Gas Exchange

In a compressible gill the pressure of the gas will be equal to that of the surrounding 483

water. The gas in the gill may be considered to include only oxygen and nitrogen, as the carbon dioxide produced in metabolism will readily dissolve in the water. Immediately after a visit to the surface, the composition of the gas in the gill will approximate that of air, that is, about 20% oxygen and 80% nitrogen and will be in equilibrium with the dissolved gases in the surrounding water. As an insect uses oxygen, a diffusion gradient will be set up so that this gas will tend to move into the gill. At the same time, the proportion of nitrogen in the gas will have increased and, therefore, nitrogen will tend to diffuse out of the gill in order to restore equilibrium. However, oxygen diffuses into the gill about three times as rapidly as nitrogen diffuses out. Thus, inward movement of oxygen will be more important than outward movement of nitrogen in restoring equilibrium. The effect of this is to prolong considerably the life of the gas store and, therefore, the duration over which an insect can remain submerged. The rate of diffusion, which is a function of the surface area of the gill exposed to the water, will obviously be greater in the case of films of gas spread over the body surface than in subelytral gas stores whose contact with the surrounding water is relatively slight. Ultimately, however, in a compressible gill the rate of oxygen use by the insect will reduce the proportion of oxygen in the gill below a critical level, and the insect will be stimulated to return to the surface.

For example, adult water beetles of the family Dytiscidae and back swimmers (Notonecta spp.: Hemiptera) are generally medium to large insects, which in summer, when water temperatures may approximate 20-25°C, show great activity. Under these conditions, the gas stored beneath the elytra (and also on the ventral body surface in Notonecta) must be regularly exchanged by visits to the water surface. However, in winter, when the water is cold (or may even be continuously frozen for several months as occurs, for example, on the Canadian prairies), the insects are more or less inactive. During this period, the gas store satisfies most or all of the insects' oxygen requirements.

In contrast, the volume of a plastron is constant but small. Hence, a plastron does not serve as a store of oxygen but solely as a gas gill. In certain adult Hemiptera (e.g., Aphelocheirus) and Coleoptera (e.g., Elmis) the plastron is held in place by dense mats of hydrofuge hairs. The hairs number about 200 x 106/cm2 in Aphelocheirus, are bent at the tip, and are slightly thickened at the base. As a result, they can resist becoming flattened (which would destroy the plastron) by the considerable pressure differences that may arise between the gas in the plastron and the surrounding water as the insect moves into deeper water or uses up its oxygen supply. Spiracular gills are mostly found on the pupae of certain Diptera (e.g., Tipulidae and Simuliidae) and Coleoptera (e.g., Psephenidae) but occasionally occur on beetle larvae (Hinton, 1968). In almost all instances they include a plastron. In many dipteran pupae the gill is a long, hollowed-out structure (Figure 15.8C) that carries a plastron over its surface. The plastron is held in place by means of rigid cuticular struts and connects via fine tubes with the gas-filled center of the gill (and hence the spiracle).

Because the volume of the plastron remains constant, the nitrogen does not diffuse out. However, the rate and direction of diffusion of oxygen will depend on differences in the oxygen content of the plastron (always somewhat less than maximum because of use of oxygen by the insect) and the surrounding water. Therefore, to ensure that oxygen always diffuses into the plastron, the water around it should be saturated with the gas. If the water is not saturated with oxygen, then the latter will either diffuse into the plastron relatively slowly or may even diffuse out, leading eventually to asphyxiation of the insect. It is not surprising, then, to discover that plastron respiration is used by aquatic insects living in fast-moving streams, at the edges of shallow lakes, and in the intertidal zone. Plastron-bearing species known to inhabit water whose oxygen content may fluctuate daily, for example, that

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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