New data on the habits of the ants of the genus Veromessor

AMERICAN MUSEUM
NOVITATES

PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY
CITY OF NEW YORK MARCH 20, 1953 NUMBER 1612

NEW DATA ON THE HABITS OF THE
ANTS OF THE GENUS VEROMESSOR

By Wit.iiaM S. CREIGHTON!

The major part of this paper deals with Veromessor pergande1,
but it has been possible to include observations on the habits
of four other species as well. The field work on which this paper
is based is part of a survey of southwestern ants which was made
possible by a Guggenheim Fellowship. I wish to express my
sincere thanks to the John Simon Guggenheim Memorial Founda-
tion for this opportunity and privilege.

It will avoid repetition to cite here certain characteristics
shared by the species discussed in this paper. All these ants are
remarkably docile. They will not sting, even when one tries to
make them do so. They bite only on severe provocation. When a
nest is broken open they show none of the frenzied excitement
that marks many ants at such times. They attack the intruder
methodically and without haste. If undisturbed they pay little
attention to a person who is standing near the nest or close to a
foraging column, and this makes them excellent subjects for field
observation. But, despite this imperturbability, all the species
are highly sensitive to damage to the nest. If the area about the
entrance is no more than slightly altered, the ants are apt to
abandon the nest. If the nest is broken open the ants will usually
move to another site rather than repair the damage. When they
move they do not always take all the stored seeds with them.
Other harvesters (Pheidole and Pogonomyrmex) will often mine
an abandoned nest for days after the Veromessor colony has left
it. The colonies of andrei, juliana, pergander, and stoddardi are
large. When mature they may consist of several thousand

1 Department of Biology, The City College of New York. ©

2 AMERICAN MUSEUM NOVITATES No. 1612

workers. Those of lartverst are notably smaller, consisting of a
few hundred workers at most.

Veromessor andrei (Mayr)

The writer has been able to study 32 colonies of this ant at
11 different stations. The observations cover a period from May
to September. They permit a much more statisfactory account
of the habits of andrei than was possible in earlier publications.

The nesting habits of andre are more flexible than is usually
the case with a member of the genus Veromessor. The species
is equally at home on the seashore, in lowland stream bottoms,
or in mountain valleys up to an elevation of 3500 feet. The nests
may be built in heavy clay, loam, sand, or gravel. V. andrei
may or may not build a crater, but, when it does, the crater is
low and obscure at best. When a crater is not present the ex-
cavated soil is spread around the nest opening in a circular. disc
or fan. The number of nest openings varies with the size of the
nest. Small nests usually have a single opening. Large nests
may have as many as eight entrances. The entrances are often
an inch or more across and they may. be very irregular in shape.
Occasionally an entrance will lie at the bottom of a saucer-like
depression. The construction of the chaff pile varies with the
season. There is rarely any chaff pile present during the spring
months. At such times the seed husks are scattered haphazard
around the nest. But in late June and early July, when the seeds
of various grasses ripen, the chaff pile often becomes conspicuous.
It sometimes completely blankets the nest, but more frequently
it has the form of a flattened band or ring which surrounds the
nest but does not cover it. By the end of the summer the chaff
pile has matted down to a thin carpet of hulls.

Veromessor andrei has morning and afternoon periods of forag-
ing like pergandei (q.v.), but, unlike pergandei, there is little
activity at the nest during the midday hours. At such times the
nests of andret appear to be deserted, for the ants retreat so fa
into the nest passages that they cannot be seen. The morning
period of foraging usually terminates between 9 and 10 o'clock.
The afternoon foraging is not begun until about an hour before
sunset and continues during twilight and after it has become
dark. Once the light has failed, however, the ants in the column
are less certain in their movements. The column. ultimately
breaks up into scattered groups of workers trying to find their
way back to the nest. The marriage flight of andrei occurs dur-
ing the latter part of July. Latitude seems to make little dif-

1953 HABITS OF ANTS 3

ference in the time of the marriage flight. The females of andrez
are frequently taken into established nests after the marriage
flight.

A study of material taken in the area west of Jacumba, Cali-
fornia, has shown that the subspecies casfaneus Wheeler and
Creighton is without validity. As may be recalled, Jacumba is
the type locality for castaneus. The writer was particularly anx-
ious to take castaneus at Jacumba, for the status of this form
needed clarification. The area around Jacumba was collected
for almost a month, but no colonies of casianeus could be found
there. If the types of casianeus actually came from Jacumba the
record is exceptional. On the other hand, the insect is abundant
in Cameron Valley, about 20 miles west of Jacumba.

