. Discovery. Science. 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), a
. Discovery. Science. 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 "; 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
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