Chapter

478 tracheae to flatten or collapse. In some species both inspiration and expiration are brought about by muscles; in others, only expiration is under muscular control, and inspiration occurs as a result of the natural elasticity of the body wall. During high metabolic activity, supplementary ventilation movements may occur. For example, the desert locust normally ventilates by means of dorsoventral movements of the abdomen but can supplement these by "telescoping" the abdomen and protraction/retraction of the head and prothorax (Miller, 1960). Remarkably, it has recently been reported that many insects showing no obvious signs of breathing have rapid cycles of tracheal compression in the head and thorax (Westneat et al., 2003). In analogy with the situation in the abdomen, tracheal compression in these regions is induced indirectly, by contraction of mandible and leg muscles. When these muscles relax, the elasticity in the taenidial rings returns the tracheae to their original shape and volume. This discovery may require reconsideration of the proposal that diffusion alone satisfies the gas-exchange requirements of small insects.

The diffusion gradient can be further improved by increasing the volume of air in the tracheal system that is renewed during each stroke (the tidal volume). This has been achieved through the development of large, compressible air sacs. However, simple tidal flow (pumping of air in to and out of all spiracles) is still somewhat inefficient because a considerable volume of air (the dead air space) remains within the system at each stroke. The size of the dead air space is greatly reduced by using unidirectional ventilation in which air is made to flow in one direction (usually anteroposteriorly) through the tracheal system. Unidirectional airflow is achieved by synchronizing the opening and closing of spiracular valves with ventilation movements. In the resting desert locust, for example, the first, second, and fourth spiracles are inspiratory, while the tenth (most posterior) is expiratory. When the insect becomes more active, the first four spiracles become inspiratory, the remainder expiratory. The spiracular valves do not form an airtight seal, however, so that a proportion of the inspired air continues to move tidally rather than unidirectionally (20% in the resting desert locust).

During flight, the oxygen consumption of an insect increases enormously (up to 24 times in the desert locust), almost entirely because of the metabolic activity of the flight muscles. To facilitate this activity, a massive exchange of air occurs in the pterothorax, made possible by certain structural features of the pterothoracic tracheal system and by changes in the body's normal (resting) ventilation pattern. As noted earlier, the tracheal system of the pterothorax is effectively isolated from that of the rest of the body by reduction in the diameter or occlusion of the main longitudinal tracheae. Autoventilation of flight muscle tracheae also occurs. This is ventilation that results from movements of the nota and pleura during wing beating, and it brings about a considerable flow of air into and out of the thoracic tracheae. During autoventilation, normal unidirectional flow, where such occurs, becomes masked by the massive increase in tidal flow in the pterothorax. To achieve this tidal flow, in the desert locust, spiracles 2 and 3 remain permanently open. Spiracles 1 and 4-10, however, continue to open and close in synchrony with abdominal ventilation so that some unidirectional flow occurs. The rate of abdominal ventilation movements also increases during flight to about four times the resting value, but these movements probably serve primarily to increase the rate of flow of hemolymph around the body, bringing fresh supplies of metabolites to the flight musculature. However, keeping the central nervous system well supplied with oxygen also appears to be important.

Autoventilation is used by many Odonata, Orthoptera, Dictyoptera, Isoptera, Hemiptera, Lepidoptera, and Coleoptera, but is of little importance in Diptera and Hy-menoptera. Its significance can be broadly correlated with body size, the type of muscles used in flight (Chapter 14, Section 2.1), and the extent of movements of the thorax during

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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