The Function of the Air Sacs of Insects

Abstract

THIN-WALLED expansions of the tracheæ known as air sacs occur in those insects which have well-developed powers of flight. Misconception as to the function of these structures is common. They have been described as pumps, as reservoirs, and as balloons. The first term alone seems to give an accurate description of their function, for Lee has made it clear that they must ventilate the tracheæ: each air sac, when compressed, expels the air from the tracheæ between it and the spiracle. As reservoirs the sacs could be of little use to aerial insects, for the oxygen they contain would last but a very short time during flight, while there is an unlimited supply of air only a little farther from the tissues.^{1 2}