More than a dozen colonies of castaneus were found in Cam-
eron Valley, and substantial series were taken from nine of them.
The character of this material was disconcerting, for it showed
that unjustifiable conclusions had been drawn from the limited
type series of castaneus. On the basis of the types, castaneus
was said to be lighter in color and smaller in size than andre.
It was also stated that castaneus has longer antennal scapes,
a more feeble impression at the epinotal suture, and a less
prominent ventral tooth on the petiole. When adequate material
was examined, none of these distinctions held up, with the pos-
sible exception of color. It is now clear that they were the re-
sult of comparing the smaller workers of castaneus with the
larger ones of andretz. When the two forms were compared
size for size, the only difference was the uniformly light color of
the Cameron Valley specimens. It may be added that this light
color marked the females as well as the workers of the Cameron
Valley colonies.

There is no reason why color should not distinguish a sub-
species, provided that it can be correlated with a distinctive
geographical distribution. As far as could be determined, the
light-colored form is the only one in the Cameron Valley. But
it should be remembered that, while all the workers in the Cam-
eron Valley nests are light in color, those in colonies taken else-
where are not always dark. Indeed, the number of colonies in
which all the workers are dark seems to be comparatively small.
Much more often there is considerable variation of color within
the nest, and not infrequently the lightest workers in such colo-
nies are so nearly like those of castaneus that no distinction would

4 AMERICAN MUSEUM NOVITATES NO. 1612

be possible without considering the whole colony. These inter-
mediate colonies occur at random over the entire range of andret,
except in the Cameron Valley. But, since this valley is only
about 6 miles long and not more than 3 miles wide, it represents
an extremely small part of the range of andret. For the range of
this ant parallels the Pacific coast for almost 1000 miles and at
one point extends eastward to Arizona and Nevada. Under such
circumstances it is impossible to attribute much geographical
significance to the Cameron Valley colonies. A more acceptable
view is that their light color results from some ecological factor
(possibly aridity) which is more uniform in the Cameron Valley
than in other parts of the range.

The range of andrez is more extensive than the older records
indicate. The ant is surprisingly abundant in the Siskiyou
Mountains of Oregon. Ten colonies were found in the mountains
6 miles west of Jacksonville at an elevation of 1500 feet. V.
andre also occurs in Baja California. The writer secured two
colonies in a stream bottom 10 miles north of San Vicente, at an
elevation of 600 feet. Two colonies were also found at San An-
tonio del Mar, on the landward side of the large dunes which
flank the beach. Oddly enough no colonies of andrez were en-
countered in the Sierras of Baja California, although it might be
expected that the vertical range of andrez would rise as it passes
to the south. |

Veromessor chamberlini (Wheeler)

Dr. M. R. Smith writes me that he has seen specimens of
V. chamberlini that came from the California mainland. This is
gratifying but scarcely surprising. As far as ants are concerned,
there is little evidence of endemicity in the case of the islands
off the California coast. It is to be hoped that field observations
on this species can be made, for at present nothing is known of
its habits.

Veromessor juliana (Emery)

_ As far as the writer is aware, nothing has been published on
the habits of juliana. It was, therefore, gratifying to discover
three large nests of this species at Bahia San Quintin, Baja
California. These nests were constructed in light, sandy soil
at the edge of the bay and placed so that they were not more
than 4 or 5 feet above high-tide level. The area in which the ants

1953 HABITS OF ANTS 5

were nesting was covered by a dense mat of Mesembryanthemum
crystalinum. Through this mat the ants had cleared crooked
trails about 1 inch wide. When foraging they kept strictly to
these trails. There were several entrances to each nest, and
these were often concealed by a Mesembryanthemum plant.
There was no sign of excavated soil around the nest entrances
nor any special arrangement of seed husks to form a chaff pile.
several of the entrances were partially blocked with seed husks
which had been abandoned at the mouth of the entrance. In
general the foraging activities of julzana are like those of per-
gandet (q.v.). It is interesting to note, however, that juana will
forage on days of heavy overcast, which pergandet seldom does.
It is not likely that juliana could exist on the Pacific coast of
Baja California unless it could adapt itself to considerable
fog.

