354 Birge, Distribution of the pelagic Crustacea of lake Mendota. 



2) The dredge had an opening- of 400 square centimeters and a 

 net made of fine cloth known as ,,India linen". The average number 

 of Diaptomus caught in the series of six hauls was, in round numbers, 

 4200; of Cyclops, 2000; D. hyalina, 220; D. pulicaria, 100. The last 

 species almost disappeared in the last days, Aug. 14. In the other 

 series it averaged about 125. 



3) The following table shows the distribution expressed in per- 

 centages of the total number caught. 



Diaptomus Cyclops D. hyalina D. pulicaria 



Depth Average Day Night Average Average Average 

 0-3 m 53 / 57 ft / 47 / 43 / 42 / 1.9 / 



5.3 







40.8 

 15.5 

 1518 0.09 0.08 0.1 0.1 0.45 0.0 



In Diaptomus the averages for the day and night are further 

 apart than in any other species and for that reason are given. The 

 other species are practically alike in distribution by day and night. 

 The percentages in the table are usually expressed in the nearest 

 whole number rather than give small fractious of a percent. 



4) It is evident from this table: a) The distribution of Cyclops, 

 Diaptomus, and D. Injalina is alike and strikingly different from 

 that of D. pulicaria. b) About 90/ of the first named forms are 

 in the upper 9 meters of the lake and below 12 meters there were 

 practically no Crustacea. This was also found true in the deeper 

 parts of the lake down to 22 meters. This distribution changes in 

 the autumn as the temperature of the lake falls and when the tem- 

 perature has become uniform throughout the entire depth the Crustacea 

 are distributed with approximate uniformity, c) There is no evidence 

 of a diurnal movement of the Crustacea in a vertical direction. At 

 all hours of day and night the upper 3-meter level contained 

 more Crustacea than any other. The maximum percentage found in 

 it was 78 / at 9 a. m. The lowest was 31 / , found twice at 9 a. m. 

 and 9 p. m. There was no case of an observation which showed 

 indication of a downward movement by day and an upward one by 

 night, d) There is no indication that the Crustacea seek the cooler 

 water of the deeper parts of the lake. On the contrary the upper 

 level was always crowded with Crustacea in the middle of the day 

 with a temperature of the air which was often above 30 C. The 

 Crustacea were practically confined to that part of the lake which 

 was of a high temperature - the part above the socalled ,,Sprung- 

 kluft" of temperature. 



