A Further Function of the Air Sacs in some Insects

Abstract

THE chief functions of the air sacs in insects may be listed as follows: (1) As suggested by John Hunter¹, in an insect of fixed external dimensions they will assist flight by lowering the specific gravity. (2) They increase the volume of tidal air during respiration and so reduce the distance over which oxygen must diffuse to reach the tracheal endings^2,3. Their ventilation may be effected by direct pressure from the body wall, as in the abdomen; but in the head, or other incompressible parts, collapse of the air sacs may be brought about by pressure transmitted via the hæmolymph⁴. (3) The flight muscles of many insects have large tracheæ, with elliptical cross-section, between them. The muscles are automatically ventilated during flight by the compression of these intramuscular air sacs⁵. (4) Even when the air sacs are of no great importance in ventilation they will serve to reduce the mechanical damping of the wing movements by the hæmolymph⁵. (5) The large air sacs in the front of the abdomen of Lucilia, Calliphora, etc., seem to play no essential part in respiration. They are distended with air, or collapsed, according to the state of other organs in the abdomen; in this way they ensure that the external form of the abdomen remains unchanged at all times⁶. (6) The air sacs of aquatic insects have a hydrostatic function. (7) The further function which is here proposed was suggested by observations on the adult Drosophila. When this fly emerges from the pupa the air sacs are collapsed and the body cavity contains a large amount of hæmolymph. The air sacs in this state are illustrated in Plate 1, Fig. 9 of a paper on tracheal injection⁷. By 24 h after emergence the air sacs are fully distended. They now occupy a large part of the body cavity in the head and thorax which was previously occupied by hæmolymph (Plate 1, Figs. 7, 11; Plate II, Fig. 5, 6, 7, 8)⁷.