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Some tube-dwelling species show rhythmic movements that create water currents over the body so that water in the tube is periodically renewed and the thickness of the "boundary layer" (the layer of still water adjacent to the body wall) is reduced.

In many aquatic larvae there are richly tracheated outgrowths of the body wall (hindgut in dragonfly larvae) collectively known as tracheal gills. Whether these function as accessory respiratory structures was originally controversial because even without them an insect may survive perfectly well and its oxygen consumption may not change. Tracheal gills include caudal lamellae (in Zygoptera), lateral abdominal gills (in Ephemeroptera, and some Zygoptera, Plecoptera, Neuroptera, and Coleoptera), rectal gills (in Anisoptera), and fingerlike structures, often found in tufts on various parts of the body (e.g., in some Plecoptera and case-bearing Trichoptera).

Juvenile Zygoptera have three caudal lamellae (Figure 6.11) whose functions have long been controversial (Burnside and Robinson, 1995), in large part because even without them larvae survive perfectlywell. Suggested uses of the lamellae includeswimming, especiallyto avoid predators, sacrificial structures for diverting a predator's attention, and gas exchange. There is evidence that all these possibilities may be important. For example, larvae with lamellae missing are poorer swimmers than those with a full complement; further, there are significant differences in the design oflamellae between good and poor swimmers. The lamellae have a breaking joint near their base, enabling them to be shed easily when grasped by a predator. The lamellae are also highly tracheated, and experiments have demonstrated that the lamellae normally are major sites of gas exchange and in oxygen-deficient water become especially important. For example, in Coenagrion up to 60% of oxygen uptake may occur via the lamellae (Harnisch, 1958; cited from Mill, 1974). In certain Enallagma species normal larvae can survive in water with an oxygen content of only 2.4% saturation, whereas the minimum for lamellaeless larvae is 14.5% saturation. Above this value, lamellaeless larvae live apparently normally and obtain sufficient oxygen by cutaneous diffusion (Pennak and McColl, 1944).

In larval Ephemeroptera the lateral abdominal gills are paired, segmental, normally platelike or branched structures (Figures 6.4-6.7) whose size is inversely related to the oxygen content of the surrounding water. In other orders, the gills are filamentous (Mill, 1974). In burrowing mayfly larvae and larvae of other species that live in an oxygen-poor environment, the gills are an important site of gas exchange, and about 50% of an insect's oxygen requirement may be obtained via this route. In addition, their rhythmic beating facilitates cutaneous respiration by moving a current of water over the body and reducing the thickness of the boundary layer. The frequency at which the gills beat depends both on their size and on the oxygen content of the water. In species that inhabit fast-flowing water, the gills do not beat, and removal of them does not affect oxygen consumption, indicating that most gas exchange occurs cutaneously.

The anterior part of the rectum of larval Anisoptera is enlarged to form a branchial chamber whose walls bear six rows of richly tracheated gills. Water is periodically drawn into and forced out of the rectum via the anus as a result of muscular movements of the abdomen, comparable to those that effect ventilation in terrestrial species. Contraction of dorsoventral muscles in the posterior abdominal segments decreases the abdominal volume and results in water being forced out of the rectum. A muscular diaphragm in the fifth segment may prevent hemolymph from being forced anteriorly. Water enters the rectum as the volume of the abdomen is increased, as a result of the contraction of two transverse

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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