. Animal physiology. Physiology, Comparative; Physiology, Comparative. RESPIRATION IN INSECTS AND ARACHNIDA. 253 but it is never very active. Some aquatic larvae breathe air by- means of tracheae; and they are consequently obliged, like the Whales and other aquatic Mammalia, 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 stigmata 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 end of this tube (2, Fig.
. Animal physiology. Physiology, Comparative; Physiology, Comparative. RESPIRATION IN INSECTS AND ARACHNIDA. 253 but it is never very active. Some aquatic larvae breathe air by- means of tracheae; and they are consequently obliged, like the Whales and other aquatic Mammalia, 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 stigmata 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 end of this tube (2, Fig. 153) being at the surface. 322. In most perfect Insects, we find the tracheae dilated at certain parts into large air-sacs (Fig. 151); 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 much larger portion of the trunk, than they do in the insect whose system of air-tubes has been just repre- sented,—this insect, the Nepa or Water-Scorpion being of aquatic habits, and seldom using its wings for flight. There 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 FlG- 154.—Nepa. 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 Yertebrated division, a structure bearing remarkable analogy to this (§. 326.) 323. In some of the Arachnida, such as the Ckeese-mite, the respiration is accomplished by tracheae, as in Insects; but in the Spiders it is performed by a different kind of apparatus. Instead of opening into a system of prolonged tubes, each spiracle leads to a little chamber, the lining membrane of which is arranged in a number of folds, that lie together like the leaves of a book; and thus a large surface is exposed to the
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