In many small arthropods (such as some springtails and mites) and in insect eggs, gas exchange takes place by diffusion through the cuticle, and aeropyle or chorionic airspaces, respectively. However, in most other arthropods there are specialized gas exchange structures (Fig. 3.3), and none is more widely known than the tracheal system of insects. This internal system of anastomozing tubes includes the main tracheae, which are often enlarged, especially in adult insects, to form air sacs, and which convey air from the spiracles into the system and through much of the body. These tracheal tubes divide up into finer branches, with conservation of cross-sectional area in some cases, but apparently not in others (Buck 1962; Locke 2001), eventually forming the tracheoles. These fine tubes end blindly, are usually liquid filled (though this apparently varies with oxygen partial pressure) (Wigglesworth 1935), are highly responsive to oxygen demand (Jarecki et al. 1999), and are intimately associated with the tissues. In the case of the flight muscles the tracheoles indent the plasma membrane and penetrate deeply into the muscle fibre (see Chapman 1998 for review).
Throughout the following sections, we focus almost entirely on gas exchange in terrestrial insects, mostly because of their predominance in terms of both species richness and absolute abundance. However, gas exchange in aquatic insects is no less intriguing and is based on the same principles, especially in those insects that use tracheal gills, physical (or gas) gills, or plastron respiration. The behavioural, morphological, and physiological adaptations for gas exchange in aquatic insects have been reviewed several times (Nation 1985, 2002; Chapman 1998), and the principles underlying this gas exchange are dealt with extensively by Kestler (1985).
Before proceeding with a discussion of gas exchange, it is necessary to clarify our use of the terms diffusion, convection, and ventilation. In the literature, these terms are often used either to refer to the exchange of gases more broadly, or in the case of ventilation and convection, are used interchangeably. Here, diffusion is used in the standard, physical sense; convection means mass movement of the medium; and ventilation refers to convection that is assisted by some form of pumping activity, usually associated with muscular contraction and relaxation.
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