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A se is made on all overdue University of Illinois Library L161—H41 Digitized by the Internet Archive In 2011 with funding from University of Illinois Urbana-Champaign http://www.archive.org/details/studiesonbiology15vand ILLINOIS BIOLOGICAL MONOGRAPHS VoL. XV No. 3 PUBLISHED BY THE UNIVERSITY oF ILLINOIS UNDER THE AUSPICES OF THE GRADUATE SCHOOL URBANA, ILLINOIS 1937 EDITORIAL COMMITTEE JouHN THEODORE BUCHHOLZ FRED WILBUR TANNER HARLEY JONES VAN CLEAVE UNIVERSITY OF ILLINOIS 1000—8-37—12024 St UDIES ON: THE BIOLOGY OF THE CRAYFISH CAMBARUS PROPINQUUS GIRARD WITH 46 GRAPHS BY WILLIAM Cart Van DEVENTER CONTRIBUTION FROM THE ZOOLOGICAL LABORATORY OF THE UNIVERSITY OF ILLINOIS No. 509 CONTENTS Introduction . Methods . : Distribution of the Species . Habitat Relation to Environment The Life Cycle Type of Life History. The Earliest Young . Sex Recognition : , ‘ Growth of the Juvenile Bes aatie Attainment of Sexual Maturity Copulation Woo. te +2=approximately 10=total number of molts or z. Therefore: In the case of the second size group from the right on the graph (the second most advanced), by similar calculation the number of molts undergone would be approximately 9. In the case of the third size group from the right (the third most advanced) it would be approximately 8. For the fourth size group it would be approximately 7, and for the fifth size group approximately 6. Thus the young crayfishes which are hatched in May or June pass through 6 to 10 molts during the first growing season with an average increment of 2.42 mm. cephalothorax length (equivalent to approximately 5 mm. total body length) with each molt. By mid-September they measure 12 to 27 mm. cephalothorax length which is equivalent to ap- proximately 24 to 54 mm. total body length. Other writers on the group of the crayfishes have recorded similar differences in growth rate among the juvenile population. Andrews (1904) in his work with C. affinis noted wide differences in growth rate among the season’s young. “During these 2 months and 3 weeks of summer,” he says, “some few seem not to have grown at all, while about a third of them have almost doubled their length.” Writing again in 1907 he estimates that the young of C. affinis pass through 11 to 13 28 ILLINOIS BIOLOGICAL MONOGRAPHS instars during their first summer; but he says that in some cases the young crayfishes may be only 20 mm. long in the fall, due apparently to their development being arrested during the 6th instar. He gives the body lengths of 101 specimens of C. affinis, which he raised in laboratory aquaria, as being 20 to 62 mm. in the October following their first growing season. Ortmann (1906) in his life history of C. obscurus says that the young crayfishes hatched in May or June measure 26 to 50 mm. body length by the end of September. He says: “It seems that the rate of growth of young crayfishes is very different in different individuals, some gaining through June, July, August, and September only about 15 mm. in length, others more than twice that length.” Creaser (1934) writing of C. propinquus in Michigan, says that the season’s young attain a size of 10 to 20 mm. cephalothorax length by September. This is equivalent to a body length of 20 to 40 mm. Thus in every case in which the growth of young crayfishes during the first season has been observed, it has been found that the rate of growth differs greatly among individuals and that therefore the spread between the largest and smallest individuals or groups in the population increases during the summer and reaches a climax in late September or early October at the close of the growing season. Causes of the Differences in Growth Rate.—Although all students of the group of the crayfishes agree as to the remarkable differentiation in size which takes place during the first season among the originally homogeneous juvenile population, only Andrews (1907) attempts to account for it. He suggests that insufficient food may be the cause of the lack of growth on the part of the smaller individuals. In speaking of one brood of laboratory-reared crayfishes which averaged 50 mm. body length in the fall as against an average of 41 mm. for the entire group reared in the laboratory, he attributes the advanced size of this brood to its having had more food and possibly also more favorable conditions of temperature than the remainder of the group. In another connection he says, ‘the rate of growth was very different in individuals, and in some cases seemed to depend directly upon food supply.” Andrews’ specimens were all carried through the first growing season in the laboratory, and the reasons which he gives for the retardation of the growth of a portion of them as compared to the rest are more appli- cable to laboratory-reared crayfish than they are to crayfish which grow up under natural conditions. In the natural environment temperature as a cause for retardation of growth is ruled out as all members of the juvenile population are subjected to approximately the same temperature conditions. Shortage of food is likewise not likely to affect any portion of the population except in abnormal seasons, since in the locality which BIOLOGY OF CRAYFISH—VAN DEVENTER 29 was studied by the writer, there was more than abundant food for all the crayfish there, both young and old. It seems to the writer that the differences in rate of growth are more probably due to inherent genetic growth factors than simply to environ- mental causes. The invironmental conditions undoubtedly influence the expression of these inherent genetic factors, however, and probably determine the upper limit of size which may be attained in a particular season. This was apparently the case in 1933, when the retarded spring season resulted in the attainment of a smaller total size in the case of all groups than in the preceding season of 1932. ATTAINMENT OF SEXUAL MATURITY Sexual Form of Males at the End of the First Season.—Until the end of the summer the males of the season’s young remain in the juvenile form. During August and the first half of September, however, there is a progressive change from the juvenile condition to the first form. Table 3 shows the progressive change in form among the young males during the late summer and early fall of 1932. TABLE 3.—PROGRESSIVE CHANGE OF ForRM OF YOUNG MALES Date of Total number of Number in Number in first collection young males juvenile form form Mugs We 1932 es 23 23 0 Sept. 15, 1932......... 14 8 6 Sept. 26, 1932......... 28 16 12 Oct. 6, 1932........... 51 8 43 After the beginning of October, in 1932, the first form males con- tinued to outnumber the juvenile males among the season’s young in most of the collections taken during the fall and winter. This condition of the juvenile population probably represents the normal fall and winter condition at this latitude. In 1933 the results were less conclusive. In two collections taken during September and October (Sept. 20th and Oct. 7th) the first form males outnumbered the juvenile males among the young of the season. In the other fall and winter collections the number of juvenile males exceeded the number of first form males by a ratio which was in most cases approximately two to one. It seems probable that in the fall and winter of 1933 the number of juvenile males really exceeded the number of first form males among the season’s young at all times, and that the two collections in which the opposite was true were the result of in- accurate sampling. Such a conclusion is in accord with the results of the 30 ILLINOIS BIOLOGICAL MONOGRAPHS majority of the fall and winter collections of 1933-34, and also with the generally delayed appearance of all crayfish life history phenomena in the season of 1933. A similar relation of the length of the growing season to the per- centage of males of the young of the year which reach maturity by the first fall is evident in the work of Creaser (1933) and of Smith (1911) with C. propinquus. Creaser working with this species at Ann Arbor, Michigan, where the growing season is necessarily shorter than at Urbana, Illinois, found that some males of the season’s young became first form by the beginning of fall, but that the great majority of them did not do so. Smith, in a collection taken at Douglas Lake, Michigan, on Aug. 5th found that among 16 young males, 4 were first form and 12 were still juvenile. In a collection recorded for the same day taken at Urbana there were 30 young males, 21 of which were first form and 9 juvenile. Thus in collections taken on the same day, the majority of the young males in the Michigan population were still juvenile, while the majority of those in the Illinois population had already become first form. There- fore, apparently the longer growing season characteristic of the more southern location not only enables a higher percentage of the young males to reach maturity by the end of their first growing season at Urbana, Illinois, than in central Michigan, but also causes the young of the Urbana population to show greater advancement at any particular time during the late summer. Investigators working on other species of crayfish have likewise found that a portion of the young of the year reach maturity by the end of the first season. Ortmann (1906) in the study of the closely related species, C. obscurus, in Pennsylvania found that most males of the sea- son’s young become first form by October. With C. affinis, also, Andrews (1904) found that in Maryland a portion, at least, of the young males attain maturity by the beginning of the first fall after they are hatched. Size of Males at Sexual Maturity—It was found in the present study that a cephalothorax length of 20 mm. marked approximately the boundary between sexual maturity and immaturity among the males of less than one year. The individuals measuring 20 mm. and above were generally found to be first form, indicating that they had attained sexual maturity, while those that measured less than 20 mm. were generally im- mature. This division however was by no means absolute. Individuals which had reached sexual maturity below 20 mm. were not uncommon, and occasional individuals were found which had passed this size and were still immature. The accompanying distribution curve (Graph 1) represents sexual maturity in males in a winter collection which was typical of the collec- tions taken after the end of the growing season. The unshaded portions BIOLOGY OF CRAYFISH—VAN DEVENTER 31 GraPH 1.