Discontinuous respiration in insects—V. Pressure and volume changes in the tracheal system of silkworm pupae

Abstract

Previous studies have indicated that intratracheal barometric pressure is a significant parameter of respiration which may prove useful in the analysis of insect respiration in general and discontinuous respiration, in particular ( Buck, 1958; Levy and Schneiderman, 1958, 1966c). The present report introduces the use of electronic transducers to record, continuously and simultaneously, changes in intratracheal pressure, tracheal volume, and gas exchange over periods of many hours with good time resolution. The paper also describes a technique of applying pressure pulses to the tracheal system during specific phases of the intratracheal pressure cycle and deducing the functional state of the spiracular valves from the effects of these pulses. This technique permits us to measure the ‘leakiness’ of the spiracular valves and provides information about both the kinetics of gas exchange and the movements of the spiracles. The methods described are applicable to many sorts of insects, but in the present study were applied only to diapausing pupae of Hyalophora cecropia which have discontinuous respiration. The disparate rates of O 2 and CO 2 exchange which characterize the respiration of these pupae has been attributed to the existence of partial vacuums within the tracheal system which cause a ‘sucking in’ of gas. The experiments reported here confirm the existence of these partial vacuums and demonstrate their impact on gas exchange. The experiments also reveal the existence of heretofore unsuspected respiratory movements in these pupae. These movements are passive and not under muscular control and result from volume changes in the tracheal system caused by changes in intratracheal pressure. These volume changes considerably damp intratracheal pressure changes. The functional significance of these volume changes in discontinuous respiration is analysed.