It appears that Bahia San Quintin lies near the northern
end of the range of julzana. There are several areas farther
north on the coast of Baja California where the environment is
the same as that at Bahia San Quintin. If nests of julzana were
present in these areas, they should be easy to find because of the
characteristic paths which the ants clear through the Mesem-
bryanthemum beds. No additional nests were found, despite
considerable search. It also appears, since the range of andrez
extends at least as far south as San Antonio del Mar, that some-
where between that station and Bahia San Quintin, a distance
of about 50 miles, the southern end of the range of andres may
meet the northern end of the range of juliana.

Veromessor lariversi M. R. Smith

This species has been recently described from specimens
secured by Dr. LaRivers near Pyramid Lake, Nevada. The writer
has taken this insect near Lone Pine, California (elevation 4300
feet), Wagner, Nevada (elevation 4000 feet), and Goldfields,
Nevada (elevation 5800 feet). Specimens from California were
sent to Dr. M. R. Smith for comparison with the types of
larwerst. I am glad to take this opportunity to thank Dr.
Smith, who not only made the comparison, as requested, but
generously sent me paratypes of larierst as well.

Observations on V. lariverst were made in early May, in mid-
July and in mid-October. Most of what follows is based on nests
found 4 miles west of Lone Pine, California. This area is a part

6 AMERICAN MUSEUM NOVITATES NO. 1612

of the plain that slopes upward from the foothills west of Lone
Pine to the base of Mt. Whitney. During two visits to this
station, six nests of lariverst were studied. All of these nests
were built in fine, hard-packed gravel. Beneath this hard layer
was a much looser layer of sandy gravel, but very few of the nest
passages reached this lower layer. In May each nest was sur-
mounted by a cluster of small, circular, steep-walled craters.
Each crater was not more than 3 inches in diameter and each
had a single entrance at the bottom. By July many of the craters
had disintegrated and the soil that formed them had become a
ragged pile above the nest. But in each case one or two craters
had been kept in repair. The nest passages often extended as
much as 2 feet beyond the area covered by the craters. The pas-
sages were remarkably shallow. Most of them were no more than
3 inches below the surface. At intervals along these passages
elongate seed chambers occurred. The nests taken in Nevada
did not differ significantly from those described above.

Little data on the foraging activity of larzverst could be ob-
tained, for the ants kept strictly to their nests during the entire
period when they were under observation. In May the nests
were observed in the late afternoon and about nine o’clock in the
morning. In July the observations were more extensive. They
were made at suitable intervals from before sunrise (6:30 A.M.)
until after dark (9:00 p.m.). The observations were discontinued
after nine o’clock, for by that time it was clear that the ants
were not going to forage. They were aestivating, and it was ap-
parent, from the neglected condition of the craters above the
nests, that this period of aestivation had been in progress for
some time by the middle of July. While V. andre: and pergander
will sometimes suppress the afternoon foraging period during
exceptionally hot weather, both species definitely do not aes-
tivate. It seems entirely likely that this is also true of juliana
and stoddardt, although the writer has seen too little of the last
two species to be certain. But while the aestivation of larivers«
may not be unique, at least this habit sharply separates lariverst
from pergandet, for the latter species responds to high tempera-
ture in a totally different fashion. Since Jariversi is more closely
related to pergandez, on the basis of structure, than any other
species in the genus, it is interesting that the two should have so
little in common as far as habits are concerned. When additional
studies can be made on Jariversi in the early spring, the writer
believes that it will prove to be a nocturnal forager.

1953 -- HABITS OF ANTS. | ‘ 7

Veromessor lobognathus (Andrews)

This species remains as enigmatical as ever. Dr. Robert Gregg,
who is in process of completing a very thorough survey of the
ants of Colorado, informs me that he has not been able to dis-
cover additional specimens of lobognathus. The ant is either ex-
traordinarily rare, or, as seems more likely, the locality labels
on the four types may have been incorrect.

Veromessor pergandei (Mayr)

In recent months the writer has been able to study the habits
of this interesting ant at 57 stations. As will be subsequently
shown, these stations cover most of the range of V. pergandet.
The observations extended from early March to the middle of
November. During the course of these studies certain experi-
ments were made to determine the basis for some of the habits
of pergandez. It will simplify matters to preface these experi-
ments with an account of the behavior of pergandei at the time
of its maximum activity.

since much of what follows involves a clear idea of the nests
of this species, it may be said that in a well-established nest of
V. pergandei there is usually a single entrance which lies at the
bottom of a low fan or crater of excavated soil. This crater is
seldom more than 15 inches in diameter and ordinarily not more
than 12 inches in diameter. At one edge of the crater is the chaff
pile, a mass of discarded seed husks which sometimes forms a
crescent at the periphery of the crater but more often forms a
low and irregular band beyond it.