—Showing the distribution of sexual maturity among the males in a typical winter collection (February 18, 1933). The shaded squares represent the immature, and the unshaded squares the mature individuals. The abscissa repre- sents cephalothorax length in millimeters. of the graph represent the mature males, and the shaded portions repre- sent the immature males. In this graph the fact that an approximate boundary between maturity and immaturity exists at 20 mm. cephalo- thorax length is plainly apparent. First form males below 18 mm. cephalothorax length were exceed- ingly rare, although one was taken on November 17, 1932, which was only 12.6 mm. The largest immature male taken measured 23.1 mm. cephalothorax length. This individual occurred in the collection of November 3, 1932. True second form males of a much larger size than this were found between the first and second adult molts of the adult males in the spring, but in the opinion of the writer a distinction may properly be made between the juvenile condition of the copulatory appendages, and the true second form condition as it occurs in adult males. Therefore these large second form males cannot be placed in the same class as the immature males. The boundaries of sexual maturity among the males of C. propinquus which were indicated in the present study agree with those recorded for the same species by Smith (1910-11). In her work, as in that of the writer, 20 mm. cephalothorax length formed an approximate lower boundary for sexual maturity among the males. She found that the great majority of first form males measured 20 mm. or more, and she found no first form males at all below 17 mm. Ortmann (1906) found a similar size limit in C. obscurus. He found that in this species a body length of 40 to 50 mm. (corresponding to a cephalothorax length of approximately 20 to 25 mm.) was indicative of sexual maturity among the males. The smallest first form male that he found measured 38 mm. body meee (approximately 19 mm. cephalo- thorax length). A slightly different result, however, was obtained by Creaser (1933) in his study of the C. propinquus of Michigan. If we may judge by his graphs, he found a much greater percentage of first form males measur- 32 ILLINOIS BIOLOGICAL MONOGRAPHS ing below 20 mm. cephalothorax length than occurred in the studies of either Smith or the writer. Indeed, in his collections, the size of 20 mm. seems to be totally without the significance which it was found to have in the Illinois population. The size of 16 mm. or 17 mm. would appar- ently hold something of the same significance in Creaser’s collections. This difference between the results of Creaser and those obtained by Smith and the writer for the Illinois population is largely explainable on the ground that Creaser’s population represented a smaller race of C. propinquus than that found in eastern Illinois. This theory finds support in the fact that Creaser’s largest adults rarely exceeded 30 mm. cephalo- thorax length, whereas in the race of crayfish used in the present study, a portion of the yearling adult group regularly exceeded 30 mm., and the absolute maximum of size seemed to be between 35 and 40 mm. Creaser’s crayfish seem, therefore, to have averaged from 3 to 5 mm. smaller in cephalothorax length among all size classes than the Illinois crayfish. Sizes of Females at Sexual Maturity.—There is no available criterion for determining whether or not individual females among the young of the year are sexually mature at the end of their first growing season. However, a general idea of the boundary of sexual maturity among the young females can be obtained by observing the sizes of the females seen in copulation during the fall. In the present study no females below a cephalothorax length of 20 mm. were found copulating, but apparently females of 20 to 25 mm. cephalothorax length (all young of the season) were quite as likely to be taken in copulation as were older and larger individuals. Similar results were obtained by Smith (1910-11). She also found copulation taking place with females of as low as 20 mm. cephalothorax length, undoubtedly belonging to the young of the season. Likewise in the closely related species, C. obscurus, Ortmann (1906) observed copula- tion in the fall among the sexually mature individuals of the season’s young, the smallest female taken in copulation having a body length of 43 mm. (approximately 21 mm. cephalothorax length). A further and more general criterion for sexual maturity among the females is furnished by the size of the females found bearing eggs in the spring. The evidence from this source fully verifies the existence of an approximate lower limit of sexual maturity among the females at 20 mm. cephalothorax length. Thus apparently with females as with males, this size forms the approximate boundary of sexual maturity among the young of the year in the race of C. propinquus found in eastern Illinois. BIOLOGY OF CRAYFISH—VAN DEVENTER 33 COPULATION In connection with the present study pairs of C. propinquus were taken copulating or in copulatory attitude in the months of September, October, November, and March. Smith (1910) found copulating cray- fish of this species in the field at Urbana, Illinois, in October, November, and December; and at Douglas Lake, Michigan, in July and August. Hay (1919) found C. propinquus copulating in Lake Maxinkuckee, Indiana, in November, January, and April. Turner (1926) gives the date of September Ist in Michigan and August 25th in Wisconsin for the observation of copulation in C. propinquus. Creaser (1933) says that in the vicinity of Ann Arbor, Michigan, the mating season of the species is in October and November. Thus it seems that the occurrence and duration of the mating season of C. propinquus varies widely with the locality. In more northern lati- tudes, such as Michigan and Wisconsin, it probably begins in July and August, and lasts until November, but does not occur again in the spring. In more southern latitudes such as east-central [linois and Indiana it probably begins in September and lasts until the onset of severe winter weather in December or January, then begins again in March, and may last into April. It is possible that with mild winter temperatures such as occur in some years, the mating season might last throughout the winter at the latitude of Urbana, because males will attempt to copulate at any time during the winter if warmed and placed in close confinement with females. This corresponds to the experience of Pearse (1910) who found that male crayfish during the mating season will attempt copulation almost immediately if placed in close confinement with females, and also to the experience of Bell (1906), who found increased sexual activity among the males accompanying a rise in temperature. In copulating pairs the male and female are usually approximately the same size, although occasionally the writer has seen larger males attempt- ing to copulate with smaller females in laboratory aquaria. Creaser (1934) says: “Presumably only a small percentage of females can mate as young of the year, unless the males are polygamous, because observa- tions indicate that mating pairs are of about the same size.”” The writer during two years of observation saw nothing to indicate that male cray- fish are necessarily monogamous, and Andrews (1895) says definitely that in C. affinis conjugation may be repeated by either animal with some other individual. 34 ILLINOIS BIOLOGICAL MONOGRAPHS Static CONDITION DURING THE FALL AND WINTER After the latter part of September or the early part of October growth ceases in all cases. In the present study the latest date at which a “‘soft,” freshly molted individual was taken was October 6th. This individual was a young female and measured 17.1 mm. cephalothorax length. In the collections taken during the fall and winter the five size groups in the population of the young of the year remain distinct and maintain relatively constant levels of size. In the different collections the mode of each group may vary back and forth within a range of 2 to 4 mm., but in all cases the five modes representing the five size groups maintain the same general relations to one another (Graph 3). The variation in the position of each mode is indicative of the opera- tion of two factors: (1) the individual variation among the members of each size group, and (2) the vagaries of sampling. Consistently larger collections might have produced less variable results. Other investigators have likewise found a cessation of growth in the early fall, and a static condition of the population during the fall, winter, and early spring. Andrews (1907) says of C. affinis: “After October there are no more molts until the following warm season, the lengths remaining constant through the winter.” Ortmann (1906) says that in C. obscurus the winter is passed without change. According to Creaser (1934) there is no growth among the C. propinquus of Michigan after about September 24th. And Turner (1926) writing of crayfish in general says: “During the winter, growth . . . . is at a low ebb, and no molting takes place.” In all cases this absence of molting may be easily seen from the condi- tion of the exoskeleton. In all crayfish, large and small, taken in the winter collections, the shell is dark, almost black, and is frequently covered with algal and protozoan growths. Creaser (1934) advanced the theory that the cessation of growth and molting in the early fall might be caused by falling temperature, and he related this to the fact that at Ann Arbor, Michigan, he observed no crayfish feeding during the