SURVEY OF INVERTEBRATES 119 



even during periods of activity, and that, at summer tem- 

 peratures, it was adequate for periods of rest, in most 

 species. As to the smaller species of Corixa and Hyphy- 

 drus they can, even in the summer, get all the oxygen 

 needed for their movements. Thus, the only reason why 

 aquatic insects cannot stay indefinitely under water at 

 low temperatures is that in time — and this time is not too 

 long — the air bubble disappears due to a nitrogen gradient 

 established between bubble and water, which leads to a 

 gradual diffusion of nitrogen into the water. 



The unreliability of Plateau's {I. c.) figures becomes 

 apparent if one compares his results with those of Ege 

 {I. c), which were obtained under truly anaerobic con- 

 ditions. As we said above, Plateau reported a survival 

 of 65 hours for Dytiscus and of 3 hours for Notonecta. 

 Ege's figures for the same animals are 36 hours and 5 

 minutes respectively. 



That some species of aquatic beetles are able to remain 

 active in winter even if they are prevented from renewing 

 their stores of air at the surface because of the ice is 

 explained by the fact that they catch bubbles rising from 

 plants or from the mud. ^'Even if these latter contain 

 very little oxygen they may be essential for keeping up 

 the quantity of gas on which the uptake of oxygen from 

 the water depends" (Krogh, 1941). 



The resistance of diving Hemiptera in experimental 

 anaerobic conditions seems to be about the same as that 

 of beetles. Among the most reliable figures we may 

 mention those of Wrede and Kramer (1926). These au- 

 thors found that Naucoris cimicoidcs survives 24 hours, 

 but not 45 hours, in strictly anoxic conditions. 



3. TERRESTRIAL INSECTS 



A. Anaerohiosis under natural conditions. There is 

 little reason to assume that terrestrial insects will often 

 find themselves under strictly anaerobic conditions in na- 

 ture but it is conceivable that they may, from time to 