In the late spring and early summer pergandez begins its daily
activity soon after dawn. Prior to sunrise the ants emerge from
the nest entrance and spread slowly over the crater. At this
time their movements are apt to be exceedingly slow because of
the low temperature. After sunrise their movements quicken,
and the spread of the ants away from the nest entrance is accel-
erated. Thirty to 45 minutes after the sunlight first falls on the
nest the ants push out a foraging column which usually proceeds,
without hesitation, to the seed supply. The area in which the
seeds are gathered is often 75 yards or more from the nest. In
such cases it may be 20 minutes before the first foragers reach
the seed supply. During this period all ants in the column are
outward bound. Presently, however, seed-laden workers, in-

8 AMERICAN MUSEUM NOVITATES NO, 1612

ward bound, appear in the column, and soon they are bringing
back quantities of seeds to the nest. This morning period of
foraging ordinarily lasts from two to three hours.

About mid-morning there comes a time when the workers
cease to leave the nest. This cessation is surprisingly sudden.
At one minute workers will be leaving the nest two and three
abreast. Five minutes later there will be only an occasional
worker, outward bound in the column. This soon reduces the
column to inbound workers only and shortly thereafter the col-
umn dwindles and disappears. Then follows what, for want of a
better name, may be called nest work. The ants leave the nest
entrance in short sorties which do not take them off the crater
or beyond the chaff pile. Each ant carries a bit of gravel or a seed
husk. The gravel is deposited on the crater, the seed husks on the
chaff pile. This nest work may continue without appreciable
change until the second daily foraging column takes off in the
late afternoon. Usually, however, the number of ants engaged
in nest work dwindles markedly towards noon. On very hot
days the ants may terminate all visible activity and retire into
the nest during the early afternoon hours. On most days this
does not happen but, during the early afternoon, the nest work
is often restricted to exceedingly brief sorties by very few ants.
These ants barely get out of the nest entrance, hence this limited
nest work is apt to be overlooked.

At some time in the late afternoon the volume of nest work
begins to increase. This increase is also an abrupt one. In the
course of a few minutes several dozen workers appear on the
surface of the nest. With this increase in the number of workers
comes an increase in the length of their sorties. As a result the
ants begin to spread outward over the crater. At this time most
of these workers are engaged in carrying out gravel or seed husks.
But this nest work soon diminishes in a rush of workers who
emerge onto the surface of the nest without carrying anything
in their jaws. These ants push outward beyond the crater, and
most of them take the direction in which the seed supply lies.
More and more ants follow these leaders, and soon the after-
noon foraging column is outward bound. It is seldom more than
five minutes from the cessation of nest work to the formation
of the foraging column. As a rule the afternoon foraging continues
until early dusk (three hours, more or less), at which time the
foragers straggle home for another period of nest work. This

1953 . HABITS OF ANTS 9

nocturnal nest work may continue into the early hours of the
following morning.

It is easy to suppose that this rhythmic behavior is controlled
by the daily temperature cycle. For the morning period of forag-
ing might be terminated by the rising temperature, which would
first restrict the ants to the nest area and later force them into
the nest passages. The reverse process could be expected as the
temperature dropped during the afternoon. The ants should
first show a limited activity near the nest and later begin to
forage when the ground cooled to the proper surface tempera-
ture. It was, therefore, a surprise when attempts to measure
these critical controlling temperatures showed that they do
not exist.

The method used in the attempt to determine these tempera-
tures was a rather crude one. The only thermometers with a
proper temperature range which could be obtained were small,
coil-spring type of instruments, enclosed in a metal case and pro-
vided with a rubber suction disc by which they are supposed to
be attached. This disc was removed and the opening thus left
was covered with masking tape to keep sand and gravel out of
the case. The thermometer was then placed directly on the
surface of the ground. In such a position the coiled spring was
approximately 5 mm. above the surface. Dr. Charles Lowe, of
the University of Arizona, who has read this article and gener-
ously given me the benefit of his experience with temperature
determinations of desert animals, is of the opinion that the
figures presented below as “‘surface temperatures’ are more
probably air temperatures 5 mm. above the surface. While this
should make no difference in the explanation of the behavior
of pergandet, it seems well to note that a more exact measure-
ment of the true surface temperature may give figures somewhat
higher than those presented in this article.

