Gas Exchange

muscles, the diaphragm and subintestinal muscles, which pull the tergal walls inward and 481

force the sterna downward. This is aided by the natural elasticity of the body wall and by relaxation of the dorsoventral muscles (Hughes and Mill, 1966; Mill, 1977). The rate of ventilation of the branchial chamber varies with the oxygen content of the water and metabolic rate of the insect. Cutaneous respiration is probably not significant in Anisoptera, whose body wall cuticle is generally thick. Rectal pumping also occurs in larval Zygoptera, though its function here is primarily in relation to ion uptake (Chapter 18, Section 4.2) as these insects lack rectal gills (Miller, 1993).

4.2. Open Tracheal Systems

Among insects that have an open tracheal system can be traced a series of stages leading from complete dependence on atmospheric air (i.e., where an insect must frequently visit the water surface to exchange the gas in its tracheal system) to the stage where an insect maintains around its body a supply of atmospheric gas into which oxygen can diffuse from the surrounding water at a sufficient rate to totally satisfy the insect's needs. Thus, the insect is completely independent of atmospheric air.

Surface-breathing insects must solve two problems. First, they must prevent waterlogging of the tracheal system when they are submerged, and second, they must be able to overcome the surface tension force at the air-water interface. Both problems are solved by having hydrofuge (water-repellent) structures around the spiracles. In many dipteran larvae special epidermal glands (perispiracular/peristigmatic glands) secrete an oily material at the entrance of the spiracle. The spiracles of some other aquatic insects are surrounded by hydrofuge hairs, which, when submerged, close over the opening but when in contact with the water surface spread out to permit exchange of air (Figure 15.7). In addition, insects may possess other modifications to the tracheal system to help cope with these problems, for example, reduction of the number of functional spiracles and restriction of the spiracles to special sites, typically at the tip of a posterior extension of the body (postabdominal respiratory siphon), as occurs in mosquito larvae (Figure 9.6F) and water scorpions (Figure 8.16D). In larvae of some Syrphidae (e.g., Eristalis) the siphon is very extensible, its shape and flexibility giving rise to the common name of these insects—rattailed maggots. In a few species of Coleoptera and Diptera, whose larvae live in mud, the siphon, which is rigid and pointed, is forced into air spaces in the roots of aquatic plants.

Insects have gained variable degrees of independence from atmospheric air by holding a gas store about their body. The gas store may be subelytral or may occur as a thin film

FIGURE 15.7. Hydrofuge hairs surrounding a spiracle. (A) Position when submerged; and (B) position when at water surface. [After V. B. Wigglesworth, 1965, The Principles of Insect Physiology, 6th ed., Methuen and Co. By permission of the author.]

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

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