302 Aquatic Societies 



and well fringed swimming legs, that may fairly be 

 counted limnetic. There is only one limnetic insect. 

 It is the larva of Corethra — a very transparent, free 

 swimming larva, having within its body two pairs of 

 air sacs that are doubtless regulators of its specific 

 gravity. 



Fig. 183. The larva of the midge, Corethra. (After Weismann.) 



Seasonal Range. There is no period of absence of 

 organisms from the open water, yet the amount of life 

 produced there varies, as it does on land, with season 

 and temperature. In winter there are more organisms 

 in a resting condition, and among those that continue 

 active, there is little reproduction and much retardation 

 of development. Life runs more slowly in the winter. 

 Diatoms are the most abiuidant of the algse at that 

 season. 



There is least plancton in the waters toward the end 

 of winter — February and early March in our latitude. 

 The returning sun quickens the over- wintering forms, 

 according to their habits, into renewed activity, and 

 up to the optimum degree of warmth, hastens reproduc- 

 tion and development. With the overturn of the 

 waters in early spring comes a great rise in the produc- 

 tion of diatoms, these reaching their maximum often- 

 times in April. This is followed by a brisk develop- 

 ment of diatom-eating rotifers and Crustacea. Usually 

 the entomostraca attain their maximum for the year in 

 May. This rise is accompanied by a marked decline 

 in numbers of diatoms and other algae, due, doubtless, 

 to consumption overtaking production. The warmth 