Daily observations were made on three nests of pergander
near Jacumba, California, from June 17 through June 29.
During that time the morning period of foraging was observed
to terminate at surface temperatures which varied from 88° F.
to 112° F. A comparable latitude for temperature marked the
start of the afternoon foraging. The column would form at
temperatures which varied from 90° F. to 110° F. Since the
identical response was forthcoming at temperatures which
differed by as much as 30° F., it seemed highly improbable

10 AMERICAN MUSEUM NOVITATES No. 1612

that such responses could be the result of a temperature control.

Yet out of this mass of irrelevant temperature data certain
significant temperature responses emerged. The speed of the
ants is clearly determined by temperature. It increases with an
increase in temperature over a range from 50° F. to 90° F. This
range probably begins at 45° F., but below 50° F. the speed of
the ants is so slow and their motion so lacking in direction that
the writer could make no satisfactory measurement of it.
At 50° F. the ants move at a speed of about 6 inches a minute.
At 90° F. the speed has increased to 8 feet per minute. This
appears to be the best speed of which pergandei is capable, for,
while they will forage at surface temperatures up to 115° F.,
no further increase in speed was noticeable at temperatures
higher than 90° F.

It also became apparent that a surface temperature of 125° F.
restricts the ants to the nest. They never voluntarily expose
themselves to such a temperature, and it was easy to find the
reason for this behavior. In most cases workers placed on a
surface where the temperature is 125° F., or higher, will scramble
back to the shelter of the nest, or to some shaded area, within a
few seconds after exposure. In such cases they are no worse for
the experience. But if a worker of pergandei is picked up by one
leg with a fine pair of tweezers, it can be held, uninjured but
fully exposed, at one point on the heated surface. Under such
circumstances the worker will die in a surprisingly short time.
Minor workers will be dead in 20 seconds or less. Medias usually
survive from 30 to 40 seconds. Majors are more resistant,
but they succumb in a minute to a minute and a quarter.
Hence, there is the best possible reason why the workers of
pergandei avoid exposure to a surface temperature of 125° F.
Such a temperature kills the ants in a short time if they are
unable to get out of it. It may be added that this result oc-
casionally occurs under natural conditions. Minor workers
sometimes get too far away from the nest entrance, or a protec-
tive bit of shade, and die in consequence. Medias and majors
are rarely killed in this way, probably because of their greater
resistance, which gives them a longer time to get back to shelter.
But, regardless of the size of the worker, it should be clear that
if the ants waited until the surface reached a temperature of
125° F. to begin the avoidance reaction, they would run a
serious risk of being killed. This risk is reduced to a minimum

1958 - HABITS OF ANTS i1

because the avoidance reaction begins at a surface temperature
of 120° F. At this temperature the ants are not restricted to the
nest passages, but they confine themselves to the briefest of
sorties from them. On such trips they rarely get more than an
inch or two from the nest entrance and, unless they can take
advantage of some bit of shade, they return to it with under-
standable alacrity. This is the behavior which occurs when
limited nest work is done in the early afternoon hours. Thus
this response, and the cessation of all external activity, are
directly controlled by temperature. But this is quite a different
matter from the foraging cycle as a whole.

After it became clear that temperature was not responsible
for stopping the morning foraging or for starting this activity
in the afternoon, the writer began to examine the possibility
that light intensity might control the foraging rhythm. The
daily cycle of light intensity has the requisite characteristics.
There is a morning increase and an afternoon decrease com-
parable to that of the daily temperature cycle. After a few -
experiments with a light meter placed near the nest entrances,
it was clear that it would be extraordinarily difficult to obtain
significant readings for the critical light intensity, assuming that
one existed. For the ants made daily alterations in the nest
entrance and the nest area which defeated any attempt to
secure uniform readings. One example may be cited to show the
nature of the difficulties involved. At the time of these observa-
tions many of the seeds garnered by the ants possessed a ter-
minal tuft of silky fibers rather like a tiny milk-weed seed. The
ants would strip these tufts of fibers from the seeds, and they
usually placed them on the chaff pile. But one colony occasion-
ally left the tufts in the nest entrance itself, where they formed
a loose, stopper-like mass which reflected so much light that
the meter readings for that nest were temporarily doubled.

