THE RESPIRATION OF INSECTS

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Summary: The review covers the ground of external and internal respiration in insects, but deals neither with quantitative changes in metabolism, nor with the intimate processes of oxidation in the tissues.The general anatomy and histology of the tracheal system are described. The form of the tracheae and their mode of ending in the tissues is extremely varied in different insects and in different organs. The question whether the tracheal endings contain fluid or air and the problem of what forces keep the larger branches filled with gas are discussed in detail. The embryological development of the tracheal system and the nature of the “spiral thread” are briefly considered.The exchange of gases in the tracheal system is effected primarily by diffusion. This is modified by opening and closure of the spiracles (“ diffusion control ”) and by mechanical ventilation of the larger tracheal branches (“ventilation control ”). The mechanism of tracheal ventilation is discussed at length: including the part played by the air sacs and the question whether there is a directed stream of air through the tracheal system.The respiratory movements are controlled by nerve centres, variously situated in different insects; and these centres may be stimulated either by oxygen want or by carbon dioxide excess.The relative importance of the spiracles and the skin in respiration, especially in the elimination of carbon dioxide, is discussed.Under the respiration of aquatic insects the functions of the “blood gills,”“tracheal gills ” and “ cuticular gills ” are reviewed, and the problems connected with the “ hydrofuge ” structures of aquatic insects and the air stores which they carry are considered. The respiration of parasitic insects presents many parallels with that of aquatic forms.The part played by the blood of insects as a carrier of oxygen is discussed in detail.

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