Strain-gauge transducer studies on intratracheal pressure and pupal length during discontinuous respiration in diapausing silkworm pupae

Abstract

Previous studies have shown that discontinuous respiration is accompanied by intratracheal vacuums and changes in tracheal volume. Most of the time, insects showing discontinuous respiration do not breathe solely by diffusion. Nor do they actively ventilate. Instead they develop intratracheal vacuums which suck air into the tracheal system. The present experiments analyse the intratracheal pressure changes, tracheal volume changes, and muscular movements that accompany discontinuous respiration in the diapausing pupae of Hyalophora cecropia. This paper describes strain-gauge transducer recording techniques with considerable sensitivity and time resolution which reveal respiratory phenomena previously missed. The report also describes a new method for recording spiracular valve movements which permits an examination of the co-ordination of spiracular valve openings. In addition, the function of the spiracular filter which guards each spiracular opening is examined. The principle findings were these: During a carbon dioxide burst the spiracular valves are open and the intratracheal pressure is at atmospheric. The burst is followed by a decline period during which a gradual decrease in intratracheal pressure is interrupted by one or more transient pressure-rises. The decline period is followed by a constriction period during which the pressure falls at a decreasing rate to −3 to −5 mm Hg. This constriction is terminated by a period of pressure rise during which the intratracheal pressure rises in steps to atmospheric. This is followed by a flutter period consisting of a series of microcycles. Each microcycle has a pressure rise of 1 to 2 sec and a pressure fall of 1 to 80 sec. The maximum negative intratracheal pressure during a microcycle ranges from 0·025 to 0·9 mm Hg. During about 95 per cent of the entire cycle of discontinuous respiration the intratracheal pressure is below atmospheric. Three types of pupal length (tracheal volume) changes were observed. (1) Passive changes accompanying each pressure change which damp these pressure changes. (2) Active, slow abdominal muscle tone changes which start during a burst and slowly shorten the pupa until about 30 sec before the next burst whereupon the pupa rapidly returns to its original length. (3) Volleys of active contraction-relaxation cycles separated by quiescent periods which occurred in about 60 per cent of the pupae. The respiratory function of (2) and (3) are unknown, but the passive length changes have significant consequences. During the constriction period the compliance of the pupa (length change per unit pressure change) increases as the intratracheal vacuum increases. This is largely responsible for the reduction in the rate at which the intratracheal pressure falls per unit time. The frequency of spiracular valve openings as detected by intratracheal pressure recordings did not correspond to the frequency of valve movements recorded directly. Apparently only a few of the visually observed valve movements are effective in terms of extensive gas exchange. It was also observed that the two spiracular valves on the same segment were co-ordinated but were not synchronized with those on other segments. It was also demonstrated that the spiracular filter exerts a resistance to the flow of air. Thus the spiracular filter prolongs the pressure-rise portion of the microcycles and decreases the time that the spiracular valves are open when the intratracheal pressure is at atmospheric. Thus the structure of the spiracle itself alters the rates of gas exchanges and, most importantly, decreases water loss.