STUDIES IN THE LIFE CYCLE OF SIMOCEPHALUS VETULUS

Abstract

I. Life History.1. The offspring of any female of Simocephalus vetulus in the asexual phase may consist of any one or all three of the following kinds of individuals:(a) Sexual females, which produce a series of from one to seldom more than six ephippial eggs early in life, then become parthenogenetic and so remain, being then indistinguishable from other parthenogenetic females. The final ephippium is often only partially developed, showing the sexual capacity to be gradually lost.(b) Parthenogenetic females, which display no tendency to produce ephippial eggs.(c) Males.The kinds of offspring occur in no definite order, but their character is probably determined at birth, and not by subsequent conditions. (See summaries of isolation experiments 2 and 3.) Eggs which will develop into males, into highly sexually females, and parthenogenetic females, often arise in the same female and not infrequently at the same time, either early or late in her reproductive period, whether or not she has passed through the sexual state herself.2. The sequence of the generations is very indefinite:(a) The stem mother is functionally like the females produced parthenogenetically, except that she probably never gives rise to ephippial eggs. (However see Sharfenberg, 1911, p. 24.) There is not even an approximately definite number of generations in the cycle from one stem mother to another. It may be one or many.(b) The remoteness of the generations from the stem mother bears no definite relation to the percentage of males produced, the ratio of sexual to parthenogenetic females, or to the duration of the sexual state when present.(c) The sexual state is probably determined in the ovary of the preceding generation. There are almost certainly predisposing factors in the environment but it is not certainly known what they are. Food or lack of food does not offer a sufficient explanation. Sexual females and males tend to arise at the same time, presumably in response to the same environmental complex.(dCultures are indefinitely viable parthenogenetically. The species will express itself in all of its forms under a great variety of conditions. Under certain conditions sexual forms are completely inhibited. Parthenogenesis cannot be completely inhibited in cultures or even in individual females. Thus cultures of Simocephalus vetulus can never be terminated merely because of the onset of sexuality.(e) The production of mixed broods is not to be interpreted as evidence that the female producing them is undergoing transition from male producing to female producing or vice versa.3. The production of ephippia and ephippial eggs are related but not causally, both being dependent upon common internal factors. The introduction of males into a culture does not induce the production of ephippial eggs, nor does their presence have any relation to the prolongation of the sexual state when it has once appeared.II. Breeding Habits.Sexual attraction is limited not only to sexual females in the sexual state, but is confined to a limited period of a few hours before the ephippial egg is laid. It seems to be due to some kind of substance omitted by the female and borne by her exhalant respiratory current, where it is detected through a chemical sense by the male. Fertilization seems to take place in the brood pouch after the egg is laid. The presence of the spermatozoa in the brood chamber is probably the stimulus for its extrusion. In the absence of fertilization the ephippial eggs are usually resorbed in the ovary, or, if laid they undergo degeneration in the ephippium within one or two days.III. Theoretical.The immediate significance of the ephippial egg is as a stage in the life cycle resistant to adverse conditions, and not in the stem mother hatching from it, since her offspring are in no way unique. A more remote but more fundamental significance, held in common of course, with all fertilized eggs and zygotes, is that it provides for the permanent lability of the species through amphimixis. In view of the great prolificacy of the species in regard to all of its forms, and the almost universally concomitant occurrence of males and sexual females, the development of only 1 per cent. of the ephippial eggs would be quite sufficient to secure to the species all of the benefits to be derived from their two functions. It seems probable that there is a very great, inherent variability in the capacity for development of the ephippial eggs in a state of nature, and the lack of uniformity of results obtained by the various investigators in their attempts to shorten the latent period may be explainable on that basis.