Nevertheless, it seemed possible that by shading the nests
some effect should be observable if light intensity controlled
the foraging rhythm of the ants. It was found that a card
table made an excellent mask. By folding down two legs it
could be set at a slant above the nest, where it produced a
rectangle of shade 30 inches long and 20 inches wide. The shaded
area was open on one side (north) to easy observation. This
shading produced not only a decreased light intensity under the
mask but a decreased surface temperature as well. As a rule

12 AMERICAN MUSEUM NOVITATES NO. 1612

the surface temperature in the shade of the mask was at least
30° lower than that of the fully illuminated ground at its edge.
But, as is shown below, this could be turned to advantage, for
by removing the mask the light intensity could be immediately
increased to full force, while the surface temperature rose much
more slowly under such circumstances.

Since the cessation of the morning foraging and the resump-
tion of the afternoon foraging are abrupt events, it seemed ad-
vantageous to try the effect of shading at such times. It was
soon clear that shading the nest prior to the cessation of the
morning foraging produced remarkably little effect on the be-
havior of the foragers. About the only observable difference was
the confused response of some of the foragers when they entered
the shaded area. A few of them would turn back into the sun-
light but not many did so, and the morning foraging was not
prolonged to any appreciable degree by shading the nest.
This result might have been expected, for the critical light in-
tensity would affect all the workers in the column and not merely
those in the vicinity of the nest. Moreover, the morning forag-
ing is always followed by a period of active nest work. As is
shown below, this nest work was augmented when the nest was
shaded.

If the mask was placed over the nest in the early afternoon
some much more spectacular results ensued. Nest work im-
mediately increased in volume. Instead of two or three workers
hastily dodging in and out of the nest entrance, there were a
couple of dozen workers on the surface at once, all busily en-
gaged in bringing gravel and seed husks out of the nest. It
was noticeable, however, that despite the large shaded area
around the nest, very few of the workers left the crater. There
were never more than three or four ants exploring the shaded
area beyond the crater, and these explorers moved ina slow and
desultory fashion. After an hour or more of nest work there was
a sudden egress of several dozen ants onto the crater. These
ants emerged from the nest entrance with empty jaws. They
were, therefore, no longer engaged in nest work. These empty-
jawed workers did not confine themselves to the crater and
chaff pile but spread outward into the shaded area beyond the
nest. In a few minutes the shaded area was crowded with
hundreds of ants. These ants were very active, and they at-
tempted to form a foraging column on the side of the shaded

1953 HABITS OF ANTS 13

area nearest the food supply. The ants at the head of this pros-
pective column pushed out into the sunlight beyond the shade,
but, as the surface temperature of the fully illuminated soil
was above 120° F., they immediately returned to the shaded
area. At this time there was extremely slight activity or no
visible activity in the control. nests. These unshaded nests did
not start their foraging columns for an hour or more.

Since the ants were obviously trying to form a foraging column
at the edge of the shaded area, there seemed to be no reason
why they might not complete it if a “shade bridge’ were fur-
nished from the nest to the seed supply. This, however, they
refused to do. When a strip of shade was provided from the nest
to the seed supply the ants never went more than a foot or two
on it. They then returned to the nest. Subsequent observations
with patches of shade thrown onto a foraging column indicate
that the ants need direct sunlight to orient themselves in the
column. A patch of shade no more than 2 feet long causes many
of the foragers to break out of the column and wander aim-
lessly about. It is interesting to note that this same lack
of direction on the part of the foragers is usually encoun-
tered at the end of the afternoon period of foraging. As already
noted this afternoon foraging usually extends past sundown and
the workers straggle home at the approach of dusk with much
less precision than they show when the column is in direct sun-
light. If the ants ordinarily depend upon direct sunlight for
orientation in the columns, this would also explain why the
foraging columns do not continue after dark.

Having failed to produce a full-fledged foraging column under
artificial conditions, the writer turned his attention to the abor-
tive column which formed at the edge of the shade when a nest
was masked. The surface temperature of the fully illuminated
soil beyond the shadow of the mask ranged from 123° F. to
126° F. The surface temperature in the shade of the mask
ranged from 87° F. to 94° F. If the mask was abruptly removed,
all the ants in the previously shaded area rapidly returned to the
nest. This response was surprisingly spectacular. As there were
several hundred ants all trying to get into the nest at once, they
formed an almost solid mass of jet-black, shining bodies at the
nest entrance. This return to the nest, after the removal of the
mask, never took more than five minutes and was often ac-
complished by the end of three minutes. During this period the

