf
was examined on the upper and lower surfaces. The corn silk
required special attention because spiders of at least three
families constructed their silken retreats deep within the
strands. The tassel was also examined closely after which
it was given a stiff rap that occasionally dislodged unde-
tected spiders. Once this systematic inspection was com-
pleted for each plant, the plants were shaken vigorously and
dropping spiders were collected; the efficiency of the
visual examination was confirmed by only infrequently
catching spiders by this, final method.

This sampling method allowed me to record for each
specimen, the precise location on the plant or soil that was
inhabited. It also allowed for many prey records and spec-
ific interactions to be observed. To accomplish this, care-
ful notes were taken and each specimen was numbered and

14

preserved separately until its specific determination was
certain. For juvenile specimens, identification was not
accomplished until I became familiar with the fauna and
could match them with their adult counter parts.

Sample sites within the fields were selected by making
a transect through the field from one of the borders. The
starting point was usually not more than ten rows from the
field edge, and each additional site was reached by moving a
given number of rows (consistent for any sampling date, and
between 10 and 30). This sampling scheme allowed me to
analyze the "border effect" on spider density and diversity.
The number of samples that could be taken on a given day was
largely a function of the size of the plants and the density
of spiders. Early in the season, as many as 20 sets of ten
corn plants could be examined as described above, whereas at
maturity, only three sets could be completed during the
sampling period, from 0730 hrs to 1130 hrs. For analysis,
the seasons' samples for each field were divided into three
periods based on the phenology of the corn plants: Growth,
Flowering, and Mature.

Identification of spiders was accomplished through the
use of family and generic keys (Kaston 1978, 1981), the
numerous published specific keys (listed separately under
each family) and by comparison with specimens in the well
curated Florida State Collection of Arthropods (FSCA). Dr.
G. B. Edwards, of the FSCA, verified the majority of the

15

identifications, while spiders of the family Linyphiidae
were identified by Dr. Charles Dondale of Agriculture
Canada. Voucher specimens of all collected species are
deposited in the FSCA. As mentioned above, juvenile spiders
could, for the most part, be determined by comparing them to
series of specimens from the same locality that included
adults. Obtaining such series usually required collection
of specimens before or following the crop period on nearby
weedy vegetation (Whitcomb 1980). Nevertheless, for some
closely related species that occurred together in the same
area, reliable identification beyond the generic level was
not possible, especially for very small specimens.

Results

Checklist and Description of the Species

The spiders collected in the corn field are here listed
by family. After a general discussion of the each family,
the species found are listed with their authors. Species
marked with two asterisks constituted at least 5% of the
spiders collected, in at least one of the six fields, during
at least one of the sampling periods. Those which made up
more than 1% are marked with one asterisk. Those species
which are marked with an asterisk or those which are of
particular interest are discussed separately.

16

Uloboridae. These spiders, like those discussed under
the next family, use a variety of silk which is much dif-
ferent than that of other spiders (Foelix 1982). The silk
adheres to the cuticle of insect prey by virtue of its
finely hackled threads which act like Velcro to ensnare tiny
projections and setae on the insect. This is in contrast to
the adhesive glue found on the sticky webs of other spiders.
The family was revised by Muma and Gertsch (1964). Two
species of this small family of spiders were found:

*Uloborus glomosus (Walckenaer)
Uloborus sp.

Uloborus glomosus builds a horizontal or tilted orb web
that has a bluish-grey cast due to the hackled silk. It
appeared in the field only after the canopy was well
developed and then in the lower third of the canopy. Very
small juveniles as well as adult females and males were
collected. These spiders were common at all three sites but
were never abundant.

Dictynidae. Although this is a large and diverse group
of rather small spiders (Chamberlin and Gertsch 1958), only
three specimens, representing two, as yet, unidentified
species were encountered:

17

Dictynidae :

Dictyna sp. A
Dictyna sp. B

Theridiidae. The comb-footed spiders, a large and
diverse group of spiders, were well represented in the corn
field by 15 species and 250 specimens. These spiders build
irregular space webs of strong, sticky silk that are espec-
ially designed to capture crawling insects. Figure 2-1
shows the vertical stratification and usual web placement
for the common species. Many of the generic keys have been
published by Levi (1955, 1957, 1963) and Levi and Levi
(1962). Exline and Levi published the revision of Argyrodes
(1962). The specimens collected were identified as

Achaearanea near rupicola (Emerton)
**Achaearanea globosa (Hentz)

Anelosimus studiosus (Hentz)

Argyrodes f ictilium (Hentz)

Argyrodes sp. A

Argyrodes sp. B
*Coleosoma acutiventer (Keyserling)
*Latrodectus mactans (Fabricius)

Steotoda quadrimaculata

Steotoda erigonif ormis

18

Theridion albidum Banks
Theridion crispulum Simon
**Theridion f lavonotatum Becker
Theridion pictipes Keyserling
*Theridula opulenta (Walckenaer)
Tidarren sp.

Achaearanea globosa was one of the most characteristic
spiders of corn fields in Alachua County. It was common in
the 1983 Archer field, nearly absent from the 1982-A Archer
field and abundant in the remaining fields. A. globosa made
up 16% of the spiders collected during the flowering period
at the Gainesville field. This is a small spider, usually
less than 1.5mm in length as an adult female, but it is
easily recognized by the conspicuous black spot on the
dorsal surface of the abdomen.

A. globosa "s web was nearly always in close proximity
to the ground (see Figure 2-4) and close to the stem of the
plant. As shown in Figure 2-1 it is placed either in the
leaf axil, or on the lower surface of a leaf near the base
and stretching down to the next lower leaf or to the soil.
The principal prey, as recorded many times during the study,
were ants, particularly Solenopsis spp. It reproduced
freely in the field. The index of ■ dispersion (Southwood
1978) for this spider increased during the flowering and
mature periods, reflecting the pockets of offspring and the

19

Figure 2-1. Distribution and typical web structures of
spiders occurring on the corn plant. Theridiidae: 1. Ach-
aeranea globosa, 2. Coleosoma acutiventer, 3. Latrodectus
mactans, 4. Theridion f lavonotatum, 5. Theridula opulenta.
Linyphiidae: 6. E£eri,£one banks_i, 7. Fr ont i^ne l^]^a
pyramitela, 8. Ceraticelus similis.

20

rarity with which it disperses by air (see Chapter III).
Prey of the young A. globosa consisted of the plentiful
psocoptera and collembola that multiply on the shed pollen
and dying corn leaves.

On several occasions I found series of empty A. glob-
osa webs and later found a Mimetus spider nearby feeding on
yet another spider in its web. Achaearanea spp. are fre-
quent prey as well in the nests of Scelephron (Hymenoptera:
Sphecidae, Rau 1935).

Anelosimus s tudiosus is one of the most curious
spiders in Florida by virtue of its sociality (Comstock,
1948). Rather than leaving the maternal web, offspring
remain to help enlarge the web and subdue larger prey. Two
such colonies were found in the corn fields at Archer. The
colony found in 1982 contained only a female and a dozen of
her offspring, but the web found in 1983 covered the entire
plant and contained an estimated 100 spiders. Most of these
were small juveniles. The corn plant itself had turned
brown, presumably due to the light-blocking web.

Argyrodes f ictilium was only rarely encountered in the
corn fields, but was in two instances taken while it fed
upon another spider. Argyrodes spp. are believed to be
primarily kleptoparas itc (Exline and Levi 1962); that is
they live in the webs of larger spiders and feed unnoticed
on the larger spider's prey. Trail (1980), however, also
recorded A. f ictil ium as predatory in nature. The four

21

specimens that I collected were mature males and females
that might well have taken many spiders as prey before my
capturing them. In addition, I found a juvenile of another
species of Argyrodes preying upon Grammonota texana, a small
linyphiid spider. This suggests that the predatory behavior
of Argyrodes spp. may be more widespread than previously
recognized .

Coleosoma acutiventer is a bit larger than Achaearanea
gl obosa , and coal black in color with an elongate, oddly
shaped abdomen. It, too, was found preying on Solenopsis
ants and constructed its web close to the ground and the
plant stem, often in the axil of a leaf which had fallen
away from the corn plant. It was collected at Archer and
Orange Heights, but was common only at the Gainesville
field. Adult females protecting egg sacs and newly hatched
young were found at the Gainesville and Archer fields. As a
very curious observation, an adult male C. acutiventer was
found clutching a batch of eggs, yet no female specimens
were detected in the vicinity.

Latrodectus mactans , the black widow spider, was most
common at the Archer fields. No mature specimens of this
species were found in the corn field, although rather large
juveniles up to 10mm in length were collected just before
the crop was harvested. As its name suggests, this spider
is jet black, while its globular abdomen is variously marked
with red and/or yellow on the ventral surface. The silk is

22

very strong and studded with droplets of glue. When small,
the webs of this spider resemble A. g lobosa "s but as the
spiders grow larger, they climb higher and higher up the
plant constructing a large web that extends to the soil
surface. As described in Kaston (1970), the vertical
strands of silk that attach to the ground are ideally suited
for the capture of strong crawling insects such as beetles
and ants. I recorded Solenopsis spp. as prey for Latro-
dectus mactans in the corn field. In the borders of the
Archer fields were piles of old lumber and boulders among
which there were numerous L. mactans including egg producing
females. These are the refugia and nurseries for the black
widow spiders that colonize the field.

Theridion f lavonotatum was the only member of its genus
that was commonly encountered, and then only at the Gaines-
ville field where it comprised 5% of the spider community
when the crop was mature. Its irregular space web is con-
structed on the undersurface of middle canopy leaves. Unlike
the web of Theridula opulenta (discussed below), guy lines
do not extend down to the next leaf. As prey, I recorded
Cicadelidae, Aphidae, Ottitidae (leaf mining diptera), and
small parasitic hymenoptera. I found adult, reproducing
females on the corn plants by the time the corn plants
reached maturity.

During the study, I collected only six other specimens
of Theridion spp. Two of these, however, were with eggs

23

and/or young spiders in the web. Both of these species,
Theridion pictipes and T. albidum, are larger than T. f lavo-
notatum. Their irregular webs included as many as 3 corn
leaves. A Theridion pictipes web was found with a half
dozen mummies of already consumed Solenopsis invicta , but
was itself being consumed by a pirate spider, Mimetus.

Theridula opulenta is an easily recognized spider with
its transversely oblong abdomen marked with two or four
yellow spots. The web of the adult spiders can vary in
structure, but in the corn fields of Alachua Co. the struc-
ture shown in Figure 2-1 predominated. The irregular top
portion captures insects that fly or crawl into the under
surface of the leaves. Additional strands of tough silk are
also attached to the next lower leaf. Here they can inter-
cept insects that walk over the surface of the leaf below.
The prey of these spiders, as recorded in this study, are
similar to that of Theridion f lavonotatum but also include
ants as prey. Although present at all six fields, T.
opulenta was common only at the Gainesville field.

Linyphiidae. In many ways the Linyphiidae were the
most difficult family of spiders to deal with. Their size
is generally minute, with adults often measuring just a
millimeter in length. The webs too, are tiny, delicate and
inconspicuous. Countless species, many of which are poorly,
if at all, described in the literature are nearly impossible

24

for the uninitiated to identify. Many of the most important
genera have not been revised since the works of Bishop and
Crosby (1932) and Crosby and Bishop (1928, 1933). The
juveniles especially, are difficult to identify with any
degree of certainty, and many of the tiniest specimens in
the samples were surely overlooked.

Except for a few species (see Figures 2-1 and 2-3),
these spiders were closely associated with the soil and/or
the base of the corn plant, where moisture is higher and
their prey of collembola, psocoptera, and nematocerans are
concentrated. The abundance and diversity of linyphiidae
varied greatly between the six fields, apparently as a
function of the moisture level in the soil. These spiders
are avid ballooners (Click 1939, Duffy 1956, Dean and
Sterling 1985 and see Chap III) and have been treated as a
distinct ecological group by researchers (e.g. Luczac 1979)
in crop spider ecology because they frequently arrive in the
fields as adults which can begin reproducing immediately.
Duffy reported that significant numbers of Linyphiidae can
survive cultivation while Thornhill (1983) has published a
comprehensive analysis of linyphiid in British sugar beet
fields. The species I collected are as follows:

Anibontes longipes Chamberlin & Ivie
*Ceraticelus similis (Banks)
Ceraticelus sp. A

25

Ceratinops crenatus (Emerton)
Ceratinopsis sutoris
* *Eperiqone banksi Ivie & Barrows
Eriqone autumnalis Emerton
*Florinda coccinea (Hentz)
*Frontinella pyramitela (Walckenaer)
*Graminonota texana (Banks)
Meioneta micaria (Emerton)
**Meionet_a spp. (4 species)
**Tennesseelum f ormicum (Emerton)

Genus et species indet. ( ca . 7 species)

Ceraticelus similis is one of four species of Liny-
phiidae that was most frequently taken from the foliage
layer of the corn field. Its sheet web was constructed on
the under surface of corn leaves, particularly ones that had
curled and were in the lower third of the plant. The
spiders are orange-red, with a nearly spherical abdomen, and
measure 2mm or less at maturity. These spiders were
encountered with regularity at all six corn fields surveyed.

Eperigone banksi accounted for 200 of the 2300 spiders
collected from corn fields; only Pardosa mi Ivina had a
larger representation. Although E. banksi was found in all
six fields, the greatest number of these were collected at
the Orange Heights field. At Orange Heights these spiders
were abundant even during the seedling and growth stages.

26

when they were aggregated in clumps of sod left partially
turned by the cultivator. Eventually these spiders and/or
there offspring spread out to colonize the whole field.

The web consists of a delicate sheet that employs tiny
weed seedlings, clumps of soil, or the lowest leaves and
prop roots of the corn plant as supports. This design is
used by many of the linyphiid species occurring in crops
(Thornhill 1983). E^ banksi is up to 3mm in length as an
adult, and moves about on the undersurface of the web. Prey
that I observed being taken included collembola, psocoptera,
diptera, Cicadelidae and not infrequently Solenopsis.

Florinda coccinea was the largest of the linyphiidae
that I found in Alachua Co. corn fields, reaching 4mm in
length. It is easily recognized by its bright red color and
tubercled abdomen. It is a foliage level species, inhabi-
ting the lower third of the plant where it constructs a
sheet and lattice web that typically spans from the upper
surface of the leaf base to the stem and usually to a leaf
or stem of an adjacent plant. It was present in all six
fields, but was commonest at the Orange Heights and Gaines-
ville fields. No adult females were found in the corn
field, with the majority of collections being early to
middle instar juveniles. In adjacent habitat adults were
found at most times of the year, including winter.

Frontinel la pyramitela was a commonly collected spider
at the AR82-B and Gainesville fields. It was also present

27

at the other fields. Most of the specimens were less than
3mm in length, none of which were adults. The common name
of this spider is "The Bowl and Doily Spider" after a
description of the web. It consists of a flat sheet of silk
over which is constructed a loosely woven lattice of space
webs. Small flying insects collide with these fine strands
and fall upon the silken net below. The spider maneuvers on
the underside of the web to the fallen prey and bites it
through the sheet. Thus, as I have recorded here, the major
prey are tiny diptera, hymenoptera, alate aphids and cica-
delids. This construction is similar to that of F. cocci-
nea's, but is situated higher in the plant at mid level.

Grammonota texana specimens were encountered at the
base of the corn plant, but also with regular frequency in
the canopy. When found on the plant itself, the web
occupied the space created by the leaf axil or a curled
leaf. The delicate sheet web did not have an overhead
lattice. This spider was present at all six corn fields,
but with greatest frequency at the Orange Heights and 1983
Archer fields where reproducing females were found. Preda-
tion by this species was not observed.

Meioneta spp., as a group, were collected in all six
fields, being common at the Orange Heights, Gainesville and
two of the Archer fields. However, they are difficult to
differentiate at the species level, even when adult
specimens are available. The webs of the five species are

28

similar, in that they were usually associated with the prop
roots and/or the lowest leaves. A sheet web is combined
with a course, overhead lattice composed of relatively
strong threads. The adult males of this species roam on
foot searching for females, and because they are very ant-
like in form and behavior they may achieve a certain amount
of protection.

Tennesseelum f ormicum was found with regularity at
moderate levels at all six fields, but was most common at
the AR82-A and Gainesville fields. Its web and the behavior
of the males is quite similar to Meioneta's. Although I did
not find any females with eggs, reproduction in the field
likely occurred since adults of both sexes were found as
well as patches of numerous juveniles.

Theridiosomatidae. This- is a small family of minute
spiders which are classified as members of the Araneidae by
some authors (Levi 1980). The tiny orb web is built close
to the ground in moist, protected situations. Its web
differs from typical araneid webs (Kaston 1965) by having an
indistinct hub; the radii meet at 3 or 4 points rather than
a single center. Also, the spider sits not at the hub of
its orb web, but rather at the end of a strand attached to
the hub. All but one of the five specimens representing
this family were found in the Orange Heights field. The
single species was '

29

Theridiosomatidae :

Theridiosoma radiosa (McCook)

Araneidae. No other family of spiders had more
species or individuals than the Araneidae, the orb weavers.
Twenty-six species were recorded from the six Alachua County
corn fields. As would be expected, a wide range of size,
morphology, behavior, habitat selection and web structure
is found among this diverse fauna. Luczac (1975) also noted
the strong representation of Araneidae in corn fields.

Three basic variations of web orientation and con-
struction are used by the species found in the corn field.
The first of these types has the plane of the orb web paral-
lel to the ground. All of the species of this type that I
found in the corn fields have webs made of light, flexible
silk that yields to air currents, allowing the web to sway
up and down. Craig et al. (in press) suggest that this is
an adaptation for increasing the probability of capturing
nematoceran flies. The second basic design has the plane
perpendicular to the ground and constructed from relatively
tough, inflexible silk. The spider sits at the hub of such
a web during the day. The third design is also perpend-
icular to the ground, but has an additional component of a
retreat in the upper corner of the web where the spider
hides during the day. In some species that use this third

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36

design, the web is consumed at daybreak and then rebuilt
each night. Within each of these design types, there is a
stratification of species within the canopy as depicted in
Figures 2-2 and 2-3. Interestingly, the araneids which are
at the hub primarily by night occur higher on the plant than
do species which stay at the hub both day and night.

Levi has published many of the keys useful for identi-
fying the species within this family (1971, 1975, 1976,
1977, 1980). Berman and Levi (1971) revised the genus
Neoscona. The orb weavers I collected from corn fields are
as follows:

Acacesia hamata (Hentz)

Acanthepeira sp .

