. Animal physiology. Physiology, Comparative; Physiology, Comparative. 252 RESPIRATION IN INSECTS. disposed in the aquatic Mollusca, but having a free communica- tion with the external air, and having the blood minutely distri- buted by vessels upon its walls (/?, Fig. 14). In the air-breathing Annelida, such as the Earth-worm, we find a repetition of similar cavities along the body, one pair usually existing in each segment; and these open externally by small apertures, which are termed stigmata. The same is the case in the Myriapoda ; but the air-sacs now begin to send off branching air-tube
. Animal physiology. Physiology, Comparative; Physiology, Comparative. 252 RESPIRATION IN INSECTS. disposed in the aquatic Mollusca, but having a free communica- tion with the external air, and having the blood minutely distri- buted by vessels upon its walls (/?, Fig. 14). In the air-breathing Annelida, such as the Earth-worm, we find a repetition of similar cavities along the body, one pair usually existing in each segment; and these open externally by small apertures, which are termed stigmata. The same is the case in the Myriapoda ; but the air-sacs now begin to send off branching air-tubes or trachea, which spread through the body; these, however, have not much communication with each other. In Insects, this plan of structure is carried out in the most remarkable manner. The stigmata do not open into distinct air-sacs, but into canals, which lead to two large tracheee that run along the sides of the body, and are con- nected by several tubes that pass across it—one usually for each segment. From these tracheas others branch off, which again subdivide into more minute tubes: and by the ramifications of these, Fig. 152—Air- . ' J ' tube of insect, even the minutest parts of the body are pene- trated (Fig. 151). These tubes are formed upon a similar plan with the air-vessels of Plants, having a spiral fibre winding inside their outer membranous coat; by the elasticity of which fibre, the tube is kept from being closed by pressure. 321. In this manner the air is brought into contact with almost every portion of the tissue, and is enabled to act most energetically upon it; and thus the feeble circulation of these animals (§. 293) is in a great degree counterbalanced by the extraordi- nary activity of their respiration. There are no animals which consume so much oxygen, in pro- portion to their size, as Insects do when they are in motion; but when they are at rest, their respi- ration falls to the low standard, of the tribes to which they bear the greatest gener
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