Discontinuous gas exchange in Madagascar hissing cockroaches is not a consequence of hysteresis around a fixed PCO2threshold.

Abstract

It has been hypothesised that insects display discontinuous gas-exchange cycles (DGCs) as a result of hysteresis in their ventilatory control, where CO2-sensitive respiratory chemoreceptors respond to changes in haemolymph PCO2only after some delay. If correct, DGCs would be a manifestation of an unstable feedback loop between chemoreceptors and ventilation, causing PCO2to oscillate around some fixed threshold value: PCO2above this ventilatory threshold would stimulate excessive hyperventilation, driving PCO2below the threshold and causing a subsequent apnoea. This hypothesis was tested by implanting micro-optodes into the haemocoel of Madagascar hissing cockroaches and measuring haemolymph PO2and PCO2simultaneously during continuous and discontinuous gas exchange. The mean haemolymph PCO2of 1.9 kPa measured during continuous gas exchange was assumed to represent the threshold level stimulating ventilation, and this was compared with PCO2levels recorded during DGCs elicited by decapitation. Cockroaches were also exposed to hypoxic (PO210 kPa) and hypercapnic (PCO22 kPa) gas mixtures to manipulate haemolymph PO2and PCO2. Decapitated cockroaches maintained DGCs even when