Araneus miniatus (Walckenaer)
*Araneus pegnia (Walckenaer)

Araneus sp. indet.

Arqiope aurantia Lucas
*Arqiope trifasciata (Forskal)

Cyclosa caroli (Hentz)
**Cyclosa turbinata (Walckenaer)
*Eriophora ravilla
*Eustala anastera (Walckenaer)

Gasteracantha cancriformis (Linnaeus)
**Gea heptagon (Hentz)

Hyposinga prob. rubens (Hentz)

37

Araneidae--continued :

Larina directa (Hentz)
**Leucauqe venusta (Walckenaer)
*Manqora gibberosa (Hentz)

Mangora spiculata (Hentz)

Mecynogea lemniscata (Walckenaer)

Micrathena sagittata (Walckenaer)
*Neoscona arabesca (Walckenaer)

Scoloderus cordatus ( Taczanowski )

Tetragnatha caudata Erne r ton
**Tetraqnatha laboriosa Hentz

Wagneriana tauricornis (0. P. -Cambridge )

Araneus pegnia , Eriophora ravil la, and Neoscona ara-
besca form the bulk of the third type of orb web builder
described above in which the spider hides in a retreat
within a curled leaf near an upper corner of the web.
Araneus pegnia was most common in the 1982 Archer fields
where adults were collected during the final stages of the
crop period. This spider was infrequently collected at all
fields studied in 1983 and 1984, including those at Archer.

Eriophora ravil la took the place of A. pegnia at the
1983 and 1984 fields. It was particularly common at Orange
Heights in 1983 and Archer in 1984. The majority of speci-
mens collected were small juveniles, 4mm in length or less.

38

Adults, which are large spiders nearly 12mm in length, were
collected in the months following harvest on nearby shrubs.

Neoscona arabesca, on the other hand, was collected at
all six fields but especially at the Archer fields in 1983
and 1984. Although most of the specimens collected were
again small juveniles, a number of adult females and males
were found by harvest time. The prey range that I recorded
for these three species was quite similar. Among the prey
recorded were small coleoptera such as Sitophilus and Cal-
lida spp., Braconidae, Agromyzidae (leaf mining diptera),
Ottitidae, Geocoris, Orius, and Cicadelidae. Mature spiders
have been observed to take moths, but are too small during
the growth and flowering stages when Spodoptera and Helio-
this are important.

Argiope trif asciata was common in both 1982 Archer
fields and present at the other four fields at low popula-
tions, but a number of large juveniles and an adult female
were collected. The web of these large spiders (up to 17mm)
was usually attached to the lower half of the plant, and
spanning two adjoining plants or even across the furrow.
The plane of the flat orb makes an angle of about 60 degrees
with the ground and has a characteristic zigzag stabili-
menta. A frequent component of the diet is grasshoppers
(Comstock 1948), but I also recorded coleoptera and cica-
delids .

39

Cyclosa turbinata was the most abundant araneid in the
1982 Archer corn fields but was only moderately common at
the other fields. The web of this species is markedly
characteristic and is also helpful in determining the prey
used. Rather than dropping the dried remains of consumed
prey out of the web, this spider incorporates them into the
web along the vertical axis. The spider is similarly
colored and shaped as this debris, and so is difficult to
spot as it sits upon it. By placing this clump of dead
insects in a dilute solution of bleach to dissolve the silk,
I was able to extract and identify the contents. Like many
spiders, Cyclosa has powerful gnathobases which effectively
crush the cuticle of prey so that for many prey items I was
able to determine only the family: Chrysomel idae, Sito-
philus, Dolichopodidae, Agromyzidae, Ottitidae, Braconidae,
Aphidae, Cicadelidae and alate Formicidae.

Eustala anastera, in its mature stages, which I found
only on adjacent shrubs and weeds, behaves like A. pegnia
and N. arabesca by hiding in a retreat by day. But as early
instar juveniles in the corn field, these spiders were
mostly found sitting at the hub by day. They reached their
greatest abundance on the corn plants late in the season as
the plants were entering senescence, constructing their webs
among the spikes of the tassel. Even when the plants were
dried and brown these spiders continued to use the tassels
for their orb webs.

40

Gea heptagon was consistently captured at all six corn
fields. This easily recognized spider has a black, five-or
seven-sided splotch on the dorsum of the abdomen from which
it gets its name. The web is vertical and nearly always in
the bottom third of the canopy (see Figure 2-2) with some of
the support strands attached to the ground. This spider's
web of tough silk was also frequently found suspended from
weeds and debris beneath the canopy.

Despite being the most common araneid in my collections
of corn field spiders, no adults of Gea heptagon were col-
lected, and only a few late instar juveniles. Adult speci-
mens were encountered in the corn field after harvest
beneath tall weeds, but also in the early spring before the
field was mown and harrowed. The prey I found in their webs
were mostly flying insects, but I also observed this spider
to take an ant that had blundered into one of the strands
attached to the ground.

Leucauge venusta became numerous in the corn fields at
the Archer and Gainesville fields in 1984 after the canopy
had developed sufficiently to shade the understory.
However, these spiders were mostly quite small, and so did
not reach maturity before the corn was harvested. Even as
the corn plants were entering senescence and thus allowing
more light to penetrate to the soil, these shade loving
spiders began to disappear. This is a reflection of this

41

spider's normal habitat in north Florida, the shaded under-
story of hammocks.

Its web of finely meshed, exceeding delicate silk, is
constructed parallel to the ground at about mid-level in the
canopy. The web sways freely in the breeze which helps it
intercept small flying diptera (Craig et al. in press) and
which may also allow it to be supported by the wavering corn
leaves .

Tetragnatha laboriosa was the most abundant of the
Araneidae which build a horizontal web, and the second most
abundant orb weaver over all. The greatest portion of the
specimens collected were quite small and in practice could
not be reliably separated from the other species of Tetra-
gnatha. But since adults of other species were not col-
lected in the vicinity of any of the corn fields, except for
swampy Orange Heights, I feel justified in assuming that
most of those collected in the corn field were in fact T.
laboriosa. As a group, Tetragnatha is normally found in
marsh or swamp habitats; T. 1 abor iosa is the major excep-
tion.

I recorded a number of prey for this spider including
the alate forms "of Aphidae and Formicidae, Cicadelidae,
Braconidae, Cecidomyiidae, and Agromyzidae. LeSar and
Unzicker (1978) in a study conducted in Illinois also made
considerable contribution to the biology and known prey
range of Tetragnatha laboriosa. There it was the most abun-

42

dant spider in soybean fields. Culin and Yeargan (1982)
also detailed the prey range for this spider in soybeans.

T. laboriosa showed an interesting stratification in
the crop canopy as a function of its size. The smallest
spiders, which are less than 1 mm in length occur more often
in the lower canopy, and the adults, which reach 7 mm, in
the upper canopy. The correlation between body length and
the height of the web in the canopy as shown in Figure 2-5
was significant (r=.54, p < .01, N=39). This was the only
spider which showed a significant correlation (negative or
positive) between body size and position in the canopy.

Agelenidae. The funnel web spiders, or Agelenidae,
were rare spiders in the corn field itself, but were quite
common in the wooded borders. Just four specimens were
collected, and these were near the border areas of the corn
field. A Cicadelidae and an early instar nymph of a
Gryllidae were seen to be taken as prey. Identification of
these spiders is based on adult characters of the genitalia
and so I am sure only of the genus:

Agelenopsis sp.

Hahniidae. The Hahniidae were another uncommon family
of spiders in Alachua County corn fields, but were nonethe-
less successful colonists in that they reached maturity

43

Figure 2-5. Vertical stratification of Tetragnatha labor-
iosa on the corn plant as a function of body length.

44

before harvest. These spiders are easily recognized by
having six spinnerets arranged along the posterior margin of
the abdomen and by the construction of their web. The tiny,
delicate sheet web is spun over the soil; the spider itself
hides under an earthen clod or among the prop roots of the
corn plant, running out over the top of the web to secure
prey. Specimens, up to 3mm in length, were captured at the
Archer and Gainesville fields only.

Mimetidae. The Mimetidae, unlike most spiders, do not
normally eat insects. Instead they prey on other spiders,
particularly web dwelling species, by stealthily climbing
into the host's web. Numerous long, curved spines on Legs I
and II are used by the mimetid to preclude an attack by its
prey until its own venom has taken affect. I frequently
found a trail of empty webs, particularly of Achaearanea
globosa , at the end of which I found Mimetus in the web of
its most recent victim. Surprisingly, Mimetus sp. was among
the common spiders at Archer in 1983; it was pres-ent but
rare at the other fields. At least two species of Mimetidae
were collected with specimens taken from all six fields:

Ero leonina (Hentz)
Mimetus sp.

45

Pisauridae. Only twelve specimens of Pisauridae were
collected from the corn field during the study. All of the
specimens were collected as early instar juveniles from the
lower surfaces of leaves in the lower half of the plant.
Two species were identified using the keys in Carico (1972):

Dolomedes sp.

Pisaurina sp. prob. mira (Walckenaer)

Lycosidae. The wolf spiders, family Lycosidae, are

the dominant group of hunting spiders on the soil in many
agricultural situations (Whitcomb et al. 1963). Most
species are active and hunt at night when reflective eyes
make the majority easy to find with a headlight. Pardosa
spp, however are most active during the daylight hours while
others are largely subterranean, such as Geolycosa, Lycosa
carol inensis and Lycosa lenta and so the populations of
these were probably underestimated by my sampling
techniques .

Identification of lycosids to the level of species
relies heavily on characteristics of the genitalia and so
juveniles cannot always be placed with confidence. Using
the keys of Dondale and Redner (1984) I was able to identify
adult Pardosa, and with practice, I was able to separate the
juveniles of the four Pardosa species; this was made easier

46

by comparison with juveniles reared from positively ident-
ified adults. Most members of the genus Lycosa were identi-
fied as L. lenta because it was the only identified adult
species found in the corn field at night by head light.
Identification of Pirata was aided by Wallace and Exline
(1978).

Fourteen species of lycosids were distinguished from
my collections in corn fields:

Allocosa f loridana
Lycosa rabida Walckenaer
Lycosa carolinensis Walckenaer
Lycosa hellulo Walckenaer
*Lycosa lenta Hentz

Lycosa amophila
*Pardosa pauxila Montgomery ( =georgae )
Pardosa littoralis Banks
**Pardosa milvina (Hentz)
**Pardosa parvula Banks (=saxatilis)
Pirata allapahae Gertsch
Pirata sp. A
Pirata sp. B
Genus et sp. indet.

Lycosa lenta, as an adult, is a large nocturnal hunter
of the soil surface where I found it preying on other wolf

47

spiders and crickets. The majority of specimens collected
were taken at the Archer fields, all as early to middle
instar juveniles. All quantitative sampling was conducted
between 0730 and 1130 hours when L. lenta is usually inac-
tive and in hiding beneath dead corn leaves or under clods
of earth. Using a head light at night I could find L. lenta
adults at low densities, particularly in border areas and
along irrigation rows. I rarely found adults roaming in
under the corn canopy. As do all the Lycosidae, L. lenta
females carry the egg sac attached to the spinnerets until
the spiderlings emerge. The spiderlings crawl onto the
dorsum of their mother and then disperse over a period of
several days as she roams and hunts. Lycosa lenta fre-
quently prey on other lycosid spiders, either con- or heter-
ospecifics (Gowan, 1985).

Pardosa milvina, with 333 specimens collected during
the study, was the single most abundant spider in the corn
fields. All stages of this species were present in the
fields nearly throughout the year, but were scarcest at
planting and just after cultivations. Recoloni zation
occurred rapidly through ballooning and running over the
soil. Adult females can deposit spiderlings over a large
area as described above for L. lenta. Dense populations of
P. mi 1 vina were found in the cattle pastures (especially
over grazed ones with barren patches of soil) and in the

48

recently fallow fields that were integral parts of the
Archer and Orange Heights ecosystems.

The behavior of Pardosa milvina , as I observed it in
the field, was quite variable. Most specimens were
collected running over the soil, or among the basal leaves
and prop roots, but a significant number were found high on
the foliage, even on the corn silk. I recorded them to take
prey while on the plants, specifically corn leaf aphids
(Rhopalosiphum maizae) , but they also take water that col-
lects in the leaf axils during the morning dew. Another
specimen was found hanging from the corn silk while in the
process of ecdysis; a number of exuviae were also found
similarly positioned.

Pardosa as a group seem to require a moist environ-
ment; this was dramatically demonstrated at the Gainesville
field. From the middle of May through the beginning of
June, just as flowering was about to start, a three week
long period of very dry weather nearly destroyed the crop
during which the population of Pardosa milvina plummeted
(see Figure 2-11). When rain returned, the crop quickly
recovered but the P. mi 1 vina did so only slowly. In this
case I believe that the absence of cattle pastures and
fallow fields was responsible for the failure of the popula-
tion to recover. Another facet of recoloni zation is that
Pardosa ballooned only rarely during May through August (see
Chapter III).

49

Pardosa pauxila and P. parvula were common to abundant
wolf spiders, notably during 1983. During the mature period
at the Archer field, P. parvula was the most frequently
captured wolf spider, while at Orange Heights Pardosa
pauxila was among the most commonly collected. The behavior
of these spiders was very similar to P. milvina^s such as
climbing up into the plants to drink dew water and ecdysis
while hanging from the silk.

Oxyopidae. The lynx spiders, family Oxyopidae, are
largely diurnal hunting spiders which rely on a combination
of sight and acute tactile reception to capture prey. Silk
is used in construction of the egg sac and in ballooning,
but not for snaring prey. Identification of Oxyopidae is
aided by Brady (1964). One species made up the bulk of the
spiders in this family collected from corn fields:

Oxyopes salticus Hentz
**Peucetia viridans (Hentz)

Peucetia viridans, the green lynx spider, entered the
fields during April at the end of its ballooning period (see
Chapter III). It was one of the most common spiders when
the plants were small, and this population level that was
achieved in April remained relatively constant through the
season until senescence, when the spiders moved onto the

50

emerging weeds (see Chapter IV), The greatest numbers of P.
viridans occurred at the Archer fields but were virtually
absent at Orange Heights. They reached 7mm in length by
harvest time and so were the largest of the common species.
These are strong predators in the corn field preying on
Chrysomelidae , Scarabaeidae , Cucurl ionidae, Diptera and
small bees.

Adult P. v_iri^dans^, which appear in late July to
August, specialize on pollenators as prey. From the
blossoms of Eupatorium , Monarda, Indiqof era hirsutum and
Bidens alba they catch numerous honey bees and even large
wasps. The spiderlings hatch in the late fall and become
soil surface wanderers until early spring (Whitcomb 1985)
when they can be found on spring weeds, low tree branches
and shrubs. They balloon frequently during this time (see
Chapter III) and remain in the first post emergence instar.
Soon after their arrival in the corn field they molt to the
second instar and cease to move through the air. Peucetia
viridans is an example of a spider that despite not reaching
maturity while a part of the corn field community can be an
abundant spider depending on the weeds that grow nearby.
■The presence of large areas of late summer weeds will
provide numerous offspring for next year's crop.

51

Gnaphosidae. Among the webless hunting spiders, the
Gnaphosidae are elongate, fast running, largely soil surface
hunters. Gnaphosa serricata, the only common species in
northern Florida corn fields, hides beneath the soil by day
and roams by night. Pitfall traps set in the corn field
efficiently captured these arachnids. I did not record any
predation by these spiders, but a spider wasp (Pompilidae)
was captured taking a penultimate female of G. serricata.
Gnaphosa serricata was the only one of the following species
for which more than one specimen was collected, and then
only from the drier Archer and Gainesville fields (Platnick
and Shadab, 1975):

Drassodes sp.
Drassyllus sp.
*Gnaphosa serricata (L. Koch)
Poecilochroa variegata (Hentz)
Genus et sp. indet.

Clubionidae. The sac spiders, family Clubionidae,
have some common and important species in agricultural
fields. These spiders have, for example, been recorded to
feed on the eggs of Heliothis (Peck and Whitcomb 1970) and
the larvae of other lepidoptera. Most species of Clubion-
idae hide by day in well concealed sacs constructed of silk.
The sacs can be in a rolled or torn leaf, in a folded blade

52

of grass, or deep in the corn silk. If the plant is vigor-
ously shaken, and sometimes only gently, the spider will
almost always emerge from the sac running at high speed for
some distance and then halt. If an empty sac is found
during a visual search it is wise to look about for the
spider which may be hiding nearby. Most species of Clubion-
idae are nocturnal, but several groups which apparently
mimic wasps and ants are active in full sunlight. The
driest corn fields at Archer had the highest numbers of
clubionids, while the wettest. Orange Heights, had few. All
of the Clubionidae make use of their acute tactile percep-
tion in the capture of prey. These species were found and
identified using Edwards (1958), Reiskind (1969), and
Platnick and Shadab (1974a, 1974b):

Castianeira sp. A
Castianeira sp. B
**Chiricanthium inclusum (Hentz)
*Clubiona abbotii L. Koch
Clubiona procteri Gertsch
Micaria sp.

Myrmecotypus lineatus (Emerton)
Phrurotympus minutus
Scotinella sp.
*Trachelas deceptus (Banks)
Trachelas similis 0. P. -Cambridge )

53

Chiricanthium inclusum and its role in agricultural
fields was studied by Whitcomb et al. (1963) and Peck and
Whitcomb (1970) who found it to be an important predator in
cotton fields. But in Alachua County corn fields it was
common only at the Archer fields where it was among the top
ten most common spiders. From the time when the corn silk
emerged until it rotted off during the mature phase, C.
inclusum was found hidden deep in this silk. When the silk
was not available, it preferred leaves tattered by Spodo-
ptera f rugiperda or in the recessed midrib on the lower leaf
surface. But on other plants these spiders normally tie
several nearby leaves together (or use ones already tied
together by pyralid larvae) and build their silken retreat
within. Thus, home sites for this spider on the corn plant
are marginal and could be a factor that limits this spider's
numbers in the corn field. Otherwise the corn plant should
provide adequate prey, such as lepidoptera, as evidenced by
my finding reproducing adults on the corn plants.

Clubiona abbotii and C. procteri are quite similar
even as adults, and so I have not attempted to separate the
juveniles. Five adults of C. abboti i were found; one of C.
procteri. Again, these spiders were absent from the Orange
Heights field and were only sporadically common at Archer.
Yet these spiders were among the most common species

54

captured as ballooners (see Chapter III). Only two nests of
this species were found in the corn silk; the remainder were
in the depression along the midrib on the lower surface of
the corn leaf or in the leaf axil. I did not record any
feeding by these species, but one female was found with an
egg sac.