14 AMERICAN MUSEUM NOVITATES NO. 1612

surface temperature of the previously shaded area never rose
above 99° F. In order to make sure that there was no significant
lag in the thermometers, they were tested and found capable
of responding at a rate of 10° of change per minute. The greatest
recorded rise in surface temperature after the removal of the
mask was 11° in five minutes. It is clear, therefore, that the
recorded surface temperature of 99° F. was due to the slow rise
in the surface temperature and not to a lag in the thermometers.
But the ants hurriedly left an area where the surface tempera-
ture was at least 20° below the point at which the avoidance
response to heat begins. Since their rapid return to the nest is
clearly not a response to temperature, it is reasonable to believe
that this behavior resulted from the sudden increase in light in-
tensity which followed the removal of the mask. Thus, if the
light intensity is sufficiently high, it produces an avoidance
reaction closely comparable to that which is caused by a sur-
face temperature of 120° F. or more. But if this reaction to
light intensity can be experimentally elicited, it must also
occur under natural conditions, when the increasing light in-
tensity during the morning hours reaches what may be called
the avoidance level. Hence, as long as the surface temperature
is below 120° F., the termination of the morning period of
foraging is a response to light intensity and not to temperature.
Conversely, the beginning of the afternoon foraging, at sur-
face temperatures less than 120° F., is an indication that the
light intensity has dropped below the avoidance level, again a
response to light intensity and not to temperature.

Since shading the nests in the early afternoon had accelerated
the time of foraging by as much as an hour, it seemed
probable that an even greater acceleration might be ex-
pected if a nest were shaded as soon as the morning foraging
had ceased. In the afternoon the abortive foraging column
formed at the edge of the shade about an hour after the mask
had been put in place. The writer expected a comparable situa-
tion to ensue as a result of late morning shading. This, however,
was not the case. When the mask was put in place at the end of
the morning period of foraging there was an increase in the
volume of nest work which continued without significant di-
minution until well past noon. It then dwindled, but no column
formed until 3:00 p.m. There was thus about the same period of
nest work (four to five hours) whether the nest was shaded

1953 HABITS OF ANTS 15

or not. But, when the nest was shaded, the surface temperature
and the light intensity in the shadow of the mask were both fav-
orable to the formation of a foraging column. Since no column
formed for a period of at least three hours, it would appear that
the seed supply brought into the nest during the morning period
of foraging is itself a factor which affects the foraging rhythm.
These seeds have to be hulled and stored and, until this process-
ing is completed, no new foraging column forms even though
the external conditions may be favorable for it. The nest
work done by the ants is the visible manifestation of this seed
processing. Gravel is brought to the surface as the storage
chambers are excavated. Seed husks are placed on the chaff
pile as the seeds are stripped out. When no more seeds remain
to be processed the ants appear on the surface of the nest with
empty jaws, and it is not until then that a foraging column
begins to form.

To summarize the above data, it may be stated that the
foraging columns of pergandet form only when the surface
temperature is below 120° F., when the light intensity is below
its midday level and when the supply of seeds brought in from
the last foraging period has been completely processed. There
must also be a certain basic minimum of light, for this ant will
not forage in darkness. A favorable combination of these control-
ling factors will ordinarily occur only in the early morning and
the late afternoon hours. Conversely, the least favorable period
on three counts would be the midday hours. Hence pergandez is
held in, or close to, the nest during the dangerously hot hours
at the middle of the day by a triple system of controls, any one
of which will prevent the ants from exposing themselves to
lethal conditions.

The success of this system of controls is obvious from the
distribution of pergandet. The species shows its maximum
abundance in the hottest and most arid regions on the North
American continent. V. pergandez flourishes throughout the
region around the head of the Gulf of California. In western
Sonora it is frequently encountered as far south as Hermosillo.
South of that station the incidence decreases, and the southern
limit of the range appears to lie a little north of the latitude of
Guaymas. In November of 1952 the writer observed many
nests of pergandet in the area between Guaymas and Imuris.
This was exceptionally easy to do, since at that time every

16 AMERICAN MUSEUM. NOVITATES NO. 1612

nest was surmounted by a large chaff pile of pale, yellow seed
husks. Because of this conspicuous marker the nests could be
noted many yards away. North of Hermosillo there were
places where a dozen nests would be in view at once. Yet be-
tween Guaymas and Cocorit not a single nest could be found,
despite much searching.

