88 THE LIFE OF DEV1D8 LAKE 



former are so extremely delicate that they eonld scarcely be shunned 

 by the Crustacea because of their inability to feed upon them, as 

 Birge believes; altho, as he suggests, there may be factors other 

 than size which cause them to avoid it. He undoubtedly refers to 

 another species (L. birgei?). with filaments about 20 ce wide, 

 from that occurring in Devils Lake ( L. contorta), which has fila- 

 ments about 1.5 ec in width. 



As to whether or not the Crustacea do feed on Lyngbya, I 

 cannot sa.y, as I have paid little attention to their feeding habits. 



The only other obvious difference between conditions in 1911-14 

 and 1923 is the increase in concentration of the lake water, which, 

 a priori, one would expect to be inimical to fresh water animals, 

 rather than the reverse. Differences in plankton abundance in dif- 

 ferent years are of frequent occurrence elsewhere and have been 

 previously discussed in this paper.* The curves in general, show 

 two maxima, one in May-June and another larger one in August- 

 September. 



The sex ratio in the Crustacea presents some interesting fea- 

 tures. In Diaptomus, during the summer,** when reproduction is 

 most active, the females outnumber the males in the ratio of 662:118, 

 while the ratio for the remainder of the year is 905 :942. In Cyclops 

 the summer ratio is 167:112, and for the balance of the year 32:10. 

 While the Tuimbers of the latter ratio are too small to have much 

 significance, there is evidently here not the same difference for dif- 

 ferent seasons as occurs in Diaptomus. When Moina first appears 

 it is represented almost exclusively by females, males usually ap- 

 pearing about the end of August. In 1911 and 12 the numbers of 

 Moina are two small to have much significance. In 1913 males 

 first appeared on August 27, from which time until the last ap- 

 pearance of the genus on October 26 the ratio was 131 females to 66 

 males. In 1914, out of a total of 59 individuals taken from August 

 2 to September 14, no males were observed. 



The above data are based on collections from Main Lake only. 

 and, of course, on sexually mature specimens. 



Kofoid (1908) finds similarly large variations in the sex ratio 

 of various species of Entomostraca of the Illinois River. 



Diaptomus. There are several species of this genus in the 

 complex, but most of them are of only occasional occurrence, D. 

 sicilis being by far the most common. No attempt has been made 

 to differentiate the various species in the plankton counts 

 (pi. 22 and fig 24). 



Diaptomus is one of the most characteristic forms of the plane- 

 ton. It occurs in considerable numbers thruout the year, and is, 



*Soe page 44. 



**The data for the "summer" poriods are as follows: 1911, 7/7-fl/ll ; 1913, 

 8/21-9/.'if>; 1914. 6/8-9'/14 (end of collecting period). 