Trachelas deceptus and T. similis are also not easily
distinguished as juveniles. Here five adults of T. deceptus
were identified and one of T. similis. Trachelas are gene-
rally robust spiders with the first pair of legs longer than
the others and with the cephlothorax and forward legs a deep
maroon color which fade to pale towards the posterior of the
animal. These spiders were regularly encountered only at
the Archer fields (both species), hiding deep in the leaf
axils, especially of the lowest leaves. I found no repro-
duction occurring and did not record any prey.

Anyphaenidae. In many ways the Anyphaenidae resemble
the clubionids and it can be troublesome for the novelist to
distinguish the two. The most important and reliable char-
acter is the placement of the spiracular slit on the venter
of the abdomen. In clubionids it is just anterior to the
spinnerets, while in the anyphaenids it is placed a half or
two thirds of the way anteriorly towards the epigastric
furrow. The spiracle is very faint and requires a good

55

microscope to see. Another character which is sometimes
useful is the notched trochanter (Roth 1964). Among the
species found only Aysha spp. was collected infrequently;
the others were rare. The silk retreats, which are also
similar to the clubionid's, were most frequently found in
the tassel. Again Aysha spp. were frequently caught on the
traps used to monitor aerial dispersal indicating that the
corn plants do not provide a suitable niche. Four species
were identified using the keys published in Platnick (1974):

Aysha velox (Becker)
Aysha gracilis (Hentz)
Teudis mordax ( 0. P. -Cambridge )
Wulfila saltibunda (Hentz)

Thomisidae. The crab spiders, family Thomisidae, are
so named for their elongated anterior legs and sideways
movement characteristic of true crabs. Thomisid spiders
position themselves at key positions on their substrate so
as to maximize the encounter rate with potential prey. For
many species this means sitting in the flower of a plant to
capture pollenators and other flower feeding insects.
Despite their cryptic coloration, species which frequently
hunt at the inflorescence of a plant are not restricted to
that position. They can also hunt from the growing term-
inals or leaf petioles (Plagens 1981). The identification

56

of the Thomisidae is greatly aided by the revision by
Gertsch (1939). Only one of the following five species
collected was common enough to warrant separate discussion:

Misumenoides formosipes (Walckenaer)
**Misumenops celer (Hentz)

Misumenops oblongus (Keyserling)
Xysticus sp . A
Xysticus sp. B

Misumenops celer was frequently the most abundant
foliage layer hunting spider. The majority of captures were
during the flowering and early mature periods when the
spiders were almost always captured on the inflorescences,
either the male tassel or the female silk. The age struc-
ture of this species was greatly skewed towards the smallest
instars which arrived by ballooning and then departed the
same way once the flowers stopped attracting prey (see
Chapter III). For these early juveniles, the principal
prey, as I recorded it, were thrips, followed by minute
diptera. Larger spiders were captured while feeding on
Orius , Geocoris , Agromyzidae and other hunting spiders
including conspecif ics. Plagens (1981) found hymenopterous
parasitoids also to be a major part of this spider's diet.
Adult males and females were collected in the corn fields
but no females with eggs were found.

57

Philodromidae. For many years the family Philo-
dromidae was classified as a subfamily within the Thomisi-
dae. However, recent work on the ultras tructure of the eyes
combined with a host of other character differences has
convinced most arachnol ogi st s that they are correctly
separated (Homann 1971,1975). The hunting strategies differ
markedly also. Philodromids run very fast an legs provided
with adhering scopulae. They pursue prey for short distan-
ces after detecting their presence through vibrations and
are rarely found on or near the flowers. Many species are
found on the stems and bark of woody plants. I captured
just three specimens of this family in the corn field, but I
may well have missed some. They run to the other side of
the plant stem or leaf if they detect movement and can cling
to surfaces with surprising tenacity when the plant is
shaken. Identification was aided by Dondale and Redner
(1961, 1968).

Philodromus keyserlingi Marx

Salticidae. As sophisticated as the Araneidae are in

.the construction of webs, so are the Salticidae in the

visual pursuit of prey. The eyes are certainly the most

sophisticated of any arthropod eye and rival the vertebrate

eye in their ability to detect form, color, movement.

58

orientation and distance (Homann 1971). Not only does this
aid them in locating prey, but also in the safe capture of
prey. By grasping prey such as caterpillars or ants at
precisely strategic points (Edwards et al. 1974) the prey's
defenses are rendered ineffective.

The Salticidae were common and well represented by
species at the Gainesville and Archer fields, but were
nearly absent from the Orange Heights field. Several
species are avid ballooners (Horner 1975 and Chapter III)
even as middle to late instars which can mature quickly on
abundant prey in their new surroundings and begin producing
offspring. Eighteen species of salticids from corn fields
were identified:

Corythalia canosa (Walckenaer)

Habronatus brunneus (Peckham & Peckham)

Habronatus trimaculatus Bryant
**Hentzia palmerum (Hentz)

Marpissa dentoides Barnes

Lyssomanes viridis (Walckenaer)
**Metaphidippus galathea (Walckenaer)

Peckhamia americana (Peckhams)

Phidippus apacheanus Chamberlin & Gertsch

Phidippus audax (Hentz)
*Phidippus regius C.L.Koch

Phidippus clarus Keyserling

59

Salticidae--continued :

Phidippus pulcherrimus Keyserling

Phidippus putnami (Peckhams)

Synaqeles sp.

Thiodina sp.

Zyqoballus ruf ipes Peckhams

Hentzia palmerum was the second most abundant salticid
collected in the corn fields. The males of this species
have the chelicerae greatly elongated and porrect (directed
forward). Early juveniles through adults of this species
were found in the corn fields, particularly on the silks or
tassels. At Orange Heights, this spider was nearly absent
from the corn plants, but they became fairly common on the
weeds in that field by September (see Chapter IV). The prey
for this species consisted largely of soft bodied insects
such as agromyzids, aphids, Orius and thrips.

Metaphidippus galathea vied with Misumenops celer as
the most common foliage hunter. Like both M. celer and H.
palmerum these spiders concentrated their activity on the
inflorescences. Some arrived in the corn fields early
enough to mature and begin producing offspring. First
instar juveniles that emerge before the flowering is over
find abundant thrips and diptera in the silk and on the
tassels. Those that emerge later move to the emerging weeds

60

such as Indiqof era, Cassia and Amborsia where they become
one of the most common species (see Chapter IV), Horner and
Starks (1972) found that M. galathea has a relatively short
life cycle, reaching maturity in 6 to 7 months. Female
spiders produced a mean 8,3 egg sacs over her life span.

Like salticids in general, Metaphidippus hunts primar-
ily by sight, taking an assortment of prey similar to
Hentzia's but it can adopt alternate hunting strategies. I
found M, galathea pulling diptera larvae from the rotting
frass left by corn ear worms.

Just two specimens of Metaphidippus were collected
from the corn plants at the Orange Heights field where it
was also less common than H. palmerum on the weeds following
harvest ,

Phidippus regius is a member of a large genus of
common, large jumping spiders. These spiders produce large
quantities of offspring (Edwards 1980) that balloon fre-
quently (see Chapter III) and so it is not surprising that
they occur with regularity in the corn field. Some speci-
mens of this and the other five identified species reach the
middle instars. No adult Phidippus of any species were
collected. Like Hentzia and Metaphidippus these spiders
were most frequently collected in the silk or on the
tassels. Prey included diptera and Cicadelidae. Muniappan
and Chada (1970) found P. audax to be a potential biological
control agent of green bugs (Aphididae) in sorghum.

61

Community Structure

Species Abundance. The rank abundances of the indi-
vidual species during the growth, flowering and mature
stages are presented for each of the six fields in Figures
2-6, 2-7, 2-8, 2-9, 2-10 and 2-11. Just one spider was
common in all six fields, namely Pardosa mi Ivina. Four
spiders, Misumenops celer, Achaearanea globosa, Tetragnatha
laboriosa and Metaphidippus galathea, were common in five of
the six fields. Gea heptagon, Peucetia viridans, and Ulobo-
rus glomosus were common in four of the six fields while
eight species were common in three of the fields. Most
species which were common in one field were at least present
in the other fields. Thus, differences between fields were
mostly in the relative frequencies of the major species and
in the list of rarely captured species.

Spider Families. The family composition of spiders in
the six fields are presented in Figures 2-12, 2-13 and 2-14.
Even at this taxonomic division, there were considerable
differences between the six fields. This could be a reflec-
tion of the general strategies for surviving used by the
different spider families contrasting with the physical and
biotic conditions presented by the surrounding habitats.
The Lycosidae, for example are favored by the presence of

62

40

30

D

Major Species

Minor Species

,ft. msi!i-}tiit.im!mi<mmmt*iawmmmm

15

21

-i»f -;;{» hAmPMimmrmm,

1.0
0.8
0.6
0.4
0.2
0

4.0

3.0 s:

10

15

Ml.m•m\l>•WJlL.m.^m•>l.KV>l^Jv.w.^^l>■l.^Jl■.^vM^y^^^

2.0

1.0

20

25 28

10 15 20

RANK IN ABUNDANCE

25 30

Figure 2-6. Principal species and percent rank abundance of
spiders in field corn during three phenological periods at
Archer, Florida, 1982, Field A. Growth (A), Flowering (B),
Mature (C).

63

15

I I Major Species

l^ll Minor Species

0.6
0.4
0.2
0

20 24 33

15 20 24 45

1-4.0
3.0
2.0

[ 1.0
0

RANK IN ABUNDANCE

Figure 2-7. Principal species and percent rank abundance of
spiders in field corn during three phenological periods at
Archer, Florida, 1982, Field B. Growth (A), Flowering (B),
Mature (C).

64

30

20

10

ydWMUJJMIlSIJia,

Minor Species

1^ Major Species

UMMiUJI J,IIWi.i'i lltiiiiuminiiniiiii.ui.iiij»»iHin»..wr

0.8

0.6

0.4

0.2

10

15

20

25

RANK IN ABUNDANCE

Figure 2-8. Principal species and percent rank abundance of
spiders in field corn during three phenological periods at
Archer, Florida, 1983. Growth (A), Flowering (B), Mature
(C).

50

40

■ 2.0

Major Species

65

10 15 20

RANK IN ABUNDANCE

37

Figure 2-9. Principal species and percent rank abundance of
spiders in field corn during three phenological periods at
Orange Heights, Florida, 1983. Growth (A), Flowering (B),
Mature (C).

66

40
30
20
10
0

o J5

■Q (0

■a S

II i

Major Species
^y Minor Species

1.0
0.8
0.6
0.4
0.2
0

RANK IN ABUNDANCE

Figure 2-10. Principal species and percent rank abundance
of spiders in field corn during three phenological periods
at Archer, Florida, 1984. Growth (A), Flowering (B), Mature
(C).

67

RANK IN ABUNDANCE

Figure 2-11. Principal species and percent rank abundance
of spiders in field corn during three phenological periods
at Gainesville, Florida, 1984. Growth (A), Flowering (B),
Mature (C).

68

nearby cattle that produce habitat and prey for Pardosa spp.
The Linyphiidae are favored by wet conditions, like those
surrounding the Orange Heights field. And the visually
dependent Salticidae are favored by old field weeds and
shrubs where light and prey are plentiful.

Species Diversity. Species diversity, measured either
as H' or as the total number of species, reached its peak in
the corn fields just as the corn reached the mature stage.
The diversity measurements for all six fields and each
phenological period are presented in Figure 2-15. The
highest diversity measurement was in the mature stage at the
Gainesville field when I found 61 species and H'=4.08. As
would be expected the diversity is lowest when the corn
plants are small.

As the corn plants grow, many attributes of the
field's complexity emerge. A gradation of light, humidity
and wind is produced by the developing canopy. An array of
geometries for the web building spiders is created, as well
as an increasing variety and abundance of potential prey.
Also, the weeds begin to appear between the corn plants
providing sources of other potential prey as well as addi-
■tional structural supports. Robinson (1981) and others have
demonstrated the importance of geometry in determining
diversity and species composition.

69

Ochec Families

Figure 2-12. The major families of spiders collected in
field corn at Archer, Florida during 1982. Field A (Top)
and Field B (Bottom).

70

Other Families

Figure 2-13. The major families of spiders collected in
field corn during 1983 at Archer, Florida (top) and Orange
Heights, Florida (Bottom).

71

Other Families

Figure 2-14. The major families of spiders collected in
field corn during 1984 at Archer, Florida (top) and Gaines-
ville, Florida (Bottom).

72

>

'q

in

.Si
'u
a;

D.
CO

a

b

AAA

• • w

w w

c

d

• •-

e

f

G F M G F M

Phenological Period

Figure 2-15. Species diversity of spiders as measured by H'
in six corn fields in Alachua County, Florida. Archer, 1982
Field A (A); Archer, 1982 Field B (B); Archer, 1983 (C);
Orange Heights, 1983 (D); Archer, 1984 (E) and Gainesville,
1984 (F). Phenological periods: G = Growth, F = Flowering,
M = Mature .

73

Stratification. Six layers within the corn field
canopy are here defined: (1) the soil surface, (2) the base
of the corn plant where it meets the soil, including the
prop roots, (3) the lowest third of the foliage, (4) the
middle foliage from the fifth to the ninth leaves (numbered
from the base), which includes the developing ear, (5) the
top foliage and tassel, and finally (6) the weeds beneath
the corn plants. The results of this breakdown are pre-
sented in Figures 2-16, 2-17, and 2-18.

The seasonal dynamics of each of these strata is a
function of each individual species' population dynamics.
But since several species may be dependent on the same
environmental and biotic variables, the strata dynamics can
reveal the nature of those variables. Thus, a number of
these variables were revealed through this analysis when
combined with general field observations.

Consider first the two lowest strata, that is those
spiders which inhabit the soil surface, and those that live
around the base of the corn plant. An important point here
is that a single species is not necessarily restricted to
any one strata, but rather the opposite is true; most
species were found in more than one layer. Excluding
Orange Heights, these strata have a small representation
during the growth stage and then a modest decline in numbers
by the flowering stage and finally a sharp increase through
the mature stage. There are at least three things that

74

contribute to this movement. The first of these factors is
the cultivation of the field. One of the main purposes of
cultivations is weed control. This is achieved by turning
the soil over and breaking it up so as to bury most of the
weeds, expose the roots of others and to allow the soil
surface to become dry and inhospitable for germinating weed
seeds. This process is detrimental to spider populations
at or near the soil surface by burying many of them direct-
ly, but also by burying their food supply, namely the
insects associated with decaying plant debris from the
previous season. Cultivation has a similar deleterious
effect on the Carabidae in crop fields (House and All 1981).

Second, the decay of plant debris in the soil, which
is delayed during the cool and damp winter, accelerates at
planting when warm conditions return, thereby increasing the
supply of detritivores available for the soil surface spider
community. This material is rapidly exhausted however, and
is not replaced until flowering, when great quantities of
corn pollen are shed, much of which falls to the ground.
The pollen feeds numerous Psocoptera and Collembola which
later feed and reproduce upon the dying corn leaves at
maturity .

The dynamics of these two lowest strata of spiders in
the Orange Heights field are radically different from that
in the other five fields largely because of the contrasting
edaphic conditions. Under wet soil conditions, like those

75

15

10

Vr^

K^ Soil Surface
dm]] Base of Plant
[^ Lower Foliage
[~~1 Middle Foliage
m Upper Foliage
Wi Weeds

Growth Flowering Mature

Growth Flowering Mature

PHENOLOGICAL PERIOD

Figure 2-16. Stratification of spiders in the corn field
canopy during 1982 at Archer, Florida, Field A (A) and
Field B (B).

76

10

^ s

A

Sfl^.

T.lJjT.'?

'\:y'-'\;-'

1

^i'-iW^^

TOP

'S^'^'Si

20

15

10

Growth Flowering Mature

ty^fSH

pgfiw

B

^^ Soil Surface

fM Base of Plant

I I Lower Foliage

^ Middle Foliage

[^ Upper Foliage

^ Weeds

Growth Flowering Mature

PHENOLOGICAL PERIOD

Fiqure 2-17. Stratification of spiders in the corn field

canopy during 1983 at Archer, Florida, [A]
Heights, Florida (B).

and Orange

77

15

e 10

E

3

Z 5

iiiiiiii
1 1
m

A

Soil Surface

Base of Plant
Lower Foliage
twiddle Foliage
Upper Foliage
Weeds

iil

K''-

:::::::::::•:

ijSiiiiJ

tf^

1! '-~l~.','

'^^°M

upi]^

Growth Flowering Mature

15

B

c

B

0

■f 5

3

!J^T^r^;

liii

pttiiitnt

i/iiifniii

n

C°»t»°^:

Growth Flowering Mature

PHENOLOGICAL PERIOD

Figure 2-18. Stratification of spiders in the corn field
canopy during 1984 at Archer, Florida (A) and Gainesville,
Florida (B) .

at Orange Heights, the effectiveness of cultivation in con-
trolling weeds is diminished considerably. Large clods of
soil and sod remain upright and unf ragmented , weeds with
damaged root systems can regenerate, and the soil surface
remains moist and ideal for seedling regrowth. The general
ineffectiveness of weed control at Orange Heights is further
reflected in the greater numbers of spiders associated with
weeds. In addition, wet heavy soil is even more effective at
delaying the decay of plant debris during the winter thus
allowing an even larger production of detritivore prey wi.th
the advent of warm temperatures and the aeration provided by
the cultivator.

Another major difference between the vertical
stratification of spiders at Orange Heights and the other
fields is a lag in the numbers of spiders inhabiting the
middle and upper foliage. In the immediate vicinity of the
Orange Heights field only a limited area was left as annual,
full sunlight weeds which could serve as nurseries for
spiders adapted to the high light and wind conditions
present in the upper corn field canopy. The increase in
upper and middle foliage spiders at maturity coincided with
the mowing of these weeds.

The lag in the upper foliage hunters at Orange Heights
was paralleled by much greater numbers of spiders in the
lower foliage. Sources for shade dwelling spiders were

79

abundant in the area as under story herbs and shrubs in the
slash pine plantations surrounding the field.

The stratification of the web building spiders was
presented in Figures 2-2, 2-3 and 2-4.

Age Structure. The age structures of the spiders
collected in the corn fields are presented in Figures 2-19,
2-20 and 2-21. At the Archer and Gainesville corn fields,
during each phenological period, juvenile spiders outnum-
bered the adults by 2:1 or 3:1, and the seasonal trends were
approximately parallel. Orange Heights on the other hand
saw more than half the spiders collected during the growth
period represented as adults, and just under half were adult
during the flowering and mature stages. This difference is
largely related to the proportion of the spiders belonging
to the family Linyphiidae.

