Diffusion and ventilation

Oxygen enters the spiracle, passes through the length of the tracheae to the tracheoles and into the target cells by a combination of ventilation and diffusion along a concentration gradient, from high in the external air to low in the tissue. Whereas the net movement of oxygen molecules in the tracheal system is inward, the net movement of carbon dioxide and (in terrestrial insects) water vapor molecules is outward. Hence gas exchange in most terrestrial insects is a compromise between securing sufficient oxygen and reducing water loss via the spiracles. During periods of inactivity, the spiracles in many insects are kept closed most of the time, opening only periodically; this is referred to as discontinuous gas exchange. When the spiracles are closed, cellular respiration in the body causes oxygen to used up within the tracheal system and the partial pressure of oxygen drops until a certain threshold level at which the spiracles rapidly open and close at high frequency (the flutter phase). Some atmospheric oxygen enters the tracheae during the flutter phase but carbon dioxide continues to build up until it eventually it triggers the spiracular open phase, during which tracheal oxygen, carbon dioxide and water are exchanged with the outside air either by diffusion or active ventilation. Comparative study of many insects living in different environments suggests that discontinuous gas exchange evolved to reduce tracheal water loss. In insects of xeric environments, the spiracles may be small with deep atria or have a mesh of cuticular projections in the orifice. Insects living in mesic environments may exchange gases with the atmosphere more or less continuously.

In insects without air sacs, such as most holome-tabolous larvae, diffusion appears to be the primary mechanism for the movement of gases in the tracheae and is always the sole mode of gas exchange at the tissues. The efficiency of diffusion is related to the distance of diffusion and perhaps to the diameter of the tracheae (Box 3.2). Rapid cycles of tracheal compression and expansion have been observed in the head and thorax of some insects using X-ray videoing. Movements of the hemolymph and body could not explain these cycles, which appear to be a new mechanism of gas exchange in insects. In addition, large or dilated tracheae may serve as an oxygen reserve when the spiracles are closed. In very active insects, especially large ones, active pumping movements of the thorax and/or abdomen ventilate (pump air through) the outer parts of the tracheal system and so the diffusion pathway to the tissues is reduced. Rhythmic thoracic movements and /or dorsoventral flattening or telescoping of the abdomen expels air, via the spiracles, from extensible or some partially compressible tracheae or from air sacs. Coordinated opening and closing of the spiracles usually accompanies ventilatory movements and provides the basis for the unidirectional airflow that occurs in the main tracheae of larger insects. Anterior spiracles open during inspiration and posterior ones open during expiration. The presence of air sacs, especially if large or extensive, facilitates ventilation by increasing the volume of tidal air that can be changed as a result of ventilatory movements. If the main tracheal branches are strongly ventilated, diffusion appears sufficient to oxygenate even the most actively respiring tissues, such as flight muscles. However, the design of the gas exchange system of insects places an upper limit on size because, if oxygen has to diffuse over a considerable distance, the requirements of a very large and active insect either could not be met, even with ventilatory movements and compression and expansion of tracheae, or would result in substantial loss of water through the spiracles. Interestingly, many large insects are long and thin, thereby minimizing the diffusion distance from the spiracle along the trachea to any internal organ.

gut with strong musculature and obvious protection from abrasion (especially in the midgut, which has no cuticular lining). These features are most obvious in solid-feeders with rapid throughput of food as in plant-feeding caterpillars. In contrast, insects feeding on blood, sap, or nectar usually have long, narrow, convoluted guts to allow maximal contact with the liquid food; here, protection from abrasion is unnecessary. The most obvious gut specialization of liquidfeeders is a mechanism for removing excess water to concentrate nutrient substances prior to digestion, as seen in hemipterans (Box 3.3). From a nutritional viewpoint, most plant-feeding insects need to process large amounts of food because nutrient levels in leaves and stems are often low. The gut is usually short and without storage areas, as food is available continuously. By comparison, a diet of animal tissue is nutrient-rich and, at least for predators, well balanced. However, the food may be available only intermittently (such as when a predator captures prey or a blood meal is obtained) and the gut normally has large storage capacity.

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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