Discontinuous respiration in insects at low temperatures: intratracheal pressure changes and spiracular valve behavior.

Abstract

1. Experiments were performed to examine the effects of temperatures down to - 20° C on discontinuous respiration in diapausing Cecropia pupae. Methods were developed that permitted simultaneous recording of intratracheal pressure changes and spiracular valve movements at low temperatures.2. Discontinuous respiration continued down to - 5° C but ceased at some temperature between - 5 and - 10° C.3. At 0° C the constriction period of the cycle, during which valves remain closed for some time, generally was absent. The open phase of the burst was followed by a lengthy decline phase, during which valves close briefly from the fully open position, and then by the flutter period, during which valves continuously open briefly and then close. The flutter period made up most of the cycle at all temperatures except at 5° C where the flutter and constriction periods were of about equal length.4. Longer microcycles, relatively brief pressure fall periods caused by the closing of the spiracular valves, with correspondingly greater intratracheal vacuums than at high temperatures were observed in the decline phase of the burst and in the flutter period as the ambient temperature decreased to 0° C. At - 5° C microcycles were shorter and resulted in less of an intratracheal vacuum than at 0° C, presumably because the metabolic rate of the pupa decreased.5. The spiracular valves responded to oxygen and carbon dioxide at - 5° C but not at - 10° C or below. Thus neuromuscular coordination of spiracular function persisted at - 5° C but ceased at some temperature between - 5° and - 10° C.6. Studies of pupal weight loss indicated that spiracular valves remained constricted at low temperatures. Under the experimental conditions pupae froze at some temperature between - 5° and - 10° C. It is suggested that in nature the spiracular valves freeze in the closed position.