Abundant Linyphiidae at Orange Heights were favored
for at least two reasons, both related to the wet soil
conditions. First the soil and weeds were disrupted less
than at the other fields because of the lower efficiency of
the cultivator. This permitted a greater number of these
largely soil surface dwelling species to escape destruction
and to find suitable habitat. Second, since the Linyphiidae
are generally favored by a damp microclimate, ballooning
spiders would be attracted by the favorable physical condi-
tions that would not appear in the other fields until after
the canopy had developed. Also, the Linyphiidae are favored

80

15

- 10

E

I 5

15

10

O-l

lAdult Spiders
L_ — IJuvenileSpiders

Growth Flowering Mature

|Adult Spiders
I iJuvenile Spiders

Growth Flowering Mature

PHENOLOGICAL PERIOD

Figure 2-19. Age structure of spiders in corn fields during
1982 at Archer, Florida, Field A (A) and Field B (B).

a 10

c

5

A

^ Adult Spiders

Juvenile Spiders

'

^WiM

M

^M

Growth Flowering Mature

Growth Flowering Mature

PHENOLOGICAL PERIOD

Figure 2-20. Age structure of spiders in corn fields during
•1983 at Archer, Florida (A) and Orange Heights, Florida (B).

82

15

10

E

i 5

^M Adult Spiders
I J Juvenile Spiders

Growth Flowering Mature

15

JS

<^ 10

£

i 5

Growth Flowering Mature

PHENOLOGICAL PERIOD

Figure 2-21. Age structure of spiders in corn fields during
1984 at Archer, Florida (A) and Gainesville, Florida (B).

by the swampy conditions in the habitat surrounding the
Orange Heights field, and so were a major component of the
species pool available for colonizing the corn field.

Guild Analysis. A guild, as defined by Root (1973),
is a group of functionally similar species. The species
included in a guild should exploit a similar resource in a
similar way (Landers and MacMahon 1980). Of course the
breadth of a guild then will depend on the degree of overlap
between the guild species, since, it is assumed, that no two
species exploit exactly the same resource in exactly the
same way.

For this analysis six guilds were defined: (1) soil
surface hunters; (2) foliage hunters, diurnal; (3) foliage
hunters, nocturnal; (4) web spinners on the soil surface;
(5) web spinners around base of plant; (6) web spinners from
foliage layer. The families and genera that are included in
these guilds are listed in Table 2-2, and the results of
this analysis are shown in Figures 2-22, 2-23, 2-24, 2-25,
and 2-26.

Again, excluding the Orange Heights field, the dynam-
ics of these guilds through the season are remarkably
similar between fields and years, as seen in Figures 2-23,
2-25 and 2-26. This similarity can be a result of several
different phenomena. The similarity may simply be a result
of one or two dominant species in the "guild" that behave

84

Table 2-2. The guild definitions of spiders occurring in
the corn fields of north central Florida.

SOIL SURFACE HUNTERS

Lycosidae: Lycosa spp., Allocosa spp. , Pirata spp. ,
Pardosa spp.

Salticidae: Corythalia canosa , Habronatus spp.

Clubionidae: Scotinella spp., some Casteniera spp.

Gnaphosidae: Gnaphosa

FOLIAGE HUNTERS- -DIURNAL

Oxyopidae: Peucetia viridans , Oxyopes salticus

Thomisidae: all species

Philodromidae : all species

Salticidae: all species except those listed above under
soil surface hunters

FOLIAGE HUNTERS— NOCTURNAL
Clubionidae: Chiricanthium inclusum, Clubiona spp.
Anyphaenidae: Aysha spp., Wulf ila spp.

SOIL SURFACE WEBS

Linyphiidae: all species not listed under foliage layer
webs

Theridiidae: Steotoda spp.

Hahniidae: Neoantistea spp.

85

Table 2-2--continued

BASE OF PLANT WEBS

Linyphiidae: mostly the same species as occur in the soil
surface web guild

■Theridiidae: Latrodectus mactans, Achaeranea globosa,
sometimes Steotoda spp.

Hahniidae: occasionally Neoantistea sp.

Araneidae: Gea heptagon, occasionally other species.

Theridiosomatidae: Theridiosoma

FOLIAGE LAYER WEBS

Linyphiidae: Ceraticellus similis, Frontinella pyramitella,
Florinda coccinea, Grammonota texana

Theridiidae: Theridion spp., Theridula opulenta, Coleosoma
acutiventer, Argyrodes spp. occasionally
Achaeranea globosa

Araneidae: most species

Uloboridae: Uloborus spp.

Dictynidae: Dictyna spp.

86

5
4

in

C

JS o

Q. "J

o

^ 2

2 1
0

° Soil Surface Hunters
• Foliage Hunters-Diurnal
D Foliage Hunters-Nocturnal

Growth Flowering

PHENOLOGICAL PERIOD

Mature

•Figure 2-22. Guild structure of corn field spiders as a
function of phenological period during 1982 at Archer,'
Florida, Field A (A) and Field B (B).

87

c
- 3

O

oj 2
n

E

z

o Soil Surface Hunters
♦ Foliage Hunters -Diurnal
D Foliage Hunters-Nocturnal
A Soil Surface Webs

A Webs at Base of Plant
• Foliage Layer Webs

Growth Flowering Mature

PHENOLOGICAL PERIOD

Figure 2-23. Guild structure of corn field spiders as a
function of phenological period during 1983 at Archer,
Florida.

a>

E

5

" 3 -

Growth Flowering

PHENOLOGICAL PERIOD

Mature

Figure 2-24. Guild structure of corn field spiders as a
function of phenological period during 1983 at Orange
Heights, Florida.

89

Growth

Flowering

Mature

PHENOLOGICAL PERIOD

Figure 2-25. Guild structure of corn field spiders as a
function of phenological period during 1984 at Archer,
Florida.

90

o

E

Z

Growth Flowering

PHEN0L06ICAL PERIOD

Mature

Figure 2-26. Guild structure of corn field spiders as a
function of phenological period during 1984 at Gainesville,
Florida.

91

independently of each other, but similarly between fields.
This is a reasonable explanation for the soil surface
hunters which are dominated by Pardosa spp. , the diurnal
foliage hunters which are dominated by Misumenops celer ,
Peucetia viridans and Metaphidippus galathea, and the noc-
turnal foliage hunters which are composed primarily of
Chi ri cant hium inclusum.

But the similarity of a guild's dynamics between
fields could also result from the many included species each
responding similarly to changing conditions in the fields
that are related to crop phenology, for example, the
increasing structural complexity, the microclimatic trends
produced by the growing canopy, or changes in the abundance
and diversity of prey. This is the best explanation for the
guild dynamics of the web building spiders of the soil,
plant base and foliage. For the web builders of the soil,
the shade provided by the expanding canopy improves the
availability of moisture. The canopy provides a similar
change in the environment provided to spiders inhabiting the
base, while increasing the structural complexity for the
support of the many types of foliage layer webs.

.Spider Density

During this study I have expressed density of spiders
in numbers per 10 plants. Since the planting density was
recorded { 4.3 plants / m^ at Orange Heights and 2.4

92

2

plants/m^ at all of the other fields) the density of spiders

per unit area can easily be calculated. During the
flowering and mature phenological periods spider density
ranged from 22 spiders per 10 plants (9.5 spiders/m^) at
Orange Heights to a low of 9.3 spiders per 10 plants (2.2
spiders/m^) at Archer in 1983. The much higher density of
spiders at Orange Heights was due almost entirely to the
abundance of Eperigone banksi. This compares to densities
of 0.56 to 13.52 spiders/m^ in strawberries (Dippenaar-
Schoeman 1979) and 53 spiders/m^ in mown hayfields (Dondale
1970) .

Within Field Distribution Patterns

Since the location of each sample within the corn
fields was known and the size (10 plants) was kept constant,
analyses of the within field distributions of the spiders
were performed.

The first analysis was of the variance to mean ratio
utilizing the Index of Dispersion I^, as provided in South-
wood (1978) (see Table 2-3). If the distribution is in fact
Poisson, then the value of Ij-, will not lie outside the
limits of X^ for n-1 df as given in standard tables. Small
values indicate a regular distribution, large values an
aggregated distribution. This test could be conducted only
for the most common species. Pardosa milvina maintained a
random distribution throughout the season and tended toward
a regular distribution in two of the sample sets.

93

Table 2-3. The variance to mean ratios for sample densities
of common spiders taken in four corn fields in Alachua
County, Florida.

Phenological

Period

Field

Genus species

Growth

Flowering Mature

Archer,

Pardosa milvina

1.17

1.33

1.07

1983

Pardosa parvula

2.10

1.00

1.50

Peucetia viridans

2.21

0.74

1.15

Met aphid ippus

galathea

1.97

1.55

1.05

Misumenops celer

2.00

1.98

1.46

All Spiders

0.99

2.04

2.20

Archer ,

Pardosa milvina

1.10

0.62

0.60

1984

Peucetia viridans
Metaphidippus

0.93

0.95

0.73

galathea

0.86

1.11

0.88

Misumenops celer

1.00

1.38

1.18

Chiricanthium

inclusum

1.00

1.19

1.70

Gea heptagon

1.62

1.61

0.91

Tetragnatha

laboriosa

1.62

0.88

2.94

All Spiders

1.46

0.99

2-55

Gaines-

Pardosa milvina

0.93

0.74

0.82

ville ,

Metaphidippus

1984

galathea

-

-

0.64

Peucetia viridans

-

-

0.82

Misumenops celer

-

1.51

1.39

Achaeranea globosa

1.09

2.32

1.60

Gea heptagon

-

0.95

1.07

Leucauge venusta

-

1.37

1.47

Theridion

f lavonotatum

-

2.00

2.27

All Spiders

1.52

3.26

1.44

Orange

Pardosa milvina

1.50

0.88

1.54

Heights ,

Pardosa -pauxila

0.82

1.40

1.56

■ 1983

Gea heptagon

1.18

1.64

0.89

Eperigone banksi

5.09

1.64

4.19

Meioneta sp.

1.27

0.40

1.33

94

Significantly aggregated distributions (p < 0.5) were
recorded for Achaearanea globosa, Theridion f lavonotatum and
Tetraqnatha laboriosa. For these species, the aggregations
were the result of females producing offspring which did not
disperse far after hatching. Other species showed aggre-
gated distributions early in the season which were probably
due to uneven colonization of the field or uneven initiation
of silking and flowering. Still other spiders showed aggre-
gated distributions late in the season as the corn field
began senescing unevenly.

The highest degree of clumping was shown by Eperiqone
banksi at Orange Heights. In this case early dense
clumps of adult spiders were associated with clumps of sod
which had not been successfully turned over by the plow.
These clumps of spiders later produced numerous offspring
which apparently were not induced to disperse far. Both E.
banksi and A. globosa feed heavily on Solenopsis ants(see
Table 2-5); since web building spiders tend to move in
response to a lack of prey (Turnbull 1964) the abundance of
their prey in the corn fields may explain their failure to
disperse following eclosion.

Pieters and Sterling (1974) recorded I^ = 1.71 for all
spiders combined in Texas cotton fields.

As Taylor (1984) points out, the spatial distributions
of insects (and presumably, spiders) are dependent on both
sample size and density. Thus, if my sample size had been

95

more or less than ten plants, a different picture of disper-
sion would likely have been observed, whereas the low spider
densities in the corn field, probably kept most intra-
specific interactions (influents of spatial dispersion)
quite weak, except for Pardosa milvina.

By computing the correlation between the density of
spiders in a set of samples with the distances of those
samples from the border of the field, I hoped to discern
border effects. A negative correlation would indicate that
the spider was more abundant in the border areas, while a
positive coefficient would indicate that the spider was more
numerous in samples nearer the field center. This test was
also used for all spiders lumped together and for the number
of species in each sample.

Only one calculated coefficient was significant, that
for Peucetia viridans (r= -0.63; p < 0.5). P. viridans
ceases to disperse by ballooning (see Chapter III) at about
the time the corn plants are emerging. Thus some indivi-
duals may subsequently bridge or walk into the field from
adjacent plants, particularly the oak trees in the fence
rows (see Chapter IV). For the other spiders there was a
general, but only slight, tendency for a higher frequency in
border samples (in 50 of the 80 tests the correlation coef-
ficient was negative). The relationship between a sample's
distance from the border and the number of species in it was
not significant. This indicates that for most spiders

96

colonizing north Florida corn fields are good dispersers.
Another possibility, however, is that border effects are
minimal beyond a short distance (say 10 rows).

The Corn field as a Habitat for Spiders

In the faunal description above, the discussion under
the major species included mention of how various attributes
of the corn field ecosystem compliment or conflict with
each spider's biological and physical requirements. Each
potential colonizing spider responds to the corn field's
attributes as a function of its particular phenotype and
developmental state. But since many species were observed so
infrequently, a general discussion of the corn field attri-
butes that have the potential for structuring the spider
community is desirable.

Microclimate. The microclimatic conditions presented
to colonizing arthropods by cultivated row crops are usually
harsh and demanding (Cloudsley-Thompson 1962). Successful
colonists must withstand significant daily and seasonal
changes in the microclimate. The developing canopy is the
driving variable, shading the soil surface, decreasing wind
speeds, increasing relative humidity, and eventually de-
creasing the daily fluctuations in both temperature and
humidity. Upon senescence, the leaves of the crop droop or
abscise thus returning this ameliorated microclimate nearly

97

full circle to one that is again physically harsh. For the
corn fields in northern Florida this cycle required just
under four months time.

Moisture and humidity requirements of some species have
wider tolerances than others, or else the tolerances may
change with their development (Luczac 1979 and Cherrett
1964). The individual effects of the microclimatic vari-
ables, humidity, temperature, wind speed and insolation, are
interdependent and thus not easily separable without de-
tailed experimental evidence. Further, their effects may be
either direct or indirect. Direct effects which may pre-
clude or favor different spider species include desiccation
early in the season or disease brought on by excessive
humidity when the canopy becomes closed. Other spiders
respond to microclimatic variables as token stimuli for
locating their optimum habitat. Cherrett (1964) found that
the araneid Meta merianae, for example, oriented towards
darkness because that is where its preferred web sites and
prey are located.

Indirect effects of the corn field microclimate are
those which determine the array of available prey. I found
for example that dry conditions favored tenebrionid beetles
and ants while damp weather favored Collembola, Psocoptera
and small Diptera.

During the hottest, driest periods of the day, hygro-
philous spiders which orient towards the higher humidity

98

found deep in the corn silk or close to the undersurface of
the leaves can survive. During the extended dry period at
the Gainesville field Metaphidippus galathea, Phidippus spp,
£lli£i£^Ilthi^um incj^us_urn and Mi^s^umenops cej^er wedged
themselves deep in the corn silk. When corn is stressed for
water the leaves curl, thus creating a more protected spot
for spiders which must avoid highly desicating situations.

The corn plant is an efficient collector of dew, as an
early morning entry into the field will attest, but the
collected dew is also stored through much of the day in the
leaf axils. This water is available to spiders which can
imbibe it from the edge of the leaf base, such as Pardosa
milvina which was frequently observed taking water from this
reservoir. Small spiders can enter through the ligule while
larger, stronger species can push through it into the leaf
sheath cavity (e.g. Trachelas) . Dew and rain water that
drip off the tips of corn leaves create circular depressions
(4 - 10 mm in diameter) in the soil below that were used by
linyphiids to suspend their webs. This seemingly minor
effect might be quite important to these sheet web builders
which frequently require rather specific structures for the
erection of their tiny sheet webs (Thornhill 1983).

Agricultural practices are an important modifier of the
microclimates created by the corn field. Among the most
important of these is the use of irrigation. During drought
periods, which were severe on the sandy soil at the Archer

99

fields, crop fields present a particularly xeric habitat.
Spiders which orient towards moisture were concentrated in
the shaded areas where reduced insolation had reduced evap-
oration. On the other hand, when the field was irrigated
moisture seeking species migrated into the field, especially
from the annual weeds that had dried.

The amount of organic matter in the soil, as well as
the degree of soil compaction, will of course influence
water relations at the surface. When the weather had been
dry, clumps of plant debris were found to be damp inside;
here clubionids, gnaphosids, lycosids and linyphiids were
found taking refuge. Soil types that fissure upon drying
create refugia in which spiders such as hahniids can await
better conditions. Dry, powdery soil will favor those
species which can burrow deep enough to reach damper soil
(some gnaphosids and lycosids). Weeds beneath the corn
plants should create an even more humid microhabitat beneath
them; the moisture loving linyphiids frequently used this
habitat.

Plant spacing, row orientation, fertilization and
variety, also influence the degree to which the developing
canopy moderates the understory microclimate. Rows oriented
east and west allowed more sunshine to reach the soil in the
furrow than did rows oriented north and south, thus drying
the surface soil and debris. At Orange Heights, where the
soil was very wet to begin with, the high planting density

100

heavily shaded the soil; spiders typically associated with
pond edges (e.g. Pirata and Dolomedes ) found the resulting
habitat suitable.

Architecture. Three families of spiders, with abundant
representatives in the corn field (Linyphiidae, Araneidae,
and Theridiidae) , require a structure upon which to suspend
their webs. Not only must the geometry suit the design of
the web, but the location of the appropriate geometry must
also put the spider in a position to secure suitable prey
and in a suitable microclimate. The use of habitats by
these spiders has repeatedly been shown to be dependent on
the architecture of potential web scaffolding supports
(Cherrett 196 4 , Colebourn 1974, and Robinson 1981). Some
spider's webs are rather flexible (e.g. Tetragnatha and
Leucauge venusta) and thus can be supported by corn leaves
which rattle with the wind. Other spiders require a more
secure attachment, such as the much sturdier corn stalk.
Still other web spiders also require a retreat within a
rolled or curled leaf.

The geometry and arrangement of corn plants are depen-
dent on a number of factors, such as row spacing and orien-
•tation, planting density, varietal traits, insect and
weather related damage, and fertilization. Densely planted
fields will, besides altering the microclimate, provide
sturdy supports that are closer together. Modern varieties

101

of corn have been bred for more erect leaves, meaning that
the angle between the stalk and the leaf is smaller ( Jugen-
heimer 1976). Root damage and water logged soil may cause
prop roots to grow from the base of the corn plant. The
resulting cavities between these roots were heavily used by
spiders (especially Linyphiidae, Theridiidae and Lycosidae).
Whether these species can find suitable alternative sites
should determine whether their ability to colonize the field
is affected by the prop roots.

