Spiracle Control in Dragonflies

Abstract

HOYLE¹ has shown that carbon dioxide has a direct action on the neuro-muscular junction of the closer muscle of the second spiracle of the locust, reducing the electrical responses and the tension developed in spite of maintained stimulation of the motor axons in the transverse nerve. However, this peripheral control mechanism does not seem to play an important part in the regulation of the intact spiracle which, except during flight, remains firmly synchronized with ventilation². It might be expected to play a more important part in an insect the spiracles of which are not always synchronized with ventilation, such as the dragonfly. The adults of four species of Aeschnid dragonflies, occurring in Great Britain, have been examined and none shows ventilation synchronization in spiracles 1, 2 or 3. In the 15 species of Libellulidae from Great Britain and Uganda which have been examined, however, spiracle 1 opens with abdominal inspiration and closes completely or partially with expiration: at rest, spiracles 2 and 3 remain closed, but they may join spiracle 1 as inspiratory spiracles for a brief period after struggling or flight.