winter months. It seems probable that the phenomenon may be related to falling temperature, although in 1933, at Urbana, during the period in which the cessation of growth was noticed the days were still warm, with temperatures ranging from 15° to 20° Centigrade. The temperature control, however, if such it is, must operate in some way other than to cause the crayfish to stop eating, for the writer found food in the process of digestion in the stomachs of C. propinquus taken in December. Furthermore, the crayfish are relatively active for six weeks after growth has ceased in the fall, and Hay (1919) reports seeing BIOLOGY OF CRAYFISH—VAN DEVENTER 35 crayfish of this species feeding regularly during October and the first half of November in Lake Maxinkuckee, Indiana. It seems likely that the gradual lowering of the mean temperature for the 24-hour period which takes place during the early fall, largely on the account of the occurrence of cooler nights, may cause a slowing of the physiological processes of the animals to a level of mere maintenance, thereby bringing about a cessation of growth and molting. This seems particularly probable in view of the fact that recent studies such as that of Markus (1933) on the relation of temperature to the rate of digestion in the black bass, have shown that in cold blooded forms a very close correlation exists between temperature and the rates of physiological processes. SPRING REPRODUCTIVE ACTIVITIES Kinds of Activity—Except for the occurrence of copulation in the spring in some localities the reproductive activities of spring concern the females only. They consist of two distinct phases: (1) the laying and carrying of the eggs, and (2) the hatching and carrying of the young. It was necessary to treat fully the second phase of the female repro- ductive activities, that is, the hatching and carrying of the young, at the beginning of this study, in connection with the appearance of the first young of the season. The first phase of these activities, however, the laying and carrying of the eggs, although it was necessarily mentioned in connection with the hatching of the young, is more properly considered at this point in the description of the life cycle, since it is directly related to all of the events which take place among the adult female group at this time. Egg Laying.—During a part of the month of March the adult females are largely absent from the active population. Table 4 shows the ratio of males to females for two typical winter-time collections, two collections taken during the month of March, 1933, and two taken during the month of April, 1933. The purpose of the inclusion of the two winter collections in the table is to show the normal sex ratio as a basis for comparison TABLE 4.—SEx RATIO IN WINTER AND SPRING COLLECTIONS Date of collection Males Females Percent males | Percent females Jan. 26, 1933........ 35 24 60 40 Feb. 18, 1933....... 42 33 56 44 Mar. 4, 1933...... 56 20 74 26 Mar. 18, 1933...... 51 = 77 23 Apr. 8, 1933....... 29 22 57 43 Apr. 22, 1933....... 2 29 52 48 36 ILLINOIS BIOLOGICAL MONOGRAPHS with the March collections when the females were largely absent, and the April collections when they have rejoined the active population. From Table 4 it is seen that the males outnumber the females normally to a slight extent. This ratio in favor of the males, however, markedly increases during the month of March, and returns to normal shortly after the beginning of April. The first females bearing eggs appear coincident with the return of the adult females to the active population in early April. In 1933 the first egg-bearing females were taken in the collection of April 8th. In 1934 they appeared in the collection of April 7th. High water handicapped the work of collecting during the month of March in both years, so that a proper search for the missing females could not be made. However, since most of them reappear in early April, bearing eggs, it may be reasonably assumed that their absence during March is connected with the laying of the eggs. Andrews (1895 and 1904) tells of the extreme excitability of the females of C. affinis as egg-laying time approaches. It is reasonable to suppose, then, that during the egg-laying period and for some time prior to it the females remain hidden and inactive. Therefore, we reasonably may consider that the eggs in C. propinquus are laid in late March or early April, and that most of them are laid at about the same time, since the disappearance and reappearance of the females takes place en masse. One female, however, of sexually mature size was found without eggs on April 22, 1933, and on examination it was discovered that a normal clutch of well-developed eggs were still internal, having not yet been laid. Whether this female would have laid later, or whether she would have produced young at all, cannot be determined. However, no young were found in either 1932 or 1933 which bore evidence of belonging to so late a hatch as this would have been. The conclusion that the eggs of C. propinquus are laid in late March or early April at Urbana, Illinois, corresponds closely to the results of Smith (1910-11) for this species in the same location, and also to the results of Ortmann (1906) for the related species C. obscurus in Pennsyl- vania. Smith found the first females with eggs on March 25th. Ortmann found no egg-bearing females on March 28th or March 3lst, but found numerous ones on April 6th. In the work of these investigators, as in that of the writer, the eggs apparently were laid in almost all cases at practically the same time. Egg Carrying.—During the two years of the present study, females bearing eggs were taken only during the months of April and May. Egg- bearing females were present in the collections of April 8th, April 22nd, May 6th, and May 21st, 1933; and of April 7th, April 28th, and May 5th, BIOLOGY OF CRAYFISH—VAN DEVENTER 37 1934. After the initial appearance of egg-bearing females practically all sexually mature females were found with eggs. These results again agree closely with those obtained by other investi- gators. Smith (1910-11) found practically all sexually mature females of C. propinquus carrying eggs on April 29th, and collected the last egg- bearing female on May 18th. Pearse (1910) took C. propinquus females with eggs at Walnut Lake, Oakland County, Michigan, on May 2nd, and June 6th; at Coldwater, Michigan, on April 17th; and at Ann Arbor on April 18th. Creaser records 15 females of this species as being taken with eggs near Dexter, Michigan, on May 24th, GraPH 2.—Showing the distribution of egg-carrying among the females taken in the collection of April 8, 1933. The shaded squares represent females not carry- ing eggs and the unshaded squares those carrying eggs. The abscissa represents cephalothorax length in millimeters. Thus the egg-bearing season for C. propinquus apparently lasts during the month of April and a part or all of the month of May in the latitude of Urbana, Illinois, and probably lasts from mid-April to early June in the more northern latitude of Michigan. Size of Females Carrying Eggs—The lower size limit of the females bearing eggs corresponds to the general lower size limit of sexual maturity among both males and females, as ascertained at the time of copulation in the fall. Here again the cephalothorax length of 20 mm. forms an approximate boundary, separating, in this case, those females which bear eggs from those which do not. In the accompanying graph (Graph 2) of the female population taken in the collection of April 8, 1933, during the egg-bearing season, this boundary is clearly evident. The shaded portions of the graph represent those females which were not carrying eggs at the time the collection was made. The white portions represent the egg-bearing females. This graph represents the condition of the female population typical of the egg-bearing season. The occurrence of females below 20 mm. cephalothorax length with eggs was rare. Such females were taken occasionally, however. The 38 ILLINOIS BIOLOGICAL MONOGRAPHS smallest female found with eggs measured 16.1 mm. cephalothorax length, and was taken on May 6, 1933. The largest female taken without eggs during the egg-bearing season measured 24.9 mm. cephalothorax length and occurred in the collection of April 28, 1934. Smith (1910-11) obtained results with C. propinquus similar to those of the present study. She found no egg-bearing females measuring less than 18 mm. cephalothorax length. Likewise, Ortmann (1906) found 40 mm. body length (approximately 20 mm. cephalothorax length) to be the lower size limit of egg-bearing among females of C. affinis. Creaser (1934), however, obtained somewhat different results with the Michigan race of C. propinquus. He records the cephalothorax lengths of 14 egg-bearing females as follows: “one each, 15, 16, 17 and 18 mm.; five 19 mm.; and one each: 20, 21, 23, 24 and 25 mm.” Here the size boundary of 20 mm. cephalothorax length appears to be without signifi- cance. This is in accord with the results which he obtained concerning the size of the young males at the time of attainment of sexual maturity, and it may likewise be explained by assuming that Creaser was working with a generally smaller race of C. propinquus than that occurring at Urbana, Illinois. Number of Eggs Carried by the Females.—The numbers of eggs carried by the females were found to be roughly proportional to the sizes of the individuals. The largest female taken with eggs measured 36.1 mm. cephalothorax length and had an estimated clutch of 175 eggs. The largest clutch, estimated at 250 eggs, was borne by a female measur- ing 31.4 mm. cephalothorax length. The smallest female which was found bearing eggs measured 16.1 mm. cephalothorax length and had an estimated clutch of 40 eggs. The smallest number of eggs found on any female was 5, and occurred on a female measuring 21.7 mm. cephalo- thorax length. In this case, however, as in a number of other cases show- ing only a few eggs, the manner in which the eggs were scattered on the pleopods gave evidence that the few eggs were only the remains of a larger clutch, the majority of which had been lost. Smith (1910-11) records additional evidence which shows that the number of eggs carried is in general proportional to the size of the individual female. She found a female measuring 38 mm. cephalothorax length which was carrying 225 eggs, and another measuring 20 mm. cephalothorax length, and carrying 102 eggs. These cases agree essen- tially with the results obtained in the present study. SPRING MOLTs OF THE YEARLING GROUP Criterion for Determining the Occurrence of First Molts.