THE LIFE OF DEVILS LAKE 89 



in fact, the only species, either animal or plant, which is not wholly 

 absent from, or very rare in the plancton catches in winter time. 

 The other important genera, Cyclops and Moina, may, at times 

 outnumber it, but are on the whole of secondary importance. 



Figure 24. Seasonal distribution of Diaptomus in East Lake. 



Its distribution curve shows rather poorly marked maxima and 

 minima. In general, there is a minimum in April-May, succeeded 

 by a maximum in May- June, a minimum in July-August, a maximum 

 in August-September and a minimum in September, with one or 

 more less well defined waves in October-November. Its abundance 

 in fall and winter is in general greater than its minimum number 

 in summer, therefore temperature is not the controlling factor in 

 its distribution. Its w'aves alternate in a general way with those of 

 the nauplii (pi. 20 and 23), as might be expected, tho in midsum- 

 mer, when both Diaptomus and Cyclops show maxima, this relation 

 is not very clear. 



Diaptomus is interesting as the 'only genus, either plant or 

 animal, which is common in winter. Whil? its reproduction de- 

 creases materially at this season, immature individuals, nauplii 

 (probably of this species) pregnant females and eggs occur occasion- 

 ally beneath the ice. 



From the occurrence of a winter maximum of D. graciloides in 

 lake Plon and a summer maximum of the same species in Dobers- 

 dorfer lake Apstein (1896) concludes that temperature is ineffective 

 in influencing the development of Diaptomus. and Marsh (1896-97) 

 reaches a similar conclusion. Birge, (1897) on the contrary, con- 

 siders temperature the chief factor determining the seasonal dis- 

 tribution of Diaptomus (oregonensis )in Lake Mendota. Its maxima 

 here occur in the warmer months (May-September) with a great 

 reduction in winter. He states that "it is the first of the perennial 

 Crustacea to slacken its reproductive activity in the autumn and 

 this occurs when food is at its maximum. I can attribute this check 

 only to the fall in temjierature. Indeed, my observations show thai 

 the reproductive activity of D. oregonensis is more promptly check- 

 ed by the decline of temperature than is that of any other of the 

 perennial species." (p. 326). 



]\Iy own observations on Devils Lake indicate that the develop- 

 ment of Diaptomus is in inverse ratio to the temperature because of 