As a spider grows, it generally builds a larger and
larger snare with a likewise increase in the required dimen-
sions for the supports. Plants are not evenly spaced in the
drill row, and so a range of web spans are possible and
eventually the larger spiders, such as Neoscona arabesca and
Argiope trif asciata build webs which span across the furrow.
Many spiders which do not erect snares, frequently must
find a secure place to build a retreat, which may or may not
be provided by the corn plant. One of the most important
functions of such retreats is protection from spider-hunting
wasps. A lack of suitable retreat sites appeared to be a
factor limiting the number of Chiricanthium inclusum in the
corn fields and may also have worked against Aysha velox and
Metaphidippus galathea.

The availability of sites for webs and retreats can
change with the corn's phenology. For example, spiders
which do not rebuild their webs frequently, such as

102

Uloborus, must locate their webs so as to minimize damage by
the wind (Eberhard 1971) and so may be excluded from the
corn field until the canopy provides sufficient shelter. As
another example, M. galathea first used the undersurface of
a corn leaf for its retreat, but quickly moved to the corn
silk when flowering began, then later abandoned this site as
the dead silk rotted. On the maturing plants these jumping
spiders made their retreats among the dried stamens on the
tassel, while others abandoned the corn plants altogether
making their homes on the emerging weeds. Also, as Chew
(1961) proposes, some spiders may orient towards vegetation
upon which their camouflage functions optimally so that
microstructures (e.g. trichomes, coloration) of the plant
become important.

Planting date. Since the bionomics and life histories
of spiders are also functions of time it is obvious that
planting date can have a considerable influence on species
composition and relative abundance of spider communities
inhabiting a crop. In the literature Boiteau (1984) and
Buschman et al. (1984), for example, have reported the
significant effects of planting date on the populations of
arachnids and insects in crop fields.

The mechanisms for these effects are in some cases
quite clear. Peucetia viridans, for example, disperses by
ballooning up through 15 April so that corn fields planted

103

later in the season are largely unavailable for colonization
by this species. In most cases, however, the effects are
more complex, as those related to the phenology of adjacent
vegetation or crop fields. Large numbers of spiders are
associated with the new spring foliage of oaks, and later
disperse from these trees as the leaves mature (see Chapter
IV); the synchrony of this event with the attractiveness of
the corn field will be dependent on the planting date.

Trophic Relationships

As proposed by Whitcomb (1973) spiders play four basic
roles in a crop field ecosystem. First, they are predators
of the primary consumers of the crop, some of which are
economic pests. Second, spiders feed freely on other ento-
mophagous arthropods, including other spiders. Third,
spiders serve as food for insect predators such as Cocci-
nelidae (Whitcomb 1973) and Chrysopidae (Plagens 1981).
Finally, spiders compete with insect predators for prey,
which at times can be limiting for both groups of predators.

Primary Consumers. An understanding of the dynamics,
composition and quantity of primary consumers in the corn
field is vital to understanding the structure and function
of the spider community.

Nearly all prey consumed by the corn field spiders are
dependent on herbivory of the corn plant. Two major

104

exceptions are detritivores which feed and reproduce on the
plant residue from the previous season, and strays and
colonizers immigrating from other crop fields and nearby
vegetation. Detritivore populations are dependent on the
type and amount of crop residue, strays and colonizers on
the area of the field relative to the magnitude of the
immigrations. Both prey sources could be very important in
the establishment phase of spider community development.
For instance Pietraszko and Clercq (1982) have shown that
the addition of organic matter to the soil significantly
increases spider populations, while Luczac (1979) has shown
the importance of the first meal for first instar spiders,
a stage that frequently arrives in the field by ballooning
(see Chapter III).

Early colonizing herbivores are an important food base
for the development of a higher predaceous potential later
in the season (Gonzales and Wilson 1982). Among the first
herbivores to establish themselves on the corn plants in
northern Florida were Homoptera, primarily corn leaf aphids
(Rhopalosiphum maidis) , but also various species of leaf-
hoppers. Most frequently, I found the alate, foundress
aphids, surrounded by a dozen or less offspring, located on
the leaf sheath usually on the lower third of the plant.
Pardosa milvina was an important predator at this stage (see
Table 2-4). Other species of lycosids have been shown to be
effective biological control agents of aphids in European

105

grain crops (Chiverton 1982, Clercq and Pietraszko 1983) and
of the brown plant hopper of rice in the Philippines and
Japan ( Kartohard j ono and Heinrichs 1984). Horner (1972)
found Metaphidippus galathea to be an effective predator of
the greenbug (Aphididae) on sorghum. Occasionally large
colonies of R, maidis developed, sometimes upon the devel-
oping tassel which is protected in the whorl. Large
colonies of aphids were nearly always tended by ants, Solen-
opsis sp. or Camponotus f loridanus.

The other early arriving herbivores were the Cicadel-
idae, or leafhoppers. Very few nymphs were ever found,
which could be due either to corn being an unsuitable host
for these species (Douglas et al, 1966) and/or to the
effectiveness of the biological control exhibited by the
spiders and other predaceous insects. Cicadelids were cap-
tured as prey by a number of spiders, both wandering hunters
and web spinners. Although leafhoppers rarely cause damage
to corn directly, they are known to transmit a number of
important viral diseases (Douglas et al. 1966).

Closely following the arrival of the homopterans are
the egg laying adults of the two major lepidopterous pests
of Florida corn, Spodoptera f rugiperda (Fall Army Worm) and
Heliothis zea (Corn Ear Worm). The eggs of these moths are
potential prey for spiders such as Chiricanthium inclusum,
Metaphidippus galathea and Oxyopes sal ticus (McDaniel and

106

Table 2-4. Spider predation on pests and occasional pests
as observed in corn fields in Alachua County, Florida.
Fewer than 5 observations (small solid circles) and more
than 5 observations (large solid circles).

PREDATORS

Uloborus glomosus

Achaeranea globosa
Anelosimus studiosus
Latrodectus mactans

Theridion flavonotatum

Theridion pictipes
Theridula opulenta

Eperiqone banksi
Frontinella

pyramitella
Grammonota texana

Argiope trifasciata
Araneus pegnia
Cyclosa turbinata
Eriophora ravilla

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PREDATORS

Eustala anastera

•

Gea heptagon

Leucauge venusta

Man^ora gibberosa

Neoscona arabesca

• •

Peucetia viridans

•

•

•

Lycosa lenta

•

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•

Pardosa milvina

•

•

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Chiricanthium

inclusum

Misumenops celer

•

•

Hentzia palmerum

•

Metaphidippus

galathea

lOi

Table 2-5. Spider predation on predaceous and accessory
insects as observed in corn fields in Alachua County,
Florida. Fewer than 5 observations (small dots) and more
than 5 observations (large dots). Successfully repelled
attack (dash).

PREDATORS

Uloborus glomosus

Achaeranea globosa
Anelosimus studiosus
Latrodectus mactans

Theridion flavonotatum

Theridion pictipes
Theridula opulenta

Eperiqone banksi
Frontinella

pyramitella
Grammonota texana

Arqiope trifasciata
Araneus pegnia
Cyclosa turbinata
Eriophora ravilla

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109

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PREDATORS

Eustala anastera

•

Gea heptagon

•

•

Leucauqe venusta

•

Manqora gibberosa

•

Neoscona arabesca

•

Peucetia viridans

• •

•

•

•

Lycosa lenta

Pardosa milvina

•

•

a

•

Chiricanthium

•

inclusum

Misumenops celer

Hentzia palmerum

Metaphidippus

galathea

110

sterling 1979, 1982) although I did not observe any egg
predation. The oviposition by these two Lepidoptera differ
markedly. S. f rugiperda deposits masses of several hundred
eggs and covers them with setae; hatching larvae disperse
over a wide area and find suitable feeding sites in the
whorl or developing ears. H. zea , on the other hand,
deposits eggs singly at the feeding sites, either in the
whorl or the corn silk. Spiders which hunt in the whorl or
on the corn silk should be in a position to intercept eggs
and larvae. The egg masses of S. f rugiperda seem too widely
dispersed for spider predation to be more than a rare, but
possibly influential, event. Dispersing larvae should be
more vulnerable to spider predation, however. Whitcomb
(1967) recorded Oxyopes sal ticus and Grammonota texana as
predators of H. zea larvae.

The only other significant group of herbivores, and
thus prey for the developing spider community, were the leaf
mining Diptera of the family Ottitidae. At times these
flies were rather numerous and their damage was evident as
broken leaves resulting from the larvae boring in the leaf
midrib. Flying adult ottitids were vulnerable to sticky
snares of araneids, and were also captured by hunting
species like Metaphidippus galathea, Peucetia viridans and
Misumenops .

This rather simple herbivore community changes abruptly
with the onset of flowering. The copious production of

Ill

pollen by the tassels attracts numerous and varied pollen
feeders including Hymenoptera, Thysanoptera , Diptera and
Coleoptera. Insects which are generally classified as
predators, such as Orius insidiosus also take to feeding on
the abundant pollen (Dicke and Jarvis 1962). In addition,
oviposition by the two principal Lepidoptera species
increases, and the larvae of Nolidae and Geometridae also
appear on the tassels and occasionally the corn silks.

Despite the numerous pollen feeders, much of the pollen
falls to the ground or is caught among the plants' trichomes
or in the leaf axils. Here Psocoptera, Collembola, and
Dermaptera (among others) feed on the pollen and/or on the
molds that grow on it, thereby producing great numbers of
progeny. Many of these pollen feeding insects are taken as
prey by spiders (see Table 2-5), however the sudden over
abundance of prey simply overwhelms the spider community.
Most of these pollen feeders are of little or no interest
economically, but I believe that they divert the predatory
activity of spiders (and other predators as well) away from
the real pests, namely S. f rugiperda and H. zea. The fact
that the larvae of these pests are vulnerable to predation
for only a relatively short period before they bore into the
corn ear (Loke et al. 1983), accentuates the effect of the
abundance of alternative prey.

In addition to the abundant prey developing on the
pollen, the pentatomid pests, Nezara viridula and Euschistus

112

spp. begin ovipositing their eggs on the corn plants where
the nymphs feed on the developing ears. Despite the fre-
quent abundance of these pentatomids, I recorded just three
incidents of predation on them by spiders. Ragsdale et al.
(1981), using immunoassay techniques, recorded N. viridula
nymphs as prey for Oxyopes sal ticus , Phidippus audax and
Neoscona arabesca.

This abundance of prey in corn fields at the time of
flowering may have been responsible for the negative effect
that strip-cropping of corn with cotton had on the preda-
ceous arthropods in cotton as observed by Burleigh et al.
(1973) .

With the advent of maturity, the damage done to the
ears by Spodoptera and Heliothis draws saprophagous Diptera
that feed and oviposit in the decaying frass, while the
damaged kernels allow the entry of grain feeding Coleoptera
(e.g. Sitophilus and Oryzaephi lus ) and ants ( Solenopsis
invicta) . The adult Diptera were captured by many web
building spiders as well as the vagrant hunters. Meta-
phidippus galathea was observed feeding on maggots that it
extracted from the frass. The beetles were frequently
captured in the stronger silken webs of Neoscona arabesca,
.Eriophora ravilla, Araneus pegnia, and Cyclosa turbinata or
at the corn silk by Peucetia viridans . Also, as maturity
progresses the dried stamens from the tassels fall to the
ground and the lower leaves senesce, thus providing

113

additional food for the populations of Psocoptera and
Collembola. By this time the spider community has reached
its peak density and diversity, but soon begins to decline
as the remainder of the plants senesce and the herbivores
deplete their food supplies.

As a spider grows its prey requirements also change.
Collembola and tiny Diptera which were necessary for the
survival of first instar spiderlings may fail to even elicit
a response by the older spiders. Thus the environment must
provide an array of prey sizes in order for a spider to
complete an entire generation within the field. This
phenomenon may also limit the ability of a spider species to
show a significant numerical response to a single prey
species. After a bout of feeding, the spiders molt or
produce offspring which can no longer use the same prey
species. Thus, as proposed by Riechert and Lockley (1984),
a diverse community of spiders should be better able to
exert effective biological control of a single pest
species .

Relationship of spiders with other predators. The tro-
phic relationships that spiders have with other predators,
as mentioned previously, consist of predation, competition,
and as a source of prey for other predators.

Spiders are generally not very discriminating with
regards to their choice of prey; any insect (or spider)

114

which a spider encounters that it can handle will become
prey (Table 2-5). Prey preference tends to occur partly
result of the degree of overlap in habitat preference.
Orius spp., important predators of thrips and Lepidoptera
eggs, were frequently taken as prey by hunting spiders,
particularly in the vicinity of the corn silk by Metaphi-
dippus , Misumenops and Peucetia. The greatest number of
insect predators (including parasitoids) are taken as winged
adults by the three families of web spinners (Araneidae,
Linyphiidae and Theridiidae). This includes lebiine cara-
bids, Orius, Geocoris , and dol ichopodids , but especially the
numerous hymenopterous parasitoids. There are parasitoids
associated with essentially every one of the corn feeding
herbivores discussed previously, but the greatest number are
associated with the Rhopalosiphum maidis colonies. Large
colonies of aphids attract a large and diverse association
of predators together with their associated parasites.
Spiders, again particularly the web spinners, in the vicin-
ity of these colonies captured many of them. Unfortunately
my sampling design was not such that I could detect a
concentration of spiders in the vicinity of large aphid
colonies .

Important parasitoids and predators of the major pests
are certainly among the prey taken by spiders (Table 2-5).
Thus some spiders, particularly the web spinners which take
large numbers of winged Hymenoptera, may actually increase

115

the survival of, and level of damage produced by, Spodoptera
fruqiperda and Heliothis zea.

Ants are frequently abundant and important predators in
agricultural fields (Whitcomb 1973). Achaearanea qlobosa,
among the most common spiders in the corn fields I studied,
preys heavily on ants as do a number of other spiders (Table
2-5). Whether these spiders have a significant impact on
populations of ants is not known. Besides direct predation,
the webs of these spiders, which often act as "road blocks"
where the plant stem meets the ground, could conceivably
impede the foraging activity of the ants. Another of the
common to abundant spiders in the corn field, Eperigone
banksi, was also frequently recorded preying on ants.

Competition has long been thought to be a major force
in structuring communities of organisms (e.g. Bowers and
Brown 1982). But as Lewin (1983a, 1983b) discusses, the
theory has come under heavy fire from those who contend that
competition is only rarely of importance and can rarely be
shown to have measurable effects. Strong (1982), for
instance, feels that other forces, including predation and
stochastic events, are the principal structuring forces in
some communities. Despite a high degree of prey type
overlap and similar habitat, Riechert and Cady (1983) failed
to detect competitive release in a community of cliff-face
dwelling spiders. Considering the great number of factors I
have discussed so far, it seems that effects of competition

116

would be difficult to detect above all the "noise". Since
prey in crop fields is at times quite limited early in the
season (Price 1976), competition for limited resources
might be more important then, than later in the season when
there is an abundance of potential prey.

Riechert and Lockley (1984) hypothesize that intra-
specific competition in the form of territoriality or other
self limiting mechanisms (such as cannibalism) may be more
important among spiders than interspecific competition.
Although competition between spiders and predators may at
times be significant, the design of the present study
permits only a limited discussion. I observed two lines of
evidence that suggested competition was involved. First,
the abundant Pardosa milvina had a regular distribution (a
significant departure from random, p < 0.10) in 2 of 8
sample sets. Regular distributions can be produced as a
result of territorial exclusion. Second, in 1982, a signif-
icantly greater portion of the Pardosa milvina were found
hunting on the corn plants in field AR-82A than in field AR-
82B. And since the numbers of web spinners, particularly
theridiids, were more abundant in AR-82B, I hypothesize that
the web spinners were excluding the Pardosa milvina from the
corn plant. P. milvina were noted as prey of the theri-
diids, a possible mechanism for this exclusion.

Generalist insect predators are frequently no less
particular than spiders as to their choice of prey. It is

117

not surprising then that they will take spiders if the
opportunity arises. Whitcomb et al. (1973) reported chry-
sopid larvae and cocaine lid adults feeding on spiders and/or
their eggs, while Dennison and Hodkinson (1983) reported
spiders as a major component of the diet of Carabidae.
Besides the generalist predators which occasionally take
spiders, there are quite a number of predaceous and para-
sitic insects which specialize on spiders. The most impor-
tant order of these are the Hymenoptera, including the
Pompilidae, Sphecidae subfamily Trypoxy loninae (Krombein
1967) and Ichneumonidae (Eason et al. 1967). These spider
predators were commonly seen hunting in the Archer and
Gainesville corn fields.

Among the most frequent predators of spiders are other
spiders. Besides those that take spiders opportunistically
there are those that specialize on other spiders; Mimetus
was relatively common in all the fields.

Vertebrate predators have long been neglected as impor-
tant components of the predator community. This may be
partly due to their prevalence only in smaller fields that
have a greater area of undisturbed habitat nearby. I
observed the southern toad (Bufo terrestris) repeatedly
taking Pardosa spp. including females carrying eggs.
Remains of spiders were found in the numerous droppings of
armadillos (Dasypus novemcinctus ) which forage in the fields
at night. Lizards ( Anol is carolinensis ) and green tree

118

frogs (Hyla cinerea) were found in the corn fields hunting
on the plants. Evidence of moles and shrews was found in
the fields. Spiders form a large part of the diet of the
voracious shrews (French 1984) and insectivorous birds
(Kaczmareck et al. 1981).

Conclusion

The trophic relationships involving spiders in the corn
field are thus seen to be exceedingly complex and to affect
nearly every possible juncture in the food web, either
directly of indirectly. Of concern to the economic ento-
mologist is not so much the mere existence of these rela-
tionships, but their magnitude, and their resulting effects
on the key pests and the final yield of the crop. Given the
complexity and dynamic nature of the system it would be
impractical, if not impossible, to determine all the magni-
tudes. Thus it seems reasonable to have a preliminary, in
depth study of all the spiders inhabiting the corn field
with an eye for anything that can give some insight as to
which relationships, potentially, could have the most impact
on the dynamics of the key pests. Any sort of mass collec-
tion technique (sweep net, suction trap, beating cloth) can
not allow for a precise description of the microhabitats and
predatory relationships of the individual species. The
whole plant examination method allows for both quantitative
estimates of intensity and density together with the

119

biological information necessary to make useful judgments
with regards to future research.