——The col- lector has no difficulty in determining which individuals among the cray- BIOLOGY OF CRAYFISH—VAN DEVENTER 39 fish taken in the spring collections have undergone a molt and which have not done so. Up to the time of their first molt in the spring the shells of all of the crayfishes of both sexes, whether large or small, retain the dark color characteristic of the winter season. They frequently retain also the encrusted algal and protozoan growths which accumulate during the winter. Number of Molts Among the Mature Yearling Males——There have been various observations recorded by a number of investigators con- cerning the number of molts undergone by male crayfishes during the second season after they are hatched. In most species which have what Ortmann (1906) called the “warm water type” of life history, it has been found that the adult males undergo two molts during their second season. The first of these has been generally called the “spring” molt, while the second molt has been variously called the “summer” or “fall” molt, depending on the time of its occurrence. It will be shown in this study that during one of the years in which the investigation was carried on, this second molt of the adult males occurred so early in the season that it did not even justify the use of the term “summer” molt. There- fore it has seemed best in the present study to refer to these molts as the first and second adult molts of the males. Both of these molts involve changes of the form of the first abdominal appendages on the part of the adult males. These males have passed through the winter with first abdominal appendages of the first form. They become second form as a result of the first adult molt and then revert back to first form as a result of the second adult molt. The First Adult Molt of the Mature Yearling Males.—The first molts among the adult males in the spring appear in April or May. In 1933 the first newly molted males of this group appeared in the collection of May 6th. Out of 21 adult males taken on that day 5 were newly molted. In 1934, however, the first newly molted adult males appeared in the collec- tion of April 7th, along with the first egg-bearing females. At that time, out of 63 adult males, 22 were molted. It seems probable that the occurrence of the first adult molt in 1934 was more nearly normal than that in 1933, since the laying of the eggs by the females, and the new molts of the males represent correspondingly initial spring activities on the part of the two sexes. Also, it is well to remember again in this connection that the spring of 1933 was very much retarded while the spring of 1934 was relatively normal. The first adult male molt occupies a relatively short period of time. In 1933 the first molted individuals of this group were discovered on May 6th and at the time of the next collection on May 21st all adult males were molted. In 1934 the first newly molted adult males were taken on 40 ILLINOIS BIOLOGICAL MONOGRAPHS April 7th, and before the time of the next collection on April 28th, all had completed the molt. Smith (1910-11) gives a slightly later date for the occurrence of the first adult male molt in C. propinquus than was recorded during either of the years of this study. She gives the date for its occurrence as being from May 13th to June 2nd. However, in view of the wide diver- gence found in the time of the occurrence of this molt as between the two years of the present study, this still later period given by Smith may easily be accounted for by the particular character of the season during which she worked. Workers on other species of crayfish have obtained results similar to those obtained by Smith and by the writer for C. propinquus. Steele (1902) places the first adult molt for the males of C. virilis in Missouri from early April to mid-May. Ortmann (1906) says that with C. obscu- rus in Pennsylvania the first adult molt of the males takes place in most individuals during the first half of May, but that some individuals may begin as early as March. In both C. wirilis and C. obscurus, therefore, as well as in C. propinquus, the first adult male molt in the spring in most cases is accom- plished sometime during the months of April or May. Undoubtedly in the case of a species like C. propinquus which occurs over a wide territory this molt takes place at a relatively later date in the more northern locations. And in all cases the earliness or lateness of the occurrence of the molt in any particular area is certainly dependent on the character of the season. Second Adult Molt of the Mature Yearling Males.—After the com- pletion of the first adult male molt in the spring there are no first form males in the population, since all of the males which had previously been first form became second form at the time of the first adult molt. Any first form males which occur in the late spring or early summer, there- fore, are those which have also completed the second adult molt, since it is by this molt that the adult males again become first form. In the writer’s study of C. propinquus in 1932, a collection taken on June 29th showed 11 first form males and 9 second form males. Since these first form males had unquestionably completed the second adult molt it is evident that this molt was well advanced at that time. In the next collection, taken on July 14th, 4 first form males were taken, and one second form male. In all later collections taken during the summer all adult males were first form. It seems justifiable to conclude, there- fore, that in 1932 the second adult male molt began in the latter part of June and was completed in nearly all cases by the middle of July. In 1934 the second adult molt of the males was apparently a “second spring molt” rather than a “summer molt.” On April 7th, 41 first form BIOLOGY OF CRAYFISH—VAN DEVENTER 41 males (still retaining their old winter shells) were taken along with 22 second form (newly molted) males. These second form males were all adult in size and had become second form with the first adult molt of spring. Apparently therefore, this collection was taken just in the middle of the first adult molt of the season. At the time of the next collection, taken on April 28th, all the adult males had lost their old winter shells, indicating that they had all passed through the first adult molt, and 7 out of 38 had also passed through the second adult molt, and become first form again. The remaining 31 males were still second form. Just one week later, on May 5th, only 7 males out of a total of 62 remained second form, all others had completed their second adult molt and become first form. Thus apparently in the season of 1934, both first and second adult molts took place within a little more than a month among most of the adult male group of the population. Presumably at some time during the month of April, there was a short period during which all adult males were second form. Then, just before the collection of April 28th the second adult molt began, and it was largely completed by May 5th, just one week later. It seems probable that the remaining second form males which had not yet undergone the second adult molt by May 5th did so during the remainder of May or in the early part of June. The second form males present in the collections of June 29th and July 14th, in the summer of 1932 probably represented such ‘‘stragglers” which underwent the second adult molt and changed to first form somewhat later than the majority of the adult male group. Smith (1910-11) was unable to find evidence of any second adult molt at all among the males of C. propinquus in the Urbana population. This may now be explained on the basis that in the spring of 1911, as in that of 1934, the two molts occurred very close together. If this were true, she might easily have confused the two, and thought them a single extended molting period. Apparently the occurrence of the second adult male molt among the C. propinquus of Michigan is more delayed, judging from the work of Creaser (1933) and of Smith (1910-11). Creaser’s graph for a popula- tion sample taken on July 30th indicates that the second adult molt was largely finished by that time, since the adult males taken were mostly first form. These had mostly been second form in his graph for June 29th, showing that at that time the first adult molt had occurred but not the second. Smith found that all adult males were still second form at Doug- las Lake, Michigan, on July 6th. On August 5th only one second form male of adult size was found, all the others having reverted to first form. Thus, in Michigan the second adult molt of the males must occur during July and early August. 