Metabolism during flight is strictly aerobic. In small insects, such as Drosophila, oxygen requirements are met almost exclusively by diffusion, and spiracular opening can be regulated to meet varying aerodynamic power requirements, thus conserving water (Lehmann 2001). In large insects, aerobic demands are met by ventilation of the tracheal system in one of four ways. In some insects, such as bees and wasps, abdominal pumping, usually with unidirectional flow, is thought to be responsible for most or all of the bulk oxygen flow during flight (Weis-Fogh 1967a; Miller 1974). By contrast, in several other groups of insects, such as locusts, dragonflies, beetles, cockroaches, and moths, ventilation as a consequence of the flight muscle movements, or autoventilation, meets the gas exchange requirements of the flight musculature (Miller 1974), and has been examined in most detail in S. gregaria. During flight, tidal autoventilation, through spiracles 2 and 3, accounts for 250 lkg^h"1, and serves the flight musculature, whilst abdominal pumping accounts for a further 144 lkg-1 h_1, and is largely unidirectional through the dorsal orifice of spiracle 1 and out through spiracles 5-10 (Miller 1960; Weis-Fogh 1967a). Auto ventilation meets the oxygen demands of many large beetles, such as Goliathus regius (Scarabaeidae) one of the largest known flying insects. However, in several species, but particularly cerambycid beetles in the genus Pterognatha, there is Bernoulli entrainment of air within the tracheal system. Air is pulled through the primary tracheae because of a pressure gradient owing to higher external airflow velocities near the posterior spiracles. This draught ventilation is supplemented by autoventilation of the secondary tracheae. Although draught ventilation is known only from a few beetles, it is thought to be more widespread, particularly in fast flying insects (Dudley 2000). Finally, in the hawkmoth, M. sexta, it has been shown that airflow during flight is unidirectional as a consequence of the position of the posterior thoracic spiracle in the subalar cleft and the functioning of the flight apparatus (Wasserthal 2001). During the down-stroke, the posterior thoracic spiracles are closed automatically within the subalar cleft and thoracic deformation results in an increase in the volume of the thoracic air sacs. Air, therefore, enters through the anterior thoracic spiracle. During the upstroke, air sac volume declines and air is expired through the posterior spiracle (Fig. 3.16).
The type of ventilation used by insects could have profound implications for their response to atmospheric oxygen levels (Harrison and Lighton 1998). Insects using autoventilation can increase airflow by increasing wingstroke frequency or amplitude, but this also increases metabolic rate. In consequence, they might not be able to meet the increase in ratio of oxygen supply to demand that is required to compensate for hypoxia. By contrast, insects relying on abdominal ventilation might not face the same difficulties, and in honeybees facing
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