But because most spiders are long lived and require a
habitat with more stability than the annually cultivated and
harvested field, a complete understanding of the biology of
crop spiders requires an examination of their movement to
and from adjacent habitats, the habitat associations of the
principal species, and an investigation as to how they
survive the winter in the crop field ecosystem (Mansour et
al. 1983).

CHAPTER III
AERIAL DISPERSAL BY SPIDERS IN THE CORN FIELD ECOSYSTEM

Introduction

A recently planted agricultural field often presents a
hostile environment for arthropods. The soil has probably
been treated with insecticide, weeds are largely eliminated
by herbicides and cultivators, and the plant debris from the
past crop is buried deep. But soon after the emergence of
the seedlings, winged herbivores arrive to deposit their
eggs, which rapidly produce destructive colonies if they are
not checked by colonizing predators.

One of the first groups of colonizing predators, spi-
ders, differ from the majority of the other colonizers in
that they arrive not as winged, egg laying adults, but most
frequently as juveniles carried in the wind on silk from
their spinnerets, the trademark of spiders' success. This
has given spiders a great deal of flexibility in the struc-
ture of their life histories, and thus their potential for
colonizing disturbed habitats such as agricultural fields.
Whether a particular spider can successfully colonize a
field will depend on a number of factors, such as its

120

121

microclimatic requirements, the structural requirements for
the placement of its web or retreat, the occurrence of
suitable prey (Luczac 1979), the chance of itself failing
prey to one of many spider predators or diseases (e.g. Eason
et al. 1967, Whitcomb 1973, Rau 1935), the toxicity of
residual pesticides and the synchronization of the spider's
dispersal and its requirements.

Past work on ballooning spiders includes Duffy (1956),
a study of the aerial dispersal of spiders in a known
community of British grassland spiders. Horner (1975)
examined the phenology of ballooning salticidae in Oklahoma,
Salmon and Horner (1977) the families of ballooning spiders
in north central Texas, while the work of Yeargan (1975) is
the only major work concerning the ballooning of spiders in
an agricultural setting. His work identified the relation-
ship between various physical factors and the daily
ballooning activity of spiders caught over a California
alfalfa field. The purpose of this study was to determine
the composition of the aeronaut fauna and to gain
information about the phenology of spider movement over an
agricultural ecosystem, specifically a field corn system.

Site Descriptions

Three study sites in Alachua Co., Florida, were used
during the study, all of which were planted to corn in

122

mid- to late March and harvested in July or August. At most
sites, traps were operated during the off season as well,
but with less intensity. The study period was from January
1983 to July 1984. The Archer site sat on a deep layer of
Candler Fine Sand (USDA 1980) that supported xeric hammock
or turkey oak scrub on less disturbed sites. Irrigated
fields were rotated with peanuts, corn, watermelons and
forage grasses. Cattle and some hogs were an integral part
of this operation, often put in the field after harvest.

The Gainesville site sits on Norfolk Loamy Fine Sand,
that supported southern mixed deciduous forest in nearby
undisturbed sites. These non-irrigated fields were planted
to corn most years and were occasionally double cropped with
a winter hay or late season sorghum. No cattle were kept in
the area, with all the crop being marketed.

Orange Heights presented a much different habitat. The
mucky Mulat Sand of this site was poorly -drained and water
logged for extended periods of time. The 25 ha. field was
surrounded by slash pine plantation, with the understory of
the younger stands used for cattle. At the field center was
a pond containing the remnants of a bald cypress dome. The
summer crop is rotated between corn and soybeans; winter
vegetables are grown as a second crop.

123
Methods and Materials

Sticky wire traps were used to capture ballooning spi-
ders from over the fields. The design (Figure 3-1), modi-
fied from Duffy (1956) and suggestions from Greenstone
(personal communication), consists of an upright furring
strip (2.4 m) and a top cross piece (1.5 m) that supports a
fine steel wire 1.2 m in length. The wire is coated with
Tack Trap, (Animal Repellents Inc., Griffin, Ga.) a sticky
polymer of isobutylene, for 1.0 m of its length.

A spider ballooning in the air clings to a long thread
of silk. Should any portion of that length become stuck to
the wire, the spider will climb the silk until reaching the
sticky wire where it will be caught. A band of Tack Trap on
the upright prevents spiders from crawling up from the
ground .

Two to four traps were operated at each field and were
serviced at 7 to 14 day intervals by carefully examining the
wire for adhering spiders, teasing them from the Tack Trap
with fine twigs or pine needles and placing them in mineral
spirits. Once the sticky material had been removed, the
specimens were transferred to 70% isopropyl alcohol for
identification and storage. A new coating of Tack Trap would
then be applied. Table 3-1 lists the field sites and the
number of trap-days accumulated for each site and month.

Voucher specimens are deposited in the Florida State
Collection of Arthropods.

124

Eye Hook

V Steel Wire

Coated With Tac-Trap®

Barrier of Tac-Trap*^

Figure 3-1. Trap used for studying aerial dispersal in
spiders. Upright consists of a furring strip (5 cm X 12 cm
X 2 m), cross bar of pine molding (10 mm X 10 mm X 1.2 m).

125

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126

Results

During the 18 month study period, a total of 1,805
spiders, representing 15 families and at least 53 species,
were captured. The families Araneidae (orb weavers) and
Linyphiidae (sheet web weavers) had the largest portions of
both species and numbers, but several other families made
sizable contributions to the total. Figure 3-2 shows the
proportions of the total capture that belonged to the major
spider families.

On a seasonal basis, the intensity of ballooning activ-
ity, as indicated by Figures 3-3 and 3-4, ranged from a low
of 3.7 spiders per 10 trap days during March 1984 at Archer,
to a high of nearly 15 per 10 trap days at the same Archer
field during April 1983. This indicates great fluctuation
in activity from year to year. The great peak of Linyphii-
dae during April of 1983 was due especially to a single
species, Er_i3_one a.utumnaj^_i£, that occurred with low
frequency the following year. Also, both the Lycosidae
(primarily Pardosa milvina) and the Oxyopidae (lynx spiders,
mostly Peucetia viridans) were also noticeably absent from
the 1984 captures. Even still, the intensity of ballooning
on a seasonal basis was relatively constant, at least for
Archer 1983. Again, the Linyphiidae and Araneidae are well
represented in the captures throughout the year, but as with

127

Figure 3-2. Family proportions of ballooning spiders cap-
tured from January, 19 8 3. through July, 1984 in Alachua
County, Florida using sticky wire traps.

12i

^ 1.0

B

Plj'i^'ji^'l Llnyphlldae
lixijiiivi-SI Araneldae

Lycosidae

Oxyopidae

bv!".y''''1 Anyphaenldae

t5:;S?r::a Clublonldae
Other

Families

JAN FEB MAR APR HAY JUN JUL
1984

Figure 3-3. Ballooning intensity of spiders by month at
Archer, Florida during 1983 (A) and 1984 (B) as indicated by
sticky wire traps.

129

SEP OCT NOV DEC JAN
1984

Llnyphlldae

Araneldae
Lycosldae

Oxyopidae
^'-'.V'';^"] Anyphaenidae

p=^'3S:3 Clublonidae
I I Other Families

APR MAY JUN JUL
1984

Figure 3-4. Ballooning intensity of spiders by month in
Alachua County, Florida at Orange Heights, during 1983 (A)
and Gainesville (B), during 1984.

130

most of the other families, the species makeup changed
considerably during the season.

Table 3-2 lists the identified species caught on the
sticky wire traps, giving their seasonal phenology, relative
frequency and an indication as to whether juveniles, adults,
or both were represented. The age at which the species
disperse by ballooning varies considerably. Peucetia
viridans for example ballooned only as first instar spider-
lings (defined here as the period following emergence from
the egg sac and the next molt). This period extended from
November until early April and represents the over wintering
stage of this spider. Aysha spp. on the other hand
ballooned throughout the year as first through third
instars or occasionally as larger individuals, with seasonal
variation not very pronounced. Like Aysha, first to third
or fourth instars of Pardosa milvina balloon with high
frequency, but mostly during October through March. Only an
occasional specimen was caught on the traps during the
summer months even though abundant juveniles were present in
the fields (see Chapter II).

The majority of the Linyphiidae and many Theridion
spp. captured were adults or penultimate adults. These spi-
ders are mostly very small, averaging less than 2 mm in
length. However, adult specimens of at least 25 other

species were encountered, some as long as 5 mm.

131

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136

The frequency distribution of spider size classes, as
expressed by the total body length, is presented in Figure
3-5. The size class 1.0-1.5 mm contains 44% of all the
spiders captured. The next smaller size class, 0.5-1.0 mm,
had considerably fewer spiders, while no spiders smaller
than 0.5 mm were captured. An apparent logarithmic

decrease in numbers of spiders captured occurs for 0.5 mm
increments above 1.5 mm. Clausen (1983) showed that a
linear relation exists between the natural logarithm of the
wet weight and the natural logarithm of the length. Since
weight is a major component in the ability of a spider to
balloon this may explain the shape of the weight length
distribution .

During the course of the study observations of bal-
looning spiders taking to the air were made. The usual
technique used by ballooning spiders, as described in the
literature (Bristowe 1929, Blackwall 1827, and Richter
1970), is for the spider to hold the spinnerets, which are
at the posterior of the abdomen, as high off the substrate
and into the wind as possible. Once a strand of silk, long
enough to carry the spider upwards has been drawn out into
the wind, the spider releases hold of the substrate. I
observed this in Peucetia viridans. However, I observed
quite a different method in first instar Misumenops and
Hentzia, which began by hanging from a projecting leaf or
branch. When a breeze was produced, the spider was carried
upwards, much as a kite, while the drag line remained

137

a;
■a

E

P 0^ 1.d 1^ 2.6 IS 3.d 3^ 4X) 4J 5.0

TOTAL BODY LENGTH, mm

Figure 3-5. Frequency distribution of size classes of
ballooning spiders captured on sticky wire traps in Alachua
County, Florida from January, 1983 through July, 1984.

138

attached to the take off point as would a kite string. In
some cases the silk eventually became detached from sub-
strate while other times a break occurred nearer the spider
leaving an empty strand of silk attached to the take-off
point while the spider was carried away in the wind. Coyle
(1985) described a very similar ballooning behavior in the
mygalomorph spider, Ummidia. A method similar to this was
also observed in slightly larger spiders moving between
plants without becoming totally airborne. This behavior
could be related to the hanging defense of diurnal wandering
spiders, where spiders gain protection from nocturnal preda-
tors by remaining motionless while hanging by a strand of
silk at night (Carroll 19 77).

Discussion

Crop fields as a habitat for arthropods are unpredic-
table (as defined by Colwell 1974) because biotic and
abiotic conditions (e.g. carrying capacity) change rapidly
on a time sequence that is frequently neither constant nor
contingent. Greenstone (1982), using two species of
Pardosa, empirically demonstrated a relationship between a
species' habitat predictability and its tendency to balloon.
Thus it is not surprising that many of the major components
of crop spider communities in the southeastern United States
(Whitcomb et al. 1963, Dean et al. 1982, Culin and Yeargan

139

1983a) were captured with high frequency on the ballooning
traps.

If age structure and phenology are considered, five
general categories of dispersal behavior were observed
during the present study: (1) one synchronous generation per
year, with only one or two instars ballooning; Peucetia
viridans and Pirata sp. were the most common examples of
this type; (2) one synchronous generation per year, but with
several instars dispersing by air; examples are Neoscona
arabesca and Lycosa lenta; (3) overlapping generations with
several instars ballooning, primarily during a short part of
the year; Pardosa milvina is the principal species in this
category; (4) overlapping generations, with only one or two
instars ballooning throughout the year; this strategy is
used primarily by the smaller species of Linyphiidae and
Theridion, where adults and penultimate adults are the
stages which balloon; (5) overlapping generations with
several instars ballooning throughout the year. Many of the
most abundant spiders of crops in northern Florida adopt
this dispersal strategy such as Misumenops celer, Chirican-
thium inclusum, Aysha velox, Metaphidippus galathea, Hentzia
palmerum, Oxyopes salticus and Tetragnatha laboriosa.

That this last strategy is used widely by crop spiders
is evidence that it is an adaptation for colonization of
environments that are unpredictable over space and time.
These captures do not simply represent local movement of

140

abundant spiders in the corn fields over which the traps
were operated. Oxyopes salticus, Misumenoides f ormosipes,
and Aysha velox, for example, are abundant in other crops
(Neal 1974 and Lockley et al., 1979) but not in corn fields
(see Chapter 2). For species which exploit a habitat with a
carrying capacity that is variable with time (characteristic
of crop fields) the best strategy is for the population to
maintain a relatively high magnitude of dispersal at all
density levels (Gadgil 1971).

Seasonal, yearly and local fluctuations in ballooning
captures of a given species can be a function of a number of
factors which modify the evolved migratory behavior. Most
obvious of these is a change in the population density due
to predation, weather -related mortality, pesticides, or a
change in the carrying capacity of the environment. I was
able to identify some of these during the course of the
study. The great number of Erigone autumnalis captured in
April 198 3 was coming from a nearby (.8 km) field of matur-
ing rye. A burst of Misumenops celer was observed just after
tasseling was complete, and another burst of M. celer and
other species as the whole crop was mature and drying.
Pardosa milvina ballooners were more frequently encountered
on traps located near areas which were heavily grazed by
cattle, since these areas are frequently heavily populated
by these spiders (see Chapter IV). Other species balloon

with, the phenological changes in the surrounding trees and

141

weeds. Land use and management practices are the major
driving force behind much of these changes and offer possi-
bilities for altering the composition of the species pool
from which a crop field will be colonized.

Thus, differences in the pool of ballooning species
(and thus the pool of potential corn field colonists)
between the three sample sites was evident as shown in Table
3-3. These differences were primarily in the relative
intensities of principal species rather than differences in
the species lists. For the most part, these differences
reflected the differences in the dominance structure of the
spider communities occurring on the weeds and adjacent vege-
tation (see chapter IV), This indicates that most bal-
looning spiders move only a short distance from their
source, but that a small portion may be carried to more
distant points. This is consistent with Price (1975) who
states that it is mostly maladaptive to disperse more than
locally; an individual has only a small chance of finding a
suitable distant source.

Weather, the main force not under the control of man,
was responsible for frequent alterations in ballooning
intensity. One such weather phenomenon, a severe freeze in
December of 1983, was probably responsible for the near
absence of some species in the spring of 1984. Also, as
similarly recorded by Yeargan (1975) and Click (1939),
during the winter months, clear, warm, balmy days produced

142

Table 3-3. Relative ballooning frequencies for spiders at' three
sites in Alachua County, Florida.

Spider

Archer

Orange Heights Gainesville

Theridion spp.

Ceraticellus similis
Eriqone autumnalis
Florinda coccinea

Araneus pegnia
Gea heptagon
Leucauge venusta
Mangora spp.
Neoscona arabesca
Tetragnatha laboriosa

Mimetus sp .

Pisaurina cf. mira

Pardosa

milvina

Lycosa c

f. lenta

Oxyopes

salticus

Peucetia

viridans

Chi ri cant hium inclusum
Clubiona spp .

Aysha spp.

Misumenoides f ormosipes
Misumenops celer
Xysticus spp .

Philodromus spp.

Hentzia palmerum
Metaphidippus galathea
Phidippus spp.
Thiodina sylvana

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143

the greatest numbers of ballooning spiders. However, the
largest species, such as Lycosa lenta were captured only
during rather windy weather. Drought and irrigation prac-
tices can work in concert to affect the movement of spiders.
Under drought conditions spiders exited unirrigated fields
and accumulated in tree-shaded areas of the field or on
deep-rooted plants that still supported insect prey. The
mild winters in north Florida were partly responsible for
relatively constant ballooning activity year round, which is
in contrast to results of studies in more temperate areas
(Horner 1975, Duffy 1956).

The males of at least 25 species (other than Linyphii-
dae) were captured on the sticky wire traps, suggesting that
ballooning is an effective means to search for females. The
morphology of male spiders may aid them in this mode of
locomotion: generally small body size with long setaceous
legs. Just as the pappus of a composite seed provides
resistance to the movement of air through it, which is
transformed into aerial lift, so do the long legs and setae
provide aerial lift to the spider when air currents move
through them. The long legs and setae should especially aid
spiders that become airborne in the fashion of Misumenops
and Hentzia described above.

Mim etus is a genus of spiders in which all stages
possess long legs and setae. These structures are generally
agreed to serve the spider in capturing its usual prey,

144

the webs of prospective prey, as evidenced by their propen-
sity for ballooning (see Table 3-2). Argyrodes fictilium,
another spider with long spindly legs that preys on other
spiders (Trail 19 80), was also captured during this study.
Interestingly, other species of Argyrodes, which are primar-
ily kleptoparasites, are not long legged and were rarely
caught ballooning despite their abundance in the areas near
the fields. As kleptoparasites, their need to move from one
host's web to another is diminished. Finally, spiders of
the genus Tetragnatha are among the greatest aeronauts.
Numerous middle instars as large as 3.5mm were captured,
while Okuma (1981) recorded their arrival on buoys 200 km
from land in the South China Sea. It may be that the long
slender legs of these spiders, also aid in their dispersal
by air.

Trapping schemes used to study biological phenomena are
not without biases, and the sticky wire traps used in this
study are no exception. Work by Greenstone (1985) has shown
that sticky wire traps consistently underestimate the
lightest spiders, particularly those weighing less than
0.4mg (based on Clausen's (1983) weight/length data these
spiders are less than 1mm in length). Thus the numbers of
juvenile Linyphiidae, Achaearanea, and Theridion ballooning
in Alachua Co. may have been considerably greater than was
indicated by the trapping scheme. Greenstone also found
that trap height had significant effects on the composition

145

ballooning in Alachua Co. may have been considerably greater
than was indicated by the trapping scheme. Greenstone et
al. also found that trap height had significant effects on
the composition of the aeronaut fauna collected. Thus a
constant height of traps probably over- or underestimates
certain species. However, the simple design of these traps
and the ease of their operation should permit their use to
greatly expand our knowledge of aerial dispersal in spiders,
the role it plays in their life history strategies, and the
role spiders play in both natural and agricultural
ecosystems .

CHAPTER IV
WEED ASSOCIATIONS OF SPIDERS IN THE CORN FIELD ECOSYSTEM

Introduction

From planting to harvest most row crops require only
part of the year, and since many spiders' life cycles
require a year or longer to complete (Gertsch 1979) it
becomes obvious that a crop spider community must be an
integral extension of communities occurring on nearby
permanent vegetation (LeSar and Unzicker 1978). Further,
because spiders possess excellent dispersal capabilities
(see Chapter III), the community composition of the source
habitats and the dynamic exchanges between systems become
the central points in understanding the dynamics and
structure of spider communities in crop fields.