42 ILLINOIS BIOLOGICAL MONOGRAPHS Investigations on other species of crayfish have generally demon- strated that the second adult male molt occurs in summer rather than in spring. Steele (1902) says that this molt occurs among the C. wirilis of Missouri between June 10th and July lst with most individuals. Ortmann (1906) found that the second molting season for adult males began in July and lasted until late September, or early October, among the Penn- sylvania population of C. obscurus. The occurrence of the second adult molt of the males at so early a date as that recorded in the spring of 1934 for C. propinquus, at Urbana, has never been established for any other species of crayfish. It is impos- sible to say whether or not such an early occurrence of this molt is normal with this species at this latitude. It seems probable, however, that the second adult molt among C. propinquus may occur early in this way, whenever the season is sufficiently favorable. Molting Among the Immature Yearling Group—Those individuals among the previous year’s generation of young which failed to attain sexual maturity by the end of the first growing season and which passed through the winter and spring in the immature state undergo their first yearling molt at the same time as the first adult molt of the males. Two females and one male of immature size were found newly molted along with the newly molted adult males on May 6, 1933. They also undergo a second yearling molt at the same time as the second adult molt of the males. With the first yearling molt the immature males do not change the form of the copulatory appendages, but retain the juvenile form. With the second yearling molt, however, these immature males attain sexual maturity and become first form. Therefore, after this molt all yearling males in the population are first form. Presumably the immature females also become mature with this second yearling molt since they attain adult size as a result of it. There is evidence that the members of this immature group also undergo a third and a fourth yearling molt during the summer. This is in accord with the statement of Creaser (1933) who says: “Between March 17th and June 29th the young of the previous year resume their growth, rapidly increasing to adult size, probably by reason of several moltings.”’ Creaser’s “young of the previous year” were for the most part immature during the fall and winter, so their behavior would in general parallel that of the immature section of the previous season's young in the present study. The Spring Molt of the Adult Females.—The adult females which have borne eggs during the spring, undergo only a single molt. This takes place immediately after the young are shed. In 1933 the first newly BIOLOGY OF CRAYFISH—VAN DEVENTER 43 molted adult females were found on May 21st. At the time of the follow- ing collection, taken on June 7th, the entire adult female population had molted. In 1934 the first newly molted adult females were found in the collection of May 5th, and in the following collection, taken on May 13th, all were molted. Smith (1910-11) obtained similar results. She secured the first molted adult females on June 2nd, and found that the entire adult female group had completed its molt by June 6th. GROWTH AMONG THE YEARLING GROUP Growth Among Mature Yearling Males and Immature Individuals.— Since the first and second yearling molts of the immature group occur at the same time as the first and second adult molts of the yearling males, these groups may be considered together in the matter of growth during the two molts. In both years of the present study there was apparently no growth among either the adult yearling males or the immature group as a result of the first molt. Examination of the composite graph (Graph 3) shows that the lines representing all size groups, both mature and immature, remain essentially level during and following the time of this molt. In contrast to the result of the first molt, the second molt of the adult males and immature individuals results in marked growth among both groups. The growth increment accompanying this molt varies from 3 mm. cephalothorax length in some individuals to as much as 8 mm. in others. The variation among individuals as to the amount of growth which accompanies this single molt stands in marked contrast to the con- dition prevailing during the juvenile growth period of the first growing season. Among the juvenile population differences in size apparently come about for the most part as a result of differences in the number of molts undergone rather than as differences in the amount of growth increment following a single molt. The distinctness of the size groups which were formed in the late summer among the juvenile population as a result of the differences in the number of molts undergone by differ- ent groups of individuals depended upon the fact that the individuals within each group had not only passed through the same number of molts, but had also undergone approximately the same amount of growth at the time of each molt. As a result of the wide individual variation in growth rate in the spring at the time of the second yearling molt, there is a tendency for individuals to “migrate” into other size groups more advanced than their own, thus bringing about a “piling up” of individuals in the more ad- vanced size groups at the expense of the less advanced ones. (Compare ~ Graphs 25 and 26 for 1933, and Graphs 45 and 46 for 1934.) 44 ILLINOIS BIOLOGICAL MONOGRAPHS The fact that the growth increment accompanying this molt was as much as 8 mm. cephalothorax length in some cases, while it was never more than half this much in the case of the juvenile molts of the first season is very interesting in view of the fact that there was apparently no growth at all in connection with the first yearling molt. Presumably the approximately doubled growth which takes place in some individuals at the time of the second yearling molt compensates for the absence of growth at the time of the first yearling molt. The only published reference to the occurrence of a molt without growth in any species of crayfish is that recorded by Creaser (1933). He mentions the possibility that young males, at the end of their first growing season in the autumn, may molt without increase in size in case they undergo a change from the juvenile condition to the first form during the molt. In the present study, however, the molt without growth occurred in the spring and involved: (1) mature first form males which underwent a change to second form at the time; (2) juvenile males which remained in the juvenile form after the molt; and (3) immature females. Therefore, the case is in no way parallel to the one given by Creaser. No full explanation of the absence of growth in connection with the first molt of spring can be offered at the present time. However, a consideration of the nature of the growth process in crayfish suggests a possible explanation. Since growth among crayfish is periodic, the anabolic processes between molts result in the accumulation of substances which are stored in the body as additions to the protoplasmic content and as stored food; but because of the physical limitations imposed by the integument the body does not increase in size until the confining shell is cast off. Then chiefly through imbibition of water the newly molted individual increases in size, reestablishing temporarily the equilibrium between formed substances and water content of the body. It seems probable that in the late winter and early spring physiological activity is reduced to a minimum because of the low temperature of the water. Under such conditions there would be no appreciable accumulation of substances. Therefore at ecdysis the imbibition of water would be negligible, and consequently with the first molt of spring there would be no appreciable change in size. Such an explanation seems particularly plausible in view of the fact that a correlation between temperature and physiological activity is probably the cause of the cessation of growth and molting among the crayfish population in the fall. The third and fourth yearling molts of the immature group appar- ently result in about the same growth increment as was found to be char- acteristic of the juvenile molts (2 to 4 mm. cephalothorax length). These BIOLOGY OF CRAYFISH—VAN DEVENTER 45 two molts, however, and the growth accompanying them will be con- sidered further in connection with the accounts of their occurrence. Growth Among the Mature Females.—The second yearling molt of the adult males and the immature individuals occurs at approximately the same time as the single molt of the adult females. This molt involves growth among the adult female population of the same general extent and character as that which takes place in connection with the second year- ling molt of the other two classes. Thus the major portion of the growth among the yearling population takes place at approximately the same time in both sexes and in all size groups, although the molt by which it is accomplished among the adult females does not correspond to the one by which it is accomplished among the members of the other two classes. ATTAINMENT OF MAXIMUM SIZE As a result of the growth occurring in connection with the second yearling molt of the adult males and the single molt of the adult females, the entire yearling adult population reaches a size of 25 to 35 mm. cepha- lothorax length (Graphs 26 and 46). At the same time the size groups which were immature during the winter and spring attain a cephalothorax length approximating or exceeding 20 mm. (Graphs 26 and 46) and become sexually mature. The 25 to 35 mm. cephalothorax length seems to constitute for the great majority of the race the maximum size which an individual may attain. During two years of observation and collecting, a small number of C. propinquus were taken which measured more than 35 mm. cepha- lothorax length. These individuals apparently represented special cases of survival and growth beyond the ordinary maximum size characteristic of the race. On June 29, 1932, a single male was taken which measured 40 mm. cephalothorax length. This was the largest individual ever found, and apparently represented the extreme upper limit of size for the race of C. propinquus occurring at Urbana, Illinois. DEATH OF THE YEARLING ADULTS Almost immediately following the second adult molt of the yearling males and the single molt of the adult yearling females, resulting in the attainment of the ordinary maximum size by these classes, the major portion of this adult population of. both sexes completely disappears. In the collections taken after the middle of June, members of the maximum size groups are almost wholly lacking. There remain three classes of individuals in the collections taken in 46 ILLINOIS BIOLOGICAL MONOGRAPHS