Birge, Crustacean des Mendota-Sees. 373 



it. The larvae of Corethra are the only animals which pass freely up 

 and down through the transition stratum of lake Mendota. 



6. The effect of light is not traceable in lake Mendota deeper 

 than 12 metres; within this limit it has a powerful influence on the 

 distribution of the Crustacea. During the day the upper metre or so 

 is occupied by swarms of young^crustacea, especially Diaptomus and 

 Daphnia hyalina and D. retrocurva. In the case of Daphnia the adults 

 are repelled by light while the young are attracted or are indifferent. 

 In Diaptomus, the young are more strongly attracted by light than 

 are the adults. On bright days therefore the young Crustacea can 

 feed on Aphanizomenon and other small plants in the upper water 

 without competition from the older animals. At night old^and young 

 become mingled, but there is no general movement toward the surface. 

 On cloudy days the adults rise toward the surface coming within 

 about '/ 2 metre of it. This rise comes immediately after the sun is 

 obscured and an immediate descent follows the reappearance of the sun. 



7. Gravity aids in causing the older and weaker adults to 

 move toward the bottom and accumulate there. This fact is espe- 

 cially noticeable in the old age of broods of Cyclops and Daphnia. 

 Diaptomus does not show it. In winter, Cyclops is found in large 

 numbers near the bottom, 50/ or more of the total catch being often 

 found in the lower three metres. It is also found in very large num- 

 bers near the bottom when the enormous spring broods are dying off 

 in early summer. Daphnia shows the same tendency to the bottom 

 in late spring on the part of those individuals which have lived over 

 winter and are near the end of life. The same thing occurs in late 

 October and early November when the summer broods are dying. 

 Throughout the year the older animals are proportionally more nu- 

 merous in the lower strata of the water. 



8. In autumn the upper strata of the water are more densely 

 populated than those below so long as reproduction is active. JSIo 

 species is distributed through the water with even approximate uni- 

 formity until its reproductive period is over and it has begun to decline 

 in numbers. In late fall and winter the Crustacea are more uniformly 

 distributed than at any other time, although even then the upper 

 three metres contain much more than their exact share. Daphnia 

 pulicaria alone shows a decided tendency to aggregate near the sur- 

 face at this time, and Cyclops, as already said, is found in swarms at 

 the bottom during winter. 



9. In spring the young of each species of Crustacea first appears 

 near the surface. They move gradually downward as numbers in- 

 crease and more room and food are needed. This downward movement 

 continues during May, until the lower water is densely populated, 

 chiefly by Cyclops. In June, as the lake warms, the bottom water 