On the western side of the Gulf of California pergandez is
abundant as far south as San Felipe. It has been taken at Santa
Borja (C. D. Haines) which is approximately on the same lati-
tude as Hermosillo. Hence, it seems probable that the range of
pergandet in Baja California is similar to its range in Sonora.
The writer believes that the northern end of the range of per-
gandet lies at or near the northern end of Death Valley. There
are many nests present at Mesquite Springs, near the upper
end of Death Valley and also at Beatty, Nevada, some 30 miles
to the east. North of these stations the country rises steadily
and the incidence of pergandet decreases sharply. The eastern
limit of the range of pergandez is very irregular. The insect
avoids the highlands of northwestern Arizona and also those
which lie along the southern border east of the Baboquivari
Mountains. The maximum eastward extension lies just north
of these mountains and brings pergandez into the Tucson area.
Whether pergandet should be regarded as occurring on the Pa-
cific coastal strip or not is problematical. It certainly does not
occur at sea level. But in southern California and in Baja
California an occasional nest is encountered on the western
slope of the mountains comparatively close to the coast. The
incidence of such nests is exceedingly low in contrast to the
abundant population which occurs east of the Sierras. All of the
region outlined above is marked by intense summer heat. It
is safe to say that nowhere in its range is pergandei free from
lethal midday temperatures during the summer months. Vet,
because of its beautifully adjusted responses, pergandei thrives
in such forbidding areas. It even contrives to live at Bad Water
in Death Valley. This station, 280 feet below sea level, is said
to be the lowest point in the Western Hemisphere. In summer it
is, undoubtedly, one of the hottest places on earth.

Before leaving pergandez it seems well to note that its marriage
flight occurs in late March and early April. The nest-founding
female of pergandet will often begin the nest beneath a covering
stone. This behavior is noteworthy, since covering stones are

1958 | HABITS OF ANTS 17

usually avoided by most xerophilous ants, presumably because
they conduct too much heat into the nest.

Veromessor stoddardi (Emery)

The writer has encountered only four nests of stoddardz.
Two colonies were found at an elevation of 200 feet on a hill
1 mile east of San Ysidro, California. Another nest was taken in
hills 4 miles south of La Mision, Baja California (elevation 700
feet). The fourth colony was secured on an upland plain 19
miles north of Ensenada, Baja California (elevation 1100 feet).
The structure of these nests was remarkably uniform. Each
nest was built in iron-hard clay soil. Each had two or three
nest openings. Each nest opening consisted of an irregular
aperture which was usually situated at the bottom of a shallow,
saucer-like depression. In no case was there any sign of excavated
soil around the nest. During May and early June the seed hulls
are left in the depressions around the nest entrances. In July,
when the ants appear to carry on more active harvesting, the
seed hulls may be built into a flattened ring around the nest.
No foraging column of stoddardi was encountered by the writer.
Nevertheless certain deductions can be made as to the seed-
gathering activities of this ant. From the quantities of seed
hulls around the nest it is clear that stoddardi is an active har-
vester. It is also clear that stoddardi does not begin to forage when
andret does. Both species were simultaneously observed at the
San Ysidro station. No activity of any sort could be detected
in the nests of stoddardi at a time (about one hour before sunset)
when andrei was actively foraging. This suggests that stoddardt
may be crepuscular or nocturnal in its foraging. The former
possibility seems more likely for, when these ants are exposed
to light, they do not show the energetic avoidance response
which marks most nocturnal species. When not foraging, the
workers of stoddardi retire well into the nest passages. Their
nests, like those of andrei, appear to be deserted at such times.
The marriage flight of stoddard: occurs in early July. The fe-
males, which are rather strikingly marked with yellow, are
comparatively small. It is worth noting that the worker caste
of stoddardi is fully as polymorphic as that of pergander. A
contrary impression is apt to result from much of the material
at present in American collections. All of this material that the
writer has examined has lacked the smaller media and minor
workers.

18 AMERICAN MUSEUM NOVITATES NO. 1612

During the course of these studies two visits were made to
San Jacinto, California, with the hope of securing topotypes of
stoddardt. The second trip was undertaken after colonies of
stoddardt had been secured elsewhere, hence the writer knew
exactly what to look for. Despite this, no colonies of stoddardi
could be found at San Jacinto, nor is this surprising. Extensive
irrigation has greatly altered the character of the region around
San Jacinto, and it seems probable that stoddardi may no longer
exist in its type locality.