340 Birge — The Crustacea of the Plankton. 



Males are found during and after the spring and fall reproduct- 

 ive periods, although in very small numbers, never exceeding 4 per 

 cent, of the number of females and rarely being as numerous as this. 

 Ordinarily it is only possible to find one or two males by careful 

 search through the entire collection. These males are somewhat 

 more abundant after the fall reproductive period than earlier, 

 and may be found as late as the middle of December or even the 

 first of January. It seems, therefore, that originally this spe- 

 cies had two main reproductive periods, in the fall and spring. 

 Each of these was probably closed by the production of males 

 and the development of ephippia. The sexual reproduction has, 

 however, almost entirely disappeared, and the species has prac- 

 tically passed into a acyclic condition. 



Apstein ('96, p. 167,) finds that Daphnia hyalina is present 

 from September to July, with a maximum in winter. This his- 

 tory is so wholly different from that of the species as found in 

 lake Mendota that no profitable comparison can be made. 



Daphnia pulex var. pulicaria Forbes. 



Figure 17. — Table G, Appendix. 



The following table gives the average number of Daphnia 

 pulicaria taken during the period of my investigation. From 

 this and from the diagram it appears that the species was pres- 

 ent in very small numbers during July and August, 1894; that 

 it then entirely disappeared until the early part of July, 1895; 

 it increased in numbers during the summer and autumn, in- 

 creased greatly during December, and was present in consid- 

 erable numbers during the winter. About the middle of April, 

 1896, a period of rapid reproduction began, the species rising 

 to a maximum in the latter part of May. At this time, and in 

 the early part of June the males appeared and not infrequently 

 numbered from one-third to one-fourth of the total catch. The 

 females developed ephippia and the sexual eggs were produced 

 early in June. The species rapidly declined after this date, al- 

 though present in somewhat larger numbers early in September. 

 Scattering individuals only were found from the first of Octo- 

 ber through the winter of 1896-97. The species entered upon 