late June and early July (Graphs 27 and 28): (1) the young of the season, most of which are still below 12 mm. cephalothorax length; (2) a small adult group, measuring about 23 to 30 mm. cephalothorax length; and (3) occasional individuals measuring from 35 to 40 mm. Apparently all or nearly all of the individuals of both sexes which were sexually mature during the winter and spring, and which produced the year’s generation of young, die following the molt by which they attain maximum size. This phenomenon is clearly shown in the graphs representing the collections for June 7th and June 25th, 1933 (Graphs 26 and 27). In the collection of June 7th the adults of the winter and spring are still present in the population, measuring from about 25 to 35 mm. cephalothorax length. The immature individuals of the winter and spring, which now have become adult, all measure 19 mm. or more, and these, because of greater individual growth in a large number of cases, show a tendency to “‘pile up” on the adult population, thus shorten- ing the range of the yearling group as a whole. In the following collection, taken June 25th, the large adults are entirely absent, and the major portion of the entire yearling group has disappeared. There remain, besides the young of the season, only a few adult individuals which measure from 23 to 30 mm. cephalothorax length. This condition continues relatively the same during the remainder of the summer. At no subsequent time does the large adult group reappear in the samples, as would be the case if they had gone into burrows or had migrated to other parts of the stream. Furthermore, a careful search made in holes in the bank along the water edge and in other parts of the stream revealed none of them. Therefore we cannot escape the conclusion that, in the season of 1933, between June 7th and June 25th, most of the large adults died, including the major portion of the entire adult population, and that portion which produced most of the season’s generation of young. The small adult group in the collection of June 25th, which ranges from 23 to 30 mm. cephalothorax length, apparently represents a portion, at least, of the group of individuals which passed through the winter and spring in the immature condition, and only became mature following their second molt in the spring. These individuals have apparently passed through a third molt, involving an increase in size of about 4 mm. cephalothorax length, between June 7th and June 25th, since on June 7th the smallest of them measured 19 mm. while on June 25th the smallest of them measured 23 mm. Definite evidence for the occurrence of this molt was found in connection with the collection of June 25th. In this collection two “soft,” freshly molted females were taken which measured 25.4 and 28.3 mm. cephalothorax length, and which therefore belonged to the surviving year- ling group. BIOLOGY OF CRAYFISH—VAN DEVENTER 47 PROBABLE NATURE OF ADULT DEATH In the habitat under consideration C. propinquus plays a relatively important role in food chains. Fishes are the most important natural enemies of this species. During the present study no attempt was made to evaluate the effects of predators on the crayfish population. The pro- gressive changes which take place in the crayfish population as the season advances, however, give no evidence of marked alteration attributable to the action of predatory species. Therefore it seems probable that the toll exacted by predators is a fairly uniform one, being relatively the same at all seasons, and affecting all classes of the population equally. The death which decimated the adult population was undoubtedly a natural death, since no observed catastrophe took place which would account for it; and the fact that it affected only one class of the popula- tion precludes the possibility of its having occurred as a result of disease. Furthermore, it was not peculiar to the season of 1933, since a similar event had apparently taken place in 1932 before the first collection was taken on June 29th, judging from the size of the adult group in the four summer collections of 1932 (Graphs 4, 5, 6, and 7). The occurrence of natural death in this species cannot be laid to the attainment of the ordinary maximum size alone, because the members of the small, remaining, yearling adult group, which measure from 23 to 30 mm. cephalothorax length in June, all attain the ordinary maximum size later in the summer, and survive at this size through the fall, winter, and the following spring. Therefore there must be some other factor which, occurring in conjunction with maximum size, acts as a contributing cause in bringing about natural death. Since natural death apparently involves only those individuals which have not only attained the ordinary maximum size, but which have also contributed to the production of the season’s young, the coincident occurrence of parenthood and maximum size offers the basis for the most plausible theory regarding the immedi- ately contributing factors which bring about their death. The occurrence of natural death among adult crayfishes of maximum size coincident with the close of the reproductive season has been recorded previously by four different investigators. Creaser (1933) found natural death occurring among the adult males of C. propinquus in Michigan after the breeding season. Chidester (1912) likewise mentions the death of adult males after the breeding season in C. bartonius bartom. He says that “in the spring the males die off in great numbers.” Andrews (1904), writing of C. affinis in captivity, says: “After sexual union many died, and it was found that the males died in larger numbers than the females.” Ortmann (1906) says concerning C. obscurus: “Another re- markable fact is that after the end of the molting season in spring no 48 ILLINOIS BIOLOGICAL MONOGRAPHS very large males are found.” He believes that the large males die, and describes in detail two cases in which he found males of the maximum size in a dying condition and yet bearing no injury of any kind. He believes it probable that the old females die in the same manner as the males, since they are entirely absent from the population after the young are shed at the beginning of summer. The records of these investigators apparently bespeak conditions which were at least partially parallel to those found by the writer. The observations of Ortmann, in particular, afford an interesting comparison with the results of the present study, in view of the fact that C. obscurus and C. propinquus are closely related species. The fact that all four investigators found natural death occurring particularly among the males, parallels the fact that the writer found a marked excess of females among the small group of “oversize” individuals measuring from 35 to 40 mm. cephalothorax length. This is especially notable in view of the fact that in the population as a whole there was a definite preponderance of males. THE SUMMER ADULT GROUP The small group of individuals which survived the decimation of the adult population and which measured from 23 to 30 mm. cephalothorax length in the collection of June 25th (Graph 27) are represented regu- larly in the summer collections. These individuals were apparently those members of the yearling group which passed through the fall, winter, and spring in the immature condition, and which attained adulthood only at the time of their second yearling molt. Therefore they did not contribute to the production of the season’s brood of young. They molted twice at the time of the first and second adult molts of the mature males in the spring and apparently molted again during the month of June. By the early part of August these individuals have all attained the ordinary maximum size of 25 to 35 mm. cephalothorax length. There- fore they evidently undergo a fourth molt during late July or early August, involving a growth of 2 to 4 mm. cephalothorax length. Evidence for the occurrence of this fourth molt was obtained in two cases during the present study. The writer collected two “soft,” freshly molted females, measuring 25.3 and 28 mm. cephalothorax length, on July 27, 1932. And in a collection taken for the writer on August 5, 1933, another “soft” female was taken, which measured 28.5 mm. cephalothorax length. All three of the individuals in question belonged to the yearling adult group. This group, therefore, enters the fall at the ordinary maximum size, and lives at this size through the winter and spring. The individuals belonging to the group copulate in the fall, along with the mature mem- bers of the season’s generation of young. The females of the group bear BIOLOGY OF CRAYFISH—VAN DEVENTER 49 eggs in the spring and produce young. Creaser (1933) expressed the opinion that the two-year-old males in the population of C. propinquus which he studied did not change into second form. In the Illinois popula- tion, however, the two-year-old males undergo the two spring molts along with the adult yearlings, changing to second form with the first spring molt and reverting to first form with the second. The females likewise undergo the single spring molt along with the adult yearling females. Most individuals of both sexes among the two-year-old group apparently undergo an increase of 2 mm. or more in cephalothorax length in connection with the second adult male molt and the adult female molt (Graphs 26 and 46). This gives to all of the individuals of this group a cephalothorax length of more than 30 mm., and to some of them more than 35 mm. Thus a portion of the group come to exceed the ordinary maximum size characteristic of the race. Following these molts most of the individuals of this group die, along with the yearling individuals which have attained the ordinary maximum size and produced young. A very few of them, however, apparently sur- vive and furnish the occasional “oversize” individuals, measuring 35 to 40 mm. cephalothorax length, which occur scatteringly in the collections throughout the year. At this size they apparently may survive through a third