Functional Design of the Tracheal System of Flying Insects as Compared with the Avian Lung

Abstract

1. The wing muscles of locusts are supplied with air by means of a system of air tubes which communicate only with the first three pairs of spiracles. Both in locusts and in dragonflies each muscle receives a direct supply ending in a large air sac, the primary supply. It is automatically ventilated by thoracic pumping due to the movements of nota and pleura which accompany the wing movements. 2. The secondary supply of transverse diffusion tracheae branches off from the primary supply, often according to geometrical patterns suited for quantitative studies. 3. During flight, thoracic pumping appears to be the only mechanism responsible for the ventilation of the thorax in grasshoppers, dragonflies, moths and beetles, while Hymenoptera and Diptera rely on the usual abdominal pumping. 4. In many wing muscles from large insects there is a wide shunt between the primary supply and the terminal air sac, resembling the shunting mesobronchus in the lung of birds. In both groups of animals the shunt is considered to be of little significance during flight. 5. In insects the shunt facilitates diffusion when the thorax is not ventilated during rest. 6. In resting birds under thermal stress, it is concluded that contraction of the epithelial sphincter muscles in the parabronchi makes possible hyperventilation of the moist air passages via the mesobronchus, removing excess heat by evaporation without causing apnoea due to the washing out of CO₂.