Spiracle Control in Adult Dragonflies (odonata)

Hoyle (i960) has recently shown that carbon dioxide has a direct action on the closer muscle of spiracle 2 of the desert locust, Schistocerca gregaria Forskal, reducing the electrical responses and the tension developed. Nevertheless, except in flight, the valve movements are nearly always synchronized with ventilation, and it seems unlikely that this action of carbon dioxide plays much part in the normal regulation of the spiracle, except in the presence of high concentrations of carbon dioxide (Miller, 19606). To a considerable extent the action appears to be over-ridden in the interests of ventilation synchronization. When, however, the mesothoracic ganglion is uncoupled from the ventilatory rhythm by section of the cord between it and the metathoracic ganglion, the frequency of motor impulses in the nerve to spiracle 2 falls and this has the effect of increasing the sensitivity of the spiracle muscle, the valve now opening in 1-2% carbon dioxide. The peripheral action of carbon dioxide may provide an unimportant means of spiracle control in the locust, but in an insect, such as the adult dragonfly, whose spiracles are not usually synchronized with ventilation, it may have a more significant part to play. To investigate this hypothesis the control of spiracle 2 in the adult dragonfly has been examined. Four species were chosen for study, each showing slight differences in spiracle behaviour. It is shown here that the spiracle is dually controlled, at the spiracle muscle by the action of carbon dioxide and from the central nervous system as a result of partial anoxia and other stimuli. Differences of spiracle behaviour in the four selected species are 9hown to correspond firstly to small differences in the morphology of the tracheal system and secondly to differences in the behaviour and habitat of the adult insects. A neuro-muscular system, in which the responsiveness of the muscle to excitation of the motor nerve is variable, necessitates the monitoring of muscle movements at all times with recordings from the motor nerve. Only by doing this will the contribution of the peripheral mechanism to spiracle control be distinguishable from that of the central nervous system. For this reason the relation between spiracle movements and nerve impulses may seem at times to be unduly laboured in the following account. While there are numerous accounts of experimental investigations of larval respiration in Odonata (summarized by Corbet, 1962), respiration in the adult has received little attention. Detailed anatomical studies of the adult spiracles and tracheal system have been made by Landois & Thelen (1867), Calvert (1893), Tillyard (1917), Wolf