summer, fall, and winter, and escape natural death until the end of their third spring. During the two years of the present study only 34 individuals of this extreme size were taken, 21 of which were females and 13 were males. This unbalanced sex ratio apparently indicates that, among the two-year-old group at least, the factors which bring about natural death are more potent among males than among females. This establishing of a differential death rate, leading to an elimination of males at an age earlier than that established for females of the same species, 1s in direct accord with the discovery of Van Cleave and Lederer (1932) who found that males of the snail Viviparus contectoides normally die shortly after reaching one year of age, while females normally attain the age of two years, and a few pass beyond the three-year limit. AGE ATTAINED BY DIFFERENT GROUPS AND NUMBER OF Broops PRODUCED On the basis of the evidence drawn from the present study we may conclude that the majority of the C. propinquus in the Illinois population produce a single brood of young and die as yearlings. This holds true for those individuals which grow rapidly and reach sexual maturity by the end of their first growing season. © Those which fail to reach sexual maturity by the first fall after they are hatched, pass through the winter in an immature state and reach sexual maturity at the beginning of their second summer. They survive 50 ILLINOIS BIOLOGICAL MONOGRAPHS through the second year and, after producing a single brood of young in their second spring, most of them die as two-year-olds. A very few individuals, however, among which the females greatly outnumber the males, survive through a third year and die at the end of their third spring. It was impossible to ascertain whether or not these individuals produced more than one brood of young during their lives. The members of this size class copulated in the fall, and the females were taken with eggs in the spring, so that they were certainly fertile. There is a possibility, however, that this class may consist of individuals which were immature during their first year, and failed to produce a brood dur- ing their second year, thus not bearing their single brood of young until the spring of their third year. If this is true, then the individuals of this species never produce more than a single brood of young, and die soon after it is liberated. If the 35 to 40 mm. individuals form an exception to this rule it still remains true for the great majority of the race. The common impression among most writers has been that all cray- fish live normally to an age of three years or more. Andrews (1907) kept some C. affinis which had been hatched in the laboratory until they were more than three years old, and he believed that some very large individuals of this species found in the field might be six or seven years old. Ortmann (1906), after extensive field study of C. obscurus, con- cluded that the normal life span of the individual in this species was about three years. Turner (1926) on the basis of these studies gave four years as the approximate life span of crayfish in general. The results obtained by Creaser (1934), however, on C. propinquus form a conspicuous exception to the conclusions of these earlier workers and agree very well with the results of the present study when due allow- ance has been made for the shorter growing season occurring in Michi- gan, where Creaser worked, as compared to Illinois where the present study was conducted. Creaser found that most of the season’s young in the Michigan popula- tion failed to become mature by the first fall after they were hatched, and that most of the yearling individuals survived into the second year. The majority of the yearling group in the Michigan population, being immature, corresponds to the small yearling group in the Illinois popula- tion which failed to become mature by the first fall, and which apparently survived into a second year. Therefore the survival of the majority of the yearling group in the Michigan population into the second year is a result which would be expected on the basis of the findings of the present study. Large numbers of the yearling individuals in Creaser’s population died during the fall or early winter of their second year. A similar though less marked mortality was noticed among members of the cor- BIOLOGY OF CRAYFISH—VAN DEVENTER Sil responding age group in the present study accompanying the onset of winter, and the greater mortality in the Michigan population may have been due to the more severe winter in the more northern latitude. Furthermore, although Creaser does not mention the fact in his paper, the graphs representing his collections show indications of the presence of a few very large individuals corresponding to the occasional individuals of 35 to 40 mm. cephkalothorax length which were found in the present study. This class is represented in Creaser’s population by individuals measuring from 30 to 35 mm. cephalothorax length. This smaller size corresponds to the generally smaller size of the Michigan race of C. propinquus as compared to the Illinois race of the same species. Thus the C. propinquus which Creaser studied, like those studied by the writer, probably die a natural death as yearlings, if they have reached sexual maturity by the first fall after they are hatched; and do not die a natural death until two years old if they have failed to reach sexual maturity by the first fall. Likewise occasional individuals among them probably live to be three years old. Apparently, therefore, Creaser’s crayfish, like those studied by the writer, with few exceptions, if any, produce only a single brood of young during their lives. Thus the question of the length of life of the individuals in this species seems to be quite definitely linked to the two factors of rapidity of growth and reproduction. Ortmann (1906) says: “Whether slow growth, including late development, influences total length of life can- not be ascertained.” However, in view of the indications of the present study, supported by the evidence of the similar study made by Creaser in a different locality, having different climatic conditions, the conclusion seems inescapable that the two principal factors which determine length of life in this species are: (1) the rapidity of growth, influencing as it does the time of attainment of sexual maturity; and (2) the engaging in the reproductive activities themselves. SUMMARY OF THE LIFE CYCLE We may summarize the life cycle of C. propinquus as it occurs in east central Illinois as follows: The young are hatched in May or June, and remain attached to the mother for about one to two weeks, depending on the season. Following the second molt they become free-swimming and possess in general the form of the adult. At this time they measure about 5 mm. cephalothorax length. They undergo a total of six to ten molts between the time of hatching and the end of the first growing season in late September or early October. By the end of the season they reach a size of 12 to 27 mm. cephalothorax length. The majority of them attain sexual maturity at this time, since the attainment of sexual maturity coincides 52 ILLINOIS BIOLOGICAL MONOGRAPHS in general with the attainment of a size of approximately 20 mm. cephalo- thorax length. Copulation occurs during the late fall and early spring and involves individuals of less than one year old if they are sufficiently large to be sexually mature. During the winter no molting takes place and the sizes of all individuals remain unchanged. The eggs are laid in late March or early April, and are carried for a period of four to six weeks depending on the temperature. The females remain in seclusion during the period when the eggs are being laid, and also while the young are attached. The adult males molt twice during the spring, changing from first form to second form with the first adult molt, and from second form back to first form with the second adult molt. The immature yearling group, both males and females, molt twice at the same times as the adult males, then a third time during May or June, and a fourth time in July or August. They attain sexual maturity as yearlings at the time of their second molt in the spring. The adult females molt only once. This molt occurs just after the young are shed in May or June. No growth takes place among the adult yearling males or the immature group as a result of their first molt of spring, but a very marked growth in both groups accompanies the second spring molt. A similar growth accompanies the single adult female molt. As a result of this growth the adult yearling males and females attain the ordinary maximum size of 25 to 35 mm. cephalothorax length. Im- mediately following the attainment of this size they die a natural death. This natural death involves all of the yearlings of both sexes which were adult during the winter and spring. Therefore it involves the majority of the yearling group. The members of the yearling population which only attained maturity as yearlings survive during a second year. They attain the ordinary maximum size of 25 to 35 mm. cephalothorax length as a result of their third and fourth yearling molts. They produce a brood of young as two- year-olds, and most of them die at the beginning of their third year, along with the adult yearlings ; but a very few, among which the females greatly outnumber the males, live over a third year. These individuals attain a size of 35 to 40 mm. cephalothorax length, which apparently constitutes an absolute maximum of size for the race. The great majority of this species, therefore, die as yearlings. A small section of the population survives during a second year, and occa- sional individuals live to be three years old. All the available evidence | indicates that most or all individuals of both sexes produce only a single brood of young. BIOLOGY OF CRAYFISH—VAN DEVENTER 53 CONCLUSIONS ENVIRON MENTAL RELATIONS 1. The young C. propinquus live in the surface water of the stream during most of their first summer. 