120 



CARNEGIE INSTITUTION OF WASHINGTON. 



Table 4 gives an account of the history of all the strains at present 

 in the laboratory, together with the records of their production of males. 



Table 4. — Male production in pure strains of Cladocera. 



Species. 



Occurrences of males. 



Previous to 

 May 1917. 



Number of strains 

 in which — 



Males 

 occurred. 



Males 

 did not 

 occur. 



Daphnia pulex . 



Daphnia longispina . . 

 Simocephalus vetulus . 



Simocephalus serru- 



latus. 

 Moina brachiata . . . . , 



None, except in some 

 discarded material 

 of this stock. 



Do 



Do 



Some at two differ- 

 ent times. 



None 



In sex-intergrade 

 strain only. 



None 



None 



None 



None 



None 



1 

 2 

 2 



3 

 4 



3 

 5 

 3 

 10 

 5 



42 



10 



In marked contrast with the extreme rarity of the occurrence of 

 males in the cultures, in May 1917 males suddenly began to appear in 

 the laboratory in numbers and in the vast majority of the strains. 

 There were males in 7 of the 12 strains of Daphnia pulex; in 12 of the 

 16 strains of Simocephalus vetulus; in all of the 5 lines of Moina; in all 

 of the 13 hues of Simocephalus serrulatus, and in all except one of the 

 6 strains of Daphnia longispina. Thus in 42 of the total number of 52 

 strains in the laboratory, males occurred within a short time and in 

 considerable numbers. Yet this material consisted of 5 different 

 species of 3 genera of Cladocera, was obtained from various locaUties, 

 including two Florida locahties separated by nearly 100 miles, and had 

 been reproducing parthenogenetically in the laboratory for periods 

 varying from 1| months to 5| years and had descended solely by par- 

 thenogenetic reproduction for from 4 to 233 generations. Of Daphnia 

 pulex, 7 Unes had been in the laboratory for 66 months and had 

 descended 225 generations during that time without producing males; 

 2 other Hues of the same species had been in the laboratory for 26 

 months and 96 generations, and 3 others for 83 generations, exclusively 

 female; yet 7 out of 12 of these strains produced males at the same time. 

 Likewise with the other species, 3 lines of Daphnia longispina reared 



