Discontinuous Gas Exchange

Originally discovered in diapausing pupae of Hyalophora cecropia and other lep-idopterans, the discontinuous gas exchange cycle (DGC) [formerly known as passive (suction) ventilation] is now known to occur in all ants, many bees, some wasps, several different families of beetles, cockroaches, grasshoppers and locusts (Lighton 1996, 1998). The DGC, which makes use of the high solubility of carbon dioxide in water, comprises three phases (Figure 15.6): constricted- or closed-spiracle phase, fluttering-spiracle phase, and open-spiracle phase. In the constricted-spiracle phase the spiracular valves are kept almost closed. As oxygen is used in metabolism, the carbon dioxide so produced is stored, largely as bicarbonate in the hemolymph and tissues but partially also in the gaseous state in the tracheal system. Thus, a slight vacuum is created within the tracheal system that sucks in more air. Eventually, the tracheal oxygen concentration falls to about 3.5% and the carbon dioxide concentration rises to about 4%. At this point, the low oxygen concentration induces "fluttering" (rapid opening and closing of the valves), the effect of which is to allow some outward diffusion of nitrogen and more air to flow into the tracheal system. However, carbon dioxide cannot escape and its concentration increases to about 6.5%, at which point the valves are opened and remain open between 15 and 30 minutes. During this period there is rapid diffusion of carbon dioxide out of the tracheal system and massive release of carbon dioxide from the hemolymph. When the concentration of carbon dioxide in the tracheal system falls to 3.0%, the valves reclose. Experiments in which gases of different composition are perfused through the tracheal system or over the segmental ganglia have shown that there is dual control over the opening of the valves. Hypercapnia (above normal carbon dioxide concentration) directly stimulates relaxation of the valve closer muscle (the valve opens as a result of cuticular elasticity), whereas hypoxia (insufficient oxygen) acts at the level of the central nervous system (probably the metathoracic ganglion). In diapausing Hyalophora pupae the periods between bursts of carbon dioxide release may be as long as 7 hours.

For diapausing pupae and certain other postembryonic stages of insects, the slight net inflow of air during the constricted-spiracle phase of the DGC may serve as a means of conserving moisture that would otherwise be lost as a result of gas exchange. However, there are many insects that show DGC but do not normally experience water-loss problems. Conversely, there are many desert insects in which DGC does not occur. This has led to the proposal that DGC evolved primarily to facilitate gas exchange in hypoxic and hypercapnic

FIGURE 15.6. Discontinuous release of carbon dioxide in pupa of Hyalophora cecropia in relation to spiracular valve opening and closing. [After R. I. Levy and H. A. Schneiderman, 1966, Discontinuous respiration in insects. IV, J. Insect Physiol. 12:465-492. By permission of Pergamon Press Ltd.]

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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