2. The adults are bottom-dwellers at all seasons. 3. Members of all size groups seek the deeper portions of the stream during the winter. 4, Individuals of this species are positive to sunlight at all seasons. LIFE CYCLE 1. The young are hatched in May or June and remain attached to the mother about one to two weeks, during which time they probably undergo two molts. 2. At the time they become free-swimming they measure about 5 mm. cephalothorax length. 3. They undergo a total of 6 to 10 molts during the first growing season, and attain a cephalothorax length of 12 to 27 mm. 4. Sexual maturity in both sexes is attained coincident with a cephalo- thorax length of about 20 mm. 5. The majority of the season’s young normally attain sexual maturity by the first fall after they are hatched at the latitude of Urbana, Illinois. 6. During the winter no growth or molting takes place and the sizes of all individuals remain unchanged. 7. Copulation occurs during the late fall and early spring at the Jati- tude of Urbana, Illinois. 8. The eggs are laid in late March or early April, and are carried for a period of 4 to 6 weeks, depending on the temperature. 9. The adult males molt twice during the spring or early summer, changing to second form with the first adult molt, and reverting to first form with the second adult molt. 10. The immature yearling group of both sexes apparently molt four times during their second year. They attain sexual maturity with the second yearling molt. 11. The adult females undergo a single molt immediately following the shedding of the young in the spring. 12. Apparently no growth takes place in connection with the first yearling molt, among either mature males or immature individuals; but marked growth occurs in connection with the second yearling molt in both groups. | 13. A similar growth takes place as a result of the single molt among the adult yearling females. 54 ILLINOIS BIOLOGICAL MONOGRAPHS 14. The portion of the young of the previous year which reached sexual maturity by the end of their first growing season, produce a brood of young the following spring, attain maximum size as a result of the second adult molt of the males and the single adult molt of the females, and die as yearlings. 15. The group which failed to attain maturity by the end of their first growing season live over a second year; attain maximum size during their second summer, produce a brood of young in their second spring, and for the most part die as two-year-olds. 16. A very few individuals, among which females predominate, sur- vive over a third year, and produce a brood of young in their third spring. 17. With the possible exception of the few which live to be three years old, the individuals of this species apparently produce only a single brood of young during their lives. BIOLOGY OF CRAYFISH—VAN DEVENTER 55 BIBLIOGRAPHY ANpREWS, E. A. 1895. Conjugation in an American Crayfish. Amer. Nat., 29:867-873. 1904. Breeding Habits of Crayfish. Amer. Nat., 38:165-206. 1907. The Young of the Crayfishes Astacus and Cambarus. Smiths. Contrib. to Knowl., 35:1-79. Beem}! C. 1906. Reactions of the Crayfish. Harvard Psych. Studies, 2:615-644. Busu, S. F. 1930. Asymmetry and Relative Growth of Parts in the Two Sexes of the Hermit Crab, Eupagurus prideauxi. Arch. f. Entw. der Organismen, 123: 39-79. BusueS: F., and Huxtey, J. S. 1930. Distribution of Growth in the Hermit Crab, Eupagurus prideauxt, Nature, 126:240-241. CHIDESTER, F. E. 1908. Notes on the Daily Life and Food of Cambarus bartonius bartont. Amer. Nat., 42:710-716. 1912. The Biology of the Crayfish. Amer. Nat., 46:279-293. CREASER, E. P. 1933. Seasonal Changes in the Male Population of Faxronius propinquus (Girard). Univ. of Mich. Occ. Paper, Mus. Zool., No. 253. 1934. Age, Growth, and Sex Ratios in the Crayfish, Faxronius propinquus. Papers Mich. Acad. Sci. Arts and Letters, 19:581-585. ENGLE, E. T. 1926. Crayfishes of the Genus Cambarus in Nebraska and Eastern Colorado. Bull. U. S. Bur. Fish., 42:87-104. Faxon, W. 1884. On the So-called Dimorphism of the Genus Cambarus. Am. Jour. Sci., 27 : 42-44. 1885. Revision of the Astacidae, Part I: The Genera Cambarus and Astacus. Mem. Mus. Comp. Zool., 10:1-186. Foster, T. D. 1932. Observations on the Life History of a Finger Nail Shell of the Genus Sphaerium. Jour. Morph., 53:473-497. GIRARD, C. 1852. A Revision of the North American Astaci, with Observations on Their Habits and Geographical Distribution. Proc. Acad. Nat. Sci. Phila., 6:87-91. GRAENICHER, S. 1913. Some Notes on the Habits and Distribution of Wisconsin Crayfishes. Bul. Wis. Nat. Hist. Soc., 10:118-123. Hacen, H. A. 1870. Monograph of the North American Astacidae. Illus. Cat. Mem. Mus. Comp. Zool., 3:1-109. Harris, J. A. 1901. The Dimorphism of Cambarus. Kans, Univ. Quarterly, 10:49-59. 1903. An Ecological Catalog of the Crayfishes Belonging to the Genus Cambarus. Kans. Univ. Sci. Bull., 2:51-187. 56 ILLINOIS BIOLOGICAL MONOGRAPHS Hay, W. P. 1896. The Crayfishes of the State of Indiana. Ann. Rpt. Ind. Geol. Survey, 20:475-507. 1919. The Crawfishes of Lake Maxinkuckee. Proc. Ind. Acad. Sci., 1918: 232-235. Hux ey, J. S. 1924. Constant Differential Growth Rates and Their Significance. Nature, 114:895-896. Markus, H. C. 1933. The Extent to Which Temperature Changes Influence Food Consump- tion in the Large Mouthed Black Bass (Huro floridana). Trans. Amer. Fish. Soc., 62:202-210. NeEwcompeg, C. L. 1929. The Crayfishes of West Virginia. Ohio Journ. Sci., 29:267-288. OrTMANN, A. E. 1905. Mutual Affinities of the Species of the Genus Cambarus, and Their Dispersal over the United States. Proc. Amer. Phil. Soc., 49:91-136. 1906. The Crawfishes of the State of Pennsylvania. Mem. Carnegie Mus., 2: 343-524. PeapsE, A. S. 1909. Observations on Copulation Among Crawfishes. Amer. Nat., 43:746-753. 1910. Crawfishes of Michigan. Publ. Mich. Geol. and Biol. Survey, 1 (Biol. Ser. 1): 9-22. Suaw, M. E. 1928. A Contribution to the Study of Relative Growth of Parts in Jonchus dorsettensis. Brit. Jour. Exp. Biol., 6:145-160. SmitH, Mrs. Hucu R. 1910-11. Unpublished Records of Cambarus propinquus at Urbana, Illinois, and Douglas Lake, Michigan. STEELE, MAry. 1902. The Crayfish of Missouri. Univ. Cincinnati Bull., 2 (Ser. 2): 1-54. TurRNER, C. L. 1926. Crayfishes of Ohio. Ohio Biol. Survey Bull., 13:145-195. VAN CLEAVE, H. J. 1932. Statistical Analysis of Quantitative Collections as a Means of Interpret- ing Life Histories. Trans. Il. State Acad. Sci., 24:228-234. VAN CLEAVE, H. J., and Leperer, L. G. 1932. A Study of the Life Cycle of the Snail Viviparus contectoides. Jour. Morph., 53:499-522. VAN CLEAVE, H. J., and Markus, H. C. 1929. Studies on the Life History of the Blunt-Nosed Minnow. Amer. Nat., 63 : 530-539. BIOLOGY OF CRAYFISH—VAN DEVENTER 57 GRAPHS The collections taken during the two years in which this study was in progress are represented by the following series of forty-four graphs. Of these, forty-three (Graphs 4 to 46, inclusive) represent individual collections, arranged chronologically. On these graphs the abscissa repre- sents cephalothorax length in millimeters, and the ordinate represents number of individuals ; the total population is indicated by a solid line, the females by a broken line, and the males by a dotted line. The first graph of the series (Graph 3) is the composite graph, show- ing the progress of the modes, representing the size groups. The abscissa represents time in weeks, while the ordinate represents the position of the modes in millimeters. The letters A, B, C, D, E, and P on the lines of this graph refer to the corresponding letters on the modes of the graphs of individual collections. Each line on the composite graph thus repre- sents the development and history of a particular size group in the popula- tion. Each number on the abscissa represents a single collection. In the following list the dates of the collections are given with the corresponding numbers shown in Graph 3: 1. June 29, 1932 23. June 7, 1933 2. July 14, 1932 24. June 25, 1933 3. July 27, 1932 25. July 10, 1933 4. August 11, 1932 26. July 22, 1933 5. September 15, 1932 27. August 5, 1933 6. September 27, 1932 28. August 31, 1933 7. October 6, 1932 29. September 20, 1933 8. October 20, 1932 30. October 7, 1933 9. November 3, 1932 31. October 24, 1933 10. November 17, 1932 32. November 4, 1933 11. December 1, 1932 33. November 18, 1933 12. December 17, 1932 34. December 6, 1933 13. January 5, 1933 35. December 20, 1933 14. January 26, 1933 36. January 6, 1934 15. February 2, 1933 37. January 20, 1934 16. February 18, 1933 38. February 17, 1934 17. March 4, 1933 39. March 4, 1934 18. March 18, 1933 40. April 7, 1934 19. April 8, 1933 41. April 28, 1934 20. April 22, 1933 42. May 5, 1934 21. May 6, 1933 43. May 13, 1934 22. May 21, 1933 €ptbhly OF 6e 8 LE 9E GSE ve EC TE IE OF 6C Ae uIM ‘ZG a8ed vas uoTyeuR[dxa 104 8t EGeSCGe ve eG. Ce 1eOt- 6! dnosn 41°PY ADULLNG S}1"PU 3° “4eeq (\eMpiarpyy aug) re ydeig aqisodwioy_¢ ydeig 8! LI Qf Sil El. tl AB, '1 Ol 6 8 L 9 f& Vig Ome dnorxy LVR AS UA I5 20 aa Graph 4- Collection of June 29, 1932 LA Graph 8- Collection of September |5,1932 rey = Ke) Ls 30 Graph 10- Collection of October 6,1932 D 60 IS a2O at 30 Graph 14-Collection of December |, 1932 61 Ey 20 he) 30 Graph 20- Collection of March 4.1933 a) = ea ; / is OVA AN P / x SSC sent NNN NEN i em teow Graph 23- Collection of April 22, 1933 i [ ; i " Ls 3 : UL aS AN Gos ~ [ Graph 24-Collection of May 6, 1933 clo [ D Ls C t ; A B P Graph 25- Collection of May 21, 1933 LO LS oj ~~ i.e) IS 20 as 30 35 40 Graph 26- Collection of June 7, 1933 Graph 27- Collection of June 25,1933 63 IS 20 as 30 oo Graph 29 - Collection of July 22, 1933 Graph 32- Collection of September 20, 1933 D 64 co} [@) 15 20 he) 30 i) 40 ad Graph 33-Collection of October 7, 1933 20 B C Sy lO C B D E 5 ; AyeN y. ya eens P \ AYA 1S 20 QS 30 Graph 37- Collection of December 61933 66 as Graph 43 - Collection of April 7, 1934 C Graph 46-Collection of May !3, 1934 D 67 UR LIBRARY OF THE Pres atl Hf fal ol rer i H LEING! 3 Hele 0650971 195 _— TN