190 



DISCOVERY 



air-passages. Let us just note in passing one con- 

 sequence of these facts, that the possible rate of 

 diffusion through long and narrow air -passages sets a 

 definite limit to the possible size of insects. 



Water-beetles 



And so we pass on to the complication which arises 

 in the case of air-breathing insects in water — Dytiscus, 

 the "diving beetle," and Notonecta, the "water- 

 boatman," for instance. It is well known that these 

 forms carry bubbles of air down with them from the 

 surface when they dive (some others collect bubbles 

 in the water), and it has long been disputed whether 

 the bubbles are used in respiration, or are merely a 

 sort of "water-wings." Let us take Ege's account 

 of Xotonecta. Notonecta has, on the thorax, three 

 pairs of breathing-holes sunk in cavities which are in 

 communication with the depressions in which the ab- 

 dominal breathing-holes lie, all being roofed in by 

 delicate hairs which form a covered way. There, a 

 layer of air always clings, and this layer is, in turn, in 

 communication with a layer of air on the wings. 

 This air-supply has, as in Dytiscus and other forms, 

 an important hydrostatic function. Without it, these 

 insects, instead of being passively borne to the surface 

 to breathe, become heavier than water and fall to 

 the bottom. Normally, when it is active, Notonecta 

 comes to the surface to breathe at intervals of about 

 6 minutes. Prevented from so doing, it can live for 

 6-7 hours in water saturated with atmospheric air. 

 This might be due to any of four possibilities : (i) that 

 the air carried down in the air-passages is sufficient 

 for that time and that the function of the air-bubbles is 

 hydrostatic only ; (2) that the air in air-passages and 

 air-bubbles is sufficient for that time ; (3) that the air 

 carried down is not sufficient, but that the animal re- 

 spires without air (using its capital) ; or (4) that the air 

 taken down is insufficient, but that the animal acquires 

 a further supply from the water by diffusion into the 

 air-bubbles. First, then, a Notonecta deprived of its 

 air-jacket lived only 15 minutes in water saturated 

 with atmospheric air, so that the first supposition is 

 impossible, and the bubbles must be of use in respira- 

 tion, since it lives 6 hours with them. Next, a Noto- 

 necta allowed to fill its air-passages in atmospheric air 

 and then enclosed in water saturated with nitrogen, 

 lived only 5 minutes, so that the total original supply 

 was exhausted in 5 minutes ; and again, allowed to 

 breathe pure oxygen and then enclosed in water satur- 

 ated with oxygen, it lived only for 35 minutes. This 

 is not so strange as it seems. The Notonecta carries 

 down a bubble of pure oxygen, and its air-passages 

 are full of oxygen. As it uses up this supply, carbon 

 dioxide will be given off from the tissues into the air- 

 bubble, and, from it, will pass immediately into the 



water on account of its great solubility in water. 

 There will be no accumulation of carbon dioxide in 

 the air-store. The bubble will continue to consist of 

 100 per cent, oxygen (while the insect constantly draws 

 from it), and no pressure difference can arise between 

 it and the water. Hence no oxygen can pass from 

 the water to the air-store, and the animal will simply 

 use up its bubble. Under these circumstances it 

 lived for 35 minutes, but there it had 5 times its 

 normal supply of oxygen, 100 per cent, instead of 20 

 per cent., so that its normal supply would have lasted 

 7 minutes. 



Now, Notonecta deprived of air does not live on its 

 capital stores, for then it would have lived longer in 



COMMON WATER-BEETLE (Dyliscus margiimlis). 



nitrogen. What does happen is this : as the insect uses 

 the oxygen available in its air-passages and bubble, 

 the tension of oxygen in the bubble will fall so low 

 that oxygen will pass from the water into the bubble. 

 Ege found that after being 2-4 minutes in the water, 

 the bubbles contain as little as from 5 per cent, to 2 

 per cent, of oxygen, so that there is a big pressure- 

 head available to drive more oxygen into the bubble. 

 The rate at which oxygen will be supplied in this way 

 will depend on the size of the bubble as well as the 

 pressure-head, and Ege has calculated that for a 

 Notonecta with a full air-supply, which will have a 

 surface of about 75 square millimetres, a pressure-head 

 of 5 per cent, oxygen would be sufficient for the rest- 

 ing needs of the animal at summer temperature. At a 

 low temperature, with reduced standard metabolism 

 and little activity, the air supplied by diffusion ought 

 therefore to be amply sufficient to maintain life, even 

 when, as in frozen pools, access to the surface is alto- 

 gether prevented. 



Where the Mechanism Fails 



In view of these facts, how does it happen that any 

 limit is set to the life of a Notonecta in water saturated 

 with atmospheric air ? This depends on the fact 