Studies devoted to spiders associated with weeds in
agricultural situations are limited. Stadlebacher and
Lockley (1983) tallied the spiders occurring on Geranium
distichum, a winter annual of southern Mississippi agricul-
tural areas. Altieri and Whitcomb included spiders in their
list of predators associated with Mexican Tea (Chenopodium
ambrosioides ) (1979) and Camphorweed (Heterotheca subaxil-
aris) (1980) in North Florida.

146

147

One of the most difficult aspects of dealing with crop
or weed spider communities, is the shear number of species,
especially if one is unwilling to lump distinct biological
species into convenient groups for the purpose of making
statistical analyses. In addition, many of the most impor-
tant species occurring in crop fields have asynchronous
generations, so that any given population in the crop or in
some adjacent habitat may consist of all instars from first
through reproducing adults- And again, since the individual
instars may have varying habitat and prey requirements, to
treat them together as biological equivalents may lead to
spurious conclusions.

Plants and weeds growing in a crop ecosystem possess a
variety of growth phenologies, such as spring and summer
annuals, herbaceous spring and summer perennials, or woody
perennials such as shrubs and fence row trees. In addition,
they vary in their assembly of primary and secondary
consumers, structures for web scaffolding and retreats, and
the microclimate of their preferred habitat. Each of these
factors can contribute to the array of spiders associated
with a given plant and the temporal component of the
association .

Given this complexity of confounding variables, the
purpose of this study was to identify the species compo-
sition of spiders occurring in association with the prin-
cipal plants in a field corn ecosystem, to detect the

148

interplant movements of the most abundant species, and to
obtain a general picture of which plants potentially act as
sources, sinks, or intermediate stops for colonizing
spiders.

Site Descriptions

Three study sites in Alachua Co., Florida, were used
during the study, all of which were planted to corn in mid
to late March and harvested in July or August. The Archer
site sat on a deep layer of Candler Fine Sand (USDA 1980)
that supported xeric hammock or turkey oak scrub on less
disturbed sites. Irrigated fields were rotated with
peanuts, corn, watermelons and forage grasses. Cattle and a
few hogs were an integral part of this operation, often put
in the fields after harvest.

The Gainesville site is located on a Norfolk Loamy Fine
Sand that supported southern mixed hardwood forest (Monk
1965) in nearby undisturbed sites. These non-irrigated
fields were planted to corn most years and were occasionally
double cropped with a winter hay or late season sorghum. No
cattle were kept in the area, with all the crop being
marketed.

Orange Heights presented a much different habitat. The
mucky Mulat Sand of this site was poorly drained and water
logged for extended periods of time. The 25 ha. field was

149

surrounded by slash pine plantation, with the understory of
the younger stands used for cattle. At the field center was
a pond containing the remnants of a bald cypress dome. The
summer crop is rotated between corn and soybeans; winter
vegetables are grown as a second crop.

Methods and Materials

The predominant weed, shrub, and tree species growing
in the vicinity of each field were sampled for spiders.
This was conducted by selecting stems or branches that
measured approximately 0.5 meters in length, carefully
pruning them from the plant while holding a 45 cm diameter
sweep net underneath, and then vigorously shaking the plant
sample inside the net. This assured that a majority of the
spiders would be dislodged and recovered in the bottom of
the net. The spiders contained in the net bag were pre-
served in alcohol for later identification in the labora-
tory. For the most part, a sample consisted of 10 to 25
such plant stems; this sample size was determined by the
abundance of spiders on the plant and the prevalence of the
plant around the field. This "net-bag" sampling technique
was impractical for grasses and thorny blackberry shrubs,
where standard use of the 45-cm sweep net was substituted.
Again the sample unit was 10 or 25 sweeps.

150

Chew (1961) and others (e.g. Jennings 1971, Duffy
1962) have noted the difficulty in comparing the spider
communities between plant species because of the difficulty
in equalizing the sample size. Despite the differing struc-
tures of individual plants this method gives a rough corres-
pondence of total leaf area sampled from each species.
Also, the actual process is slightly different for branches
that grow vertically versus those that grow horizontally.
Finally this sampling technique admittedly samples only
foliage species and does not adequately sample species that
occur on the lowest stems. Thus comparison of the weed
spider communities with the corn field communities is
restricted to species inhabiting the upper foliage layer.

All sampling was conducted in the morning hours to
preclude any time dependent differences in spider activity
and was conducted throughout the year but was more intense
during the growing season. The most abundant plant species
were sampled at weekly or biweekly intervals (when present)
but less frequent species were sampled only as time
permitted.

Spiders were identified according to Kaston (1981) and
the individually published specific keys referenced in
same. Juvenile spiders of many species could be identified
by comparison with known specimens reared from adults in the
laboratory or by association with adults. Closely similar
species which co-occurred could not be separated as

151

juveniles. Voucher specimens are submitted at the Florida
State Collection of Arthropods.

Results

During the three year period 4500 individual plant
stems were sampled, yielding 3312 spiders. The data were
analyzed in two principal ways. First the relative inten-
sity of the major species of spider on the various plants
was determined and is presented in Tables 4-1, 4-2, 4-3 and
4-4. By looking at the rows across the tables, it is
clearly seen, that some plant species have much higher
densities and diversities of spiders associated with them
than do others, while the columns demonstrate that indeed
there are specific associations between spiders and plants.

The second analysis was of the temporal dynamics of the
six most common spiders and their association with plants in
the corn field ecosystem. These results are shown in Fig-
ures 4-1 (Peucetia viridans), 4-2 (Misumenops celer) , 4-3
(Aysha spp.), 4-4 ( Chiricanthium inclusum) , 4-5 (Hentzia
palmerum) and 4-6 ( Metaphidippus galathea) . The width of
the horizontal bars indicates the approximate relative
intensity of the spider on the plant over time. Since the
density of some plants (especially annuals) changed substan-
tially during the season, the density of a spider per unit
area is function a of both the spider's intensity and the
plant's density.

152

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plants in and near the field corn ecosystem in Alachua

County, Florida- Width of horizontal bar indicates approxi-
mate relative intensity.

Figure 4-2. Temporal association of Misumenops celer with
plants in and near the field corn ecosystem in Alachua
County, Florida. Width of horizontal bar indicates approxi-
mate relative intensity.

157

Aft «T

JUL! too

ttrt OCT

Figure 4-3. Temporal association of Aysha velox with
plants in and near the field corn ecosystem in Alachua
County, Florida. Width of horizontal bar indicates approxi-
mate relative intensity.

Figure 4-4. Temporal association of Chiricanthium inclusum
with plants in and near the field corn ecosystem in Alachua
County, Florida. Width of horizontal bar indicates approxi-
mate relative intensity.

159

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Figure 4-5. Temporal association of Hentzia palmerum with
plants in and near the field corn ecosystem in Alachua
County, Florida. Width of horizontal bar indicates approxi-
mate relative intensity.

Figure 4-6. Temporal association of Metaphidippus galathea
with plants in and near the field corn ecosystem in Alachua
County, Florida. Width of horizontal bar indicates approxi-
mate relative intensity.

161

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162
Discussion

The density and diversity of spiders associated with a
plant seemed to be most closely affected by the density and
diversity of prey on the plant. The potential prey asso-
ciated with Richardia scabra , for example, was largely
limited to flower visitors, and so the spider fauna asso-
ciated with this plant is rather meager as compared to
Indigof era or Cassia. Again, the spider association on
Quercus , particularly in the spring, parallels the great
flurry of herbivores that feed on the developing leaves,
while Liquidambar styracif lua has few spring time herbivores
and thus few associated spiders.

The diversity, as both evenness and total number of
species, was also affected by the sample size and the period
of time over which the plant was present. Melilotus , Phyto-
lacca, and Lepidium are late winter to early spring plants
in northern Florida, and so smaller samples over a shorter
period of time were taken. Spider density on these plants
was relatively high (which again paralleled abundant prey)
but was restricted to fewer species.

Specific associations offer many lines of conjecture as
to possible reasons for them, however I shall present only
those that were most clearly demonstrated by the sampling
and general observations. Three major attributes of the
plants that determined which spiders were associated with

163

them were the plant's architecture, the microclimate where
the plant usually grows, and the array and relative benefits
of potential prey that the plant attracts (Morse and Fritz
1982, Riechert 1973).

Structures for webs or retreats are important to many
species of spiders (Duffy 1962, Cherrett 1964, Colebourn
1964, Robinson 1981). Araneidae were, for example, gener-
ally more abundant on the stiff, woody plants than on the
herbaceous ones. The araneids which did occur on the herba-
ceous plants were largely nocturnal species which produce
temporary webs so that damage by the wind would be mini-
mized. Selection of web sites that minimize damage by the
wind was demonstrated for Uloborus by Eberhard (1971).
Theridiidae which generally suspend their webs from the
lower surface of leaves such as Theridula and Theridion,
were more common on plants with broad leaves than on those
with finely dissected leaves. Finally, both families of sac
spiders, Clubionidae and Anyphaenidae, were more common on
the plants with finely dissected leaves, or broad leaves
tattered by lepidoptera; these plants provided better sites
for the silk retreats of these spiders.

Microclimatic conditions, especially humidity and
light, have long been accepted as major factors in deter-
mining the distribution of spiders ( Cl oudsl ey-Thompson
1962). The four tables are arranged accordingly in-co sunny
annual weeds, sunny perennial weeds, shaded herbs, shrubs

164

and saplings, and the major trees. The canopies of trees
such as oaks are sun drenched and thus probably present a
microclimate similar to the open field. This was evidenced
by the similarity between the fauna of Quercus spp. with the
sunny field weeds. There were a number of other associa-
tions that appeared to be dictated, at least in part, by the
microclimate. Among the salticidae, Hentzia palmerum and
Thiodina were more abundant than Metaphidippus galathea on
partially shaded plants, while the opposite was true on
plants in full sunlight. Also, among the similar Clubion-
idae and Anyphaenidae, Chiricanthium inclusum was more
abundant on the sunny plants than Trachelas , Clubiona, and
Aysha spp, again with the opposite true on shaded plants.

Microclimate may not be the direct or sole component
governing these associations since Bigger (1981) and
Futuyama and Gould (1979) have shown that the arthropod
community as a whole is composed of different taxonomic
groups between sunny and shaded vegetation; thus the rela-
tionships may also be related to differences in the types of
available prey.

The lack of prey associated with certain plants (e.g.
Richardia scabra ) clearly inhibited associations of many
spiders with those plants, however specific differences due
to differences in prey types are difficult to detect without
numerical data concerning the communities of potential prey.
Misumenops celer and Peucetia viridans which prey heavily on

165

pollinators were strongly associated with plants in flower,
particularly those with a densely clustered inflorescence.
Louda (1982), also studying P. viridans , found them to hunt
from the denser, flat topped clusters of Haplopappus spp.

The temporal association of the six most common spiders
with plants in the corn field system provides potentially
very useful information for understanding the life history
of these spiders and for the development of management
strategies. I shall discuss the dynamics for each of the
six species illustrated. During these discussions it is
important to know that following harvest of the corn crop,
Indiqof era and Cassia became very abundant and produced
nearly 100% cover.

Peucetia viridans (Figure 4-1), with a single, synchro-
nous generation, first occurred as large numbers of first
instar spiderlings on the fresh new foliage of oaks (Quercus
spp.) from which they moved (probably by ballooning) in
early April to the corn field, Sida rhombifolia, Celtis and
Solanum. Subsequent movements of P. viridans to Ambrosia,
Sida, Cassia and Indigofera was probably by bridging and/or
crawling. The intensity of P. viridans on most of the
plants was low by September which may have been due in part
to the absence of more preferred plants such as Monarda (bee
balm) and Eupatorium (dog fennel) (Whitcomb 1984). The
plants upon which these spiders could finally reproduce,
i.e. Sida, Chenopodium, and Eupatorium are avoided by cattle

166

and so were frequently abundant in pastures. These plants
were thus the likely sources for the first instar P. vir i-
dans the following season. The fate of those that stay on
the Indiqofera or Cassia is less certain; in the fall these
weeds are either plowed under for mulch or used as forage
for cattle.

The movements of the remaining species are more diffi-
cult to interpret because the spiders have widely
overlapping generations. Thus a low population of repro-
ducing adults on one plant may represent a source of colon-
izing juveniles while a high population of juveniles on
another plant may represent temporary residents that will
soon move to another plant. Misumenops ce 1 er for example
were most abundant as early instars on Paspa 1 um , middle
juveniles on Ambrosia and later instars and adults on Cassia
and Sida rhombif olia. Although direct movement between
these plants was not observed, it seems the most likely
explanation. Movements are probably a function of the
changes in the prey on the weeds and the changing prey
requirements of the growing spiders, but may also be related
to changes in microclimate and light reactions (Cherrett
1964) .

Aysha spp. occurred as reproducing adults only rarely
on the weeds growing in the fields but rather on adjacent,
semi-shaded trees and shrubs. Thus the adults on Quercus and
Celtis represented the source of juveniles; some stayed on

167

the trees while others ballooned to the field weeds such as
Sida , Ambrosia , and Cassia. The rarity of adults on these
plants suggests that the spiders may return to the trees
before maturing. Chiricanthium inclusum occurred sporadi-
cally as reproducing adults on shrubs, trees, and Sida, the
offspring of which colonized the field corn and then the
emerging legumes. Cassia and Indigof era ; by the end of the
season adults were found on these plants as well.

The two salticidae, Hentzia palmerum and Metaphidippus
galathea , were abundant on many of the weeds and plants
associated with the field corn ecosystem. They were also
quite common on the corn plants. In the spring H. palmerum
and especially M. galathea were common on the Quercus spp.
This reflected a general trend throughout the season with M.
galathea preferring the plants in full sunlight and H.
palmerum having slightly lower intensities but less segre-
gation between shade and sunlight plants. Like Misumenops
celer , Aysha and Chiricanthium these spiders move frequently
by ballooning (see Chapter III) in the early to middle
instars. Adults are less likely to move far, so that plants
that provide suitable habitat and prey for adults early in
the season can act as sources of dispersing juveniles for
the emerging corn crop and/or the weeds. Hence, for M.
galathea , the sources of juveniles were the oak trees, and
for H. Hsj^m e r urn , the sources were Si^da r homb i^f^oj^i.a ,
Phytolacca , and Cal licarpa.

168

The frequent dispersal of juvenile spiders between
plants and adjacent habitats (also Enders 1975) contrasts
sharply with the strategy employed by most predaceous
insects, which are mobile in the winged adult stage.

Manipulation of the weeds in and around crop fields
offers the easiest way to manipulate the communities of
spiders as well as many other predaceous insects in the crop
field. Most of these manipulations are inadvertent and are
merely a function of land use and farming practices.
Grazing of cattle, for example, tends to favor Eupatorium ,
Rubus , Chenopodium ambrosioides , Asimina and therefore the
spiders they support. Heavy grazing by cattle also tend to
increase the numbers of Pardosa milvina. Barren patches of
soil provide the preferred habitat for this cursorial
species while the dung breeds dipterous prey. The cattle
also carry water from the pond or well; the spiders subse-
quently extract this water from the dung. The range and
movement of Pardosa from these cattle pastures could be in
response to prey density (Hallander 1970) which would occur
with the rotation of the cattle between pastures.

The trees growing near crop fields may be more impor-
tant than previously recognized. Oaks especially have
tremendous numbers of spiders associated with them.
Importantly, many of these spiders migrate from the oaks in
mid spring when the populations of oak feeding herbivores

169

decline as tannic acids begin to accumulate in the leaves
(Wratten 1981). Many of these spiders (or their offspring)
end up in the corn fields. Turner (1984) also recorded the
abundance of spiders among arboreal predators.

Slash pine (Pinus ell iottii ) plantations offer few
colonists for the corn field, but as must be considered when
any plant is desired as a source of predators, there are few
herbivores common to pines and most crops.

The perimeters of crop fields normally consist of mown
grass and or weeds that allow for access and maneuvering of
farm equipment. Although these areas are generally quite
narrow in relation to the whole field, they receive fertil-
izers and irrigation water from the crop field, and so may
be particularly productive. Movement of spiders into and
out of these field borders are controlled by the changing
phenologies and composition of the weed flora but also by
the timing and frequency of mowing. Some of the spiders
remain after mowing as they do in alfalfa (Culin and Yeargan
1983a, 1983b), but many move either to the corn plants or to
adjacent habitat depending on which is more attractive at
the time. If the corn field is just entering the silking
stage, potential prey is also becoming abundant and thus the
spiders should end up there.

Manipulation of weedy habitat is one alternative for
manipulating predators (Altieri and Todd 1981), another is
strip cropping. Strip cropping has appeal because the

170

predator source (which may have a dual function acting as a
trap crop for pest species) has economic value in itself.
Fye (1972) discussed the requirements of strip crops which
are expected to be predator and parasite sources. These
include a tolerant crop, the proper insect complex, timing,
and mobility of the predators and parasites.

Besides being a source of spiders that can colonize a
crop field, adjacent flora and within field weeds can also
act as sinks for spiders that leave the crop at maturity.
Given the potentially large numbers of spiders from the
crop, it is probable that the spider fauna of the weeds is
also intricately affected by the crop. If spiders are
indeed beneficial components of the predaceous arthropods in
crop fields, it is clear that a holistic view of the crop
field ecosystem (LeSar and Unzicker 1978) and a complete
understanding of the biology and ecology of the spider
species (Mansour et al. 1983) are necessary in order to
assess and manipulate the spider community effectively.

CHAPTER V
CORN STUBBLE AS AN OVERWINTERING SITE
FOR SPIDERS IN FLORIDA

Entomologists have learned to take advantage of the
overwintering strategies used by herbivorous insects, such
as the boll weevil (Slosser et al. 1984), the southwestern
corn borer (Archer et al. 1983), and the bean leaf beetle
(Jeffords et al. 1983). Overwintering populations can be
limited by tilling, which exposes and breaks up the post
harvest trash, and/or by removing accumulated leaf litter in
the borders where the hibernating stages are found.

But studies concerning the overwintering stages and
sites of predaceous arthropods found in agroecosystems have
largely been neglected, particularly in the United States
(Desender 1982). Thus our ability to manipulate populations
of beneficial arthropods is hindered. Perhaps this is due
in part to entomologists' desire to publish the previous
summer's data during the winter months.

