116 SURVEY OF INVERTEBRATES 



Very little is known about the anaerobic functions of 

 other aquatic insect larvae. Those of Palpomyia seem 

 to withstand fairly well both natural and experimental 

 oxygen deficiency (Lindeman, 1942). Whether the same 

 applies to TricJioptera larvae has not yet been investi- 

 gated. Milne (1938) is of the opinion that the cases 

 built by the caddis flies represent an adaptation to life 

 in an environment poor in oxygen. Her contention is that 

 the current of water forced through the case sets up a cir- 

 culation in the surrounding water over a considerable 

 distance from the case and thus always brings in fresh 

 water with at least some oxygen. If a larva without a 

 case simply undulates its abdomen in the water, the 

 water movements are confined more or less to the imme- 

 diate vicinity of the animal. The only gas exchange 

 therefore would be a diffusion and this would be insuffi- 

 cient to replace all the oxygen consumed. Whether this 

 view of the caddis cases as an adaptation to life at low 

 oxygen tensions will stand the test of experimental analy- 

 sis remains to be seen. 



Other developmental stages of insects that withstand 

 quite considerable oxygen deficiencies in nature are found 

 among the epJiemcrids. Sing-Pruthi (1927) states that 

 the larva of Cloeon dipterum is not injured until the oxy- 

 gen concentration drops to 0.20 to 0.30 cc. per liter and 

 those of Hexagenia rccurvata survive in water contain- 

 ing 0.50 cc. of oxygen per liter. Fox and Simmonds 

 (1933) and Fox, Simmonds and Washbourn (1935) have 

 shown that the oxygen consumption of nymphs of va- 

 rious species of may-flies taken from swift streams that 

 are rich in oxygen is greater than that of species occur- 

 ring in ponds where the oxygen content is often abnor- 

 mally low. Whether this merely indicates that the en- 

 ergy need of various species is different or whether it 

 means that the latter have some anaerobic processes 

 going on even at relatively high tensions is not yet clear. 

 It is, in any event, interesting that Cloeon dipterum from 



