. Animal physiology. Physiology, Comparative. 276 KESPIRATION OF INSECTS AND SPIDERS. the extraordinary activity of their respiration. There are no animals which consume so much oxygen, in proportion to their size, as Insects do when they are in motion (§ 308); but when they are at rest, their respiration falls to the ^ low standard of the tribes to which they bear the greatest general resemblance. Although, as we have seen, the respiration of aquatic larvae is sometimes accomplished by means of gills, yet many ac[uatic larva? breathe air by means of tracheae; and such are consequently obliged
. Animal physiology. Physiology, Comparative. 276 KESPIRATION OF INSECTS AND SPIDERS. the extraordinary activity of their respiration. There are no animals which consume so much oxygen, in proportion to their size, as Insects do when they are in motion (§ 308); but when they are at rest, their respiration falls to the ^ low standard of the tribes to which they bear the greatest general resemblance. Although, as we have seen, the respiration of aquatic larvae is sometimes accomplished by means of gills, yet many ac[uatic larva? breathe air by means of tracheae; and such are consequently obliged, like Whales and other aquatic Mammals, to come occasionally to the surface for the purpose of gaining a fresh supply of air. The larva of the Gnat, which breathes in this manner, has one of the spiracles of its tail-segment prolonged into a tube; and it may often be seen suspended, as it were, in the water, with its head downwards, the Gnat. end of this tube (t, fig. 160) being at the surface. 322. In the greater number of perfect Insects, we find the tracheae dilated at certain parts into large air-sacs (fig. 159); these are usually largest in Insects that sustain the longest and most powerful flight; in some of which, as in the common Bee, they occupy a greater portion of the trunk than they do in the insect whose system of air-tubes has been just represented,—this insect, the JS^epa or water-scorpion, being of aquatic habits, and seldom using its wings for flight. Tliere can be little doubt that one use of these cavities is to diminish the specific gravity of the Insect, and thus to render it more buoyant in the atmosphere; but it would not seem improbable that they are intended to contain a store of air for its use while on the wing, as at that time a part of the spiracles are closed. We shall find in Birds, the Insects of the Vertebrated division, a structure bearing remarkable analogy to this (§ 326). 323. In some of the Arachnida, such as the Chcese-mite, the respi
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