Spiders, due to their long life cycles, and slow rates
of reproduction, should be particularly dependent on the
adequacy of suitable overwintering sites. The purpose of
this study was to elucidate the type and location of such
sites in the field corn ecosystem..

171

172

Site Descriptions

Two corn fields in northern Florida were examined for
overwintering spiders. The first was located near Archer,
Florida in southwestern Alachua County and was studied
during the winter of 1982/1983, The second field was
located on the Tall Timbers Research Station in northern
Leon County and was studied during the winter of 1983/1984.
The Archer site sat on a deep layer of Candler Fine Sand
(USDA 1980) that supported xeric hammock or turkey oak scrub
on near by, less disturbed sites. Irrigated fields were
rotated with peanuts, corn, watermelons and forage grasses.
Cattle, and to a lesser extent hogs, were an integral part
of this operation, often put in the field after harvest.

The Tall Timbers corn fields were planted on a sandy
clay loam soil and were surrounded by fire managed slash
pine plantation, that had been maintained with the dual
purpose of maximizing wildlife populations, particularly
quail and deer. The post harvest corn trash was thus left
in the field for the birds and animals to glean.

Methods and Materials

The trash and leaf litter in the corn fields were
examined and sifted over a white cloth. Spiders and insects
were collected and preserved in alcohol. Once it was

173

established that spiders and insects were particularly abun-
dant in the corn stubble itself, sets of 10 corn stalks and
cobs were sampled from the corn fields, but could not be
selected at random since the harvesters tend to distribute
the debris so unevenly. A total of 400 corn stalks with
ears were examined during the two winter seasons. The
precise location of hidden, inactive spiders in the stubble
was noted while the activity of spiders that were moving
about or were in webs was also noted.

All sampling at both fields was conducted between 15
December and 15 February. Identification was accomplished
by use of the keys in Kaston (1981) and by comparison with
specimens in the Florida State Collection of Arthropods
(F.S.C.A.). Voucher specimens of all species collected are
deposited in the F.S.C.A.

Results

The corn stubble provided two basic hiding places for
overwintering spiders. The leaf sheath normally remains
attached to the corn stalk, where it overlays an indentation
that runs the length of the stalk. A cross section of a
corn stalk showing this protected cavity is shown in Figure
5-1; this was the only site provided by the stubble left in
the Archer field. The corn stubble that was left in the

174

Figure 5-1. Cross section of corn stalk and leaf sheath
showing the protected cavity between them.

Figure 5-2. Cross section of corn cob with enclosing husks
showing the protected layers of air.

175

field at Tall Timbers also included the shelled cobs with up
to 20 enclosing husks. This highly protected and insulated
site was greatly preferred by the overwintering spiders and
insects at Tall Timbers as shown by their virtual absence
from the protected leaf base cavities. Figure 5-2 shows in
cross section this overwintering site.

In all, 24 species of spiders were found to be using
these sites, 10 of which had previously been found to be
among the most common and important species in the corn
field ecosystem. Besides the spiders, I also collected 25
species of predaceous insects using these same sites to pass
the winter. The spiders collected are listed in Table 5-1
and the predaceous insects in Table 5-2.

The variance in the frequency of both spiders and
insects wintering in the corn stubble was very high, thus
making useful density estimates impossible. Much of the
variance, however, was related to the moisture level in the
stubble; both insects and spiders were concentrated in the
drier stubble.

Discussion

Overwintering spiders have at least three requirements
of their quarters. First, the si^lat- must insulate them from
the coldest temperatures. Kirchner (1973) studied cold
resistance in spiders, finding that many species spending

176

Table 5-1. Spiders collected during the winter from corn
stubble at Archer and Tall Timbers in north Florida, 1982-
1984.

FAMILY
Genus species Stages Archer Tall Timbers

DICTYNIDAE
Dictyna sp. MJ X

THERIDIIDAE

Achaeranea globosa EJ,MJ X X

Coleosoma acutiventer MJ,F X

Theridion spp. LJ,M,F X

LINYPHIIDAE
Eperigone near banksi EJ X

Ceraticel lus similis M X

MIMETIDAE
Mimetus sp. M X

OXYOPIDAE
Oxyopes salticus MJ,LJ X

Oxyopes scalaris LJ X

AGELENIDAE
Aqelenopsis sp. M X

HAHNIIDAE
Neoantistea magna F X

LYCOSIDAE

Pardosa milvina EJ,MJ,LJ,M,F X X

Pardosa parvula EJ,MJ,F X

Pirata sp. MJ X

ANYPHAENIDAE
Wulfila saltibunda MJ,LJ X

CLUBIONIDAE
Castieniara sp. EJ,MJ X

Chiricanthium inclusum MJ,LJ X X

Clubiona sp.
Trachelus sp .

EJ,MJ

M J , L J

X

EG,LJ

X

MJ,LJ,M,F

X

177

Table 5-1 -- continued.

FAMILY
Genus species Stages Archer Tall Timbers

GNAPHOSIDAE

Cessonia bilineata MJ X

Poecilochroa sp . LJ X

Zelotes rusticus M X

THOMISIDAE
Misumenoides f ormosipes MJ X

SALTICIDAE
Metaphidippus galathea LJ,M X X

1. EJ=Early Juvenile, MJ=Middle Juvenile, LJ=Late Juvenile,
M=Male, F=Female.

2. All specimens from Archer were collected from under the
leaf sheath.

3. All specimens from Tall Timbers were collected from
between the layered husks around the cobs.

178

Table 5-2, Predaceous insects collected during the winter
from corn stubble at Archer and Tall Timbers in northern
Florida.

FAMILY
Genus species

Stages

Archer Tall Timbers'

FORFICULIDAE

Doru aculeatum

LABI DURI DAE
Labidura

M,F
M,F

ANTHOCORIDAE

Cardiastethus assimilis ADULTS
Lasiochilus sp? LI

LYGAEIDAE

Geocoris uliginosus ADULTS
Geocoris punctipes ADULTS

MIRIDAE

Spanoqonicus sp.

NAB I DAE

Hoplistoscelis
deceptivus

REDUVIIDAE

Zelus cervicalis

LI

ADULTS

ADULTS

CARABIDAE

Calida decora

ADULTS

COCCINELIDAE
Scimnus sp.

ADULTS

STAPHYLINIDAE
Genus spp.
( 1 1 species

ADULTS

1. LI=Late Instar, M=Males , F=Females

2. All specimens from Archer were collected from under the
leaf sheath.

3. All specimens from Tall Timbers were collected from
between the layered husks around the cobs.

179

the winter in the open or in hollow plant stems can survive
temperatures as low as -16 to -30 C. Air temperatures this
low are rarely reached in Florida, but exposed surfaces may
be considerably colder than the air due to radiative
cooling. Kirchner's study involved species inhabiting
central Europe; just how cold tolerant Florida species are
has not been investigated.

Second, the spider must avoid desication (Edgar and
Loenen 1974). Proximity to the soil and protection from the
wind should be sufficient in this regard. A third require-
ment, which has in the past been largely ignored, is the
need to avoid predation (Danks 1978). Poikil otherms are
particularly vulnerable to birds and mammals due to their
decreased metabolic activity at low temperatures. Gunnarsson
(1983) found that overwintering spiders on spruce branches
had much greater survival when foraging birds were excluded.
The preference by over wintering spiders for geometrically
complex situations such as birds' nests (Otzen and Schaefer
1980), Spanish moss (Rosenfeld 1911), and in this case, the
numerous enclosing bracts around the corn cob, should be
important in providing safety from homeothermic predators.

The condition of the stubble left in a corn field is
determined by the method used to harvest and by the post-
harvest treatment of the field. If the corn is chopped for
silage, or if the field is disced and planted to a second
crop, virtually no stubble is left. The combine with a

180

picker-head, which was used at the Archer field, leaves just
the corn stalks. On the other hand, the p icker-she 1 ler ,
which was used at Tall Timbers, leaves both the stalks and
cobs in the field.

The importance of overwintering sites for predaceous
arthropods in agroecosyst em s is undeniable. However, the
corn stubble also provides overwintering sites for herbi-
vorous species as well. Among the species I found wintering
in the stubble were Chinch Bugs ( Bl is sus insularis ) , False
Chinch Bugs (Pachybrachius vinctus) , Saw Tooth Grain Beetles
(Oxy zaephilus surinamensis ) and Rice Weevils ( Sitophi lus
oryzae) . In addition, Wright et al. (1983) recorded the
Southern Corn Billbug ( Sphenophorus cal losus ) overwintering
in the crowns of corn plants while in more northern lati-
tudes stubble helps to retain a layer of insulting snow that
increases the survival of lepidoptera pupae (Turnock 1984).

Two major pests of corn also make use of post harvest
corn trash and the remaining crowns as overwintering sites,
namely the European Corn Borer ( 0 s^ t r _i n i^a n u b i^ j^a j^ i^ s_ )
(Brindley and Dicke 1963) and the Southwestern Cornstalk
Borer ( D d^a t r a e a c r a mb i^do i^de s^ ) (Metcalf et al. 1962).
Burkhardt (1952) also reported Spodoptera f rugiperda larvae
pupating in parts of the corn plant. Thus, the use of corn
stubble in management of predator populations in areas where
these insects are important pests would be unwise, but does
point out the need for providing alternative sites. How

181

these sites might be provided while not benefiting the pest
species will require intimate knowledge of the overwintering
biology of both pests and beneficials.

CHAPTER VI
CONCLUSION

The determinates of the spider community that develops
in a corn field, as for any crop field, can be divided into
four groups. First are those associated with the biology
and ecology of each individual spider species: life cycle,
fecundity, mating and foraging behavior, range of accep-
table prey, microclimate requirements, structural require-
ments for webs and/or retreats, overwintering biology and
requirements, and dispersal characteristics. A second group
of determinates are those associated with the crop: growth
rate, structure, ability to modify the microclimate,
phenology (especially as it relates to the production of
primary consumers), and the associated herbivore complex.

The third group of determinates are associated with the
geographic location of the corn field, the soil, and the
vegetation types from which the field was derived. Remnants
of the original vegetation may still be important in deter-
mining the pool of potential colonists as well as the pool
of potential predators, parasites, and competitors of the
spiders. Also in this group are the climatic and weather
factors which can change so abruptly from area to area, and
seasonal ly .

182

183

The final group of community determinates are asso-
ciated with the management practices applied to the field
and surrounding areas: planting date; weed management in
the field; cultivation technique, timing, and frequency;
technique and timing of irrigation; type, formulation, and
timing of insecticide application; fertilization; the
previous cropping history; mowing of fence rows (timing and
frequency); movement of grazing stock in and out of adjacent
areas; weed complexes that result from mowing, grazing,
cropping, irrigation, and herbicide use; the number and
species of trees left in fence rows and adjacent areas; and
the size of the field relative to the area left in more
stable habitat that can accommodate the extended life cycles
of most spiders. These last two groups of crop spider
community determinates are responsible for most of the
variability observed in the same crop over time and space.

Another way to view these many factors governing crop
spider community development is to instead divide them into
those that effect the diversity and magnitude of species in
the pool of spiders coming from the surrounding habitat, and
into those factors which exclude or restrain the various
species in this pool from becoming major components of the
community .

Spiders as a group define a loosely linked community.
That is, the flow of nutrients and energy within the group
is limited, and at least in the field crop situation, compe-

184

tition at the interspecies level is also limited to infre-
quent periods of concomitant high density and scarce
resources (prey). Then, given the fact that many of the
factors structuring the community are constantly changing in
time, are far from independent, and that each condition
affects each of the different spider species differently, it
is clear that predictors of community composition must rely
heavily on knowledge of the biology and ecology of each
species .

The above discussion leads to several lines of further
research that are necessary to better understand the devel-
opment of spider communities in crop fields and thus
potential ways of managing the community to the benefit of
better pest control.

First of these is rather straight forward descriptions
of the biology and ecology of the spiders. Fewer than a
dozen spiders of the eastern United States have had the
details of their life cycles recorded. Such studies of
spider's natural history must include both carefully
controlled laboratory rearing and intense field observations
of behavior, habitat, prey selection, overwintering biology,
dispersal behavior and population dynamics. This informa-
tion will give us valuable clues as to how to make the
agroecosystem more or less favorable for a given species (or
a set of species if it is found that important aspects of
their life histories and habitats are similar).

185

Another area where information is needed pertains to
the influence of crop species, physiognomy, spacing, weed
management, prey abundance/diversity and agronomic tech-
niques on the resulting spider community. Carefully
controlled field plot experiments similar to those conducted
by Altieri (1979) on the influence of crop field diversity
on predator diversity, would help further answer this basic
question as to whether or not a particular crop produces a
characteristic community of spiders.

More information on dispersal behavior is necessary and
should be directed at producing an accurate, time dependent
list of spiders in a given area that are potential colonists
of crop fields. Experiments designed to determine the
significance of this list, can take advantage of the fact
that much of the variation in the pool of dispersing spiders
is due to the nature and management of the surrounding
habitat, the time of year and previous and current weather
conditions .

Spiders which balloon and occur in crop fields must
have mechanisms for continuing from season to season. The
sites and stages of the overwintering populations of all the
important species must be determined. Multiple regression
analysis might be useful in isolating the weather phenomena
responsible for seasonal mortality and population perturba-
tions as well as dispersal behavior.

186

the following events are likely have taken place: A popula-
tion of the spider must been present in the surrounding
habitat having found suitable overwinter sites, a portion of
that population must have been induced to disperse in the
direction of the crop field, there must not have been an
abundance of intervening favorable habitat, and finally the
spider must have been induced to suspend dispersing (at
least momentarily) once within the bounds of the field. The
density of the population in the plot is a function of many
factors, which could be deciphered through a combination of
careful field plot design and ecological observation
inspired by the field plot results.

But what changes in the crop field spider community are
in fact desirable or even possible? The prey selection of
web building spiders is easier to assess than that of the
vagrant hunters. From the present data, it appears that a
large portion of the diet of the small orb weavers
(Araneidae) consists of small flying insects, of which para-
sitic hymenoptera form a major part. The vagrant hunting
Clubionidae, Oxyopidae, and Salticidae on the other hand
appear to be heavy predators of lepidoptera eggs and larvae,
but determining the level of this predation may require more
technical methods such as labeling or immunoassay analysis
of field collected spiders. Inductive means of assessing
this predation might be possible through manipulation of the
levels of prey or spider.

187

Given a deduced level of potential predation by a
spider and a level of predation that is desired at the field
level, can the population be elevated to the necessary
level without inducing intraspecif ic , self limiting behavior
such as territoriality? Are the stage or stages of the
arriving spiders ( ba 1 1 ooni sts ) those which could provide
significant control and/or is there sufficient alternate
prey prior to the period when the pest species arrives?
Further, it would generally be impractical to release
spiders as in an augmentation program; the number of spiders
available per unit area of crop field being dependent on the
area and density of source habitat(s) and the size of the
field. The economics of modern agriculture tend to favor a
minimum of source habitat versus large contiguous fields.

It is generally agreed that spiders, unlike the speci-
fic predators and parasitoids, are best at controlling popu-
lations during the lag phase of establishment and should not
be expected to track exponentially expanding populations.
Thus the most practical route might be to manage the agro-
ecosystem so as to favor an assemblage of spider species of
various ages that use several stages of the pest species as
prey, and to limit those species groups of spiders which
interfere with the activities of the specific predators and
parasitoids .

Due to the high productivity of corn and the relative-
ly low use of insecticides in it, corn is a potential source

of serious pests such as Spodoptera spp., Heliothis spp.,
Diatraea spp., pentatomids, grasshoppers and spider mites
that can move into other valuable crops. Moreover, because
corn is grown on so many hectares and is part of nearly
every agroecosystem in Florida, the United States, and
virtually throughout the world, management of the pests,
predators and parasites in this crop takes on considerable
significance. Finally, legumes frequently dominate the
complex of weeds which emerges as the corn field matures;
since a large percentage of the corn field spiders move to
these weeds, they enter the population dynamics picture of
numerous other serious pests (and their parasitoids) of
agricultural systems.

Corn is also one of the most important sources of
starch in small, diversified cropping systems of the subsis-
tence farmers in developing countries. Due to their
economic situation, naturally occurring biological control
is frequently the only control; the role of spiders on these
farms might be particularly important; and achievable
because of the added benefit of habitat diversity (Riechert
and Lockley 1984).

Because spiders, like the pests we wish them to help
regulate, are mobile, we must consider ecosystem wide
dynamics and management strategies. Each crop field, weed
patch, pasture, or area of natural vegetation in the system

189

can act as either a source or a sink (trap crop) for pest
species, predators, and parasitoids as well as spiders. The
interaction of the biological characteristics of the
component species, their inter- and intraspecif ic dynamics,
and the parameters set forth by management of the ecosystem
and the prevailing climatic and weather patterns will
ultimately control the degree of pest control that is
achieved.

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BIOGRAPHICAL SKETCH

Michael J. Plagens was born on May 10th, 1954, in
Detroit, Michigan, as the first of four children, to Joseph
R. and Frances L, Plagens. In 1972 he received his diploma
from Lakeview High School in St. Clair Shores, Michigan.

After serving in the United States Army as a
Preventative Medicine Specialist, he entered the University
of Arizona in Tucson. In December of 1978 he was awarded
the Bachelor of Science degree in agriculture with High
Distinction .

In June of 1980 he was married to Paula Eidson,
daughter of Walter and Florence Eidson, of Tucson, Arizona.
She is a Registered Nurse.

In May of 1981, at the same institution, he completed
the Master of Science degree in entomology. During this
period he was employed at the U.S.D.A. Biological Control
Research Laboratory studying the biology of hymenopterous
parasites .

Beginning in August of 1981 he was a graduate student
in the Department of Entomology and Nematology at the
University of Florida. He has taught laboratories for both
entomology and general biology.

207

I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality, as
a dissertation for the degree of Doctor of Philosophy.

Willard H. Whitcomb, Chairman
Professor of Entomology &
Nematology

I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality, as
a dissertation for the degree of Doctor of Philosophy.

Je^n\R. Strayer
Professor of Entomology &
nnatology

I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality, as
a dissertation for the degree of Doctor of Philosophy.

ithan Reiskind,
Associate Professor of Zoology

This dissertation was submitted to the Graduate Faculty
of the College of Agriculture and to the Graduate School and
was accepted as partial fulfillment of the requirements for
the degree of Doctor of Philosophy.

August 1985

Dean, Ctj/Vlege of Agricu^u]

Dean, Graduate School

UNIVERSITY OF FLORIDA

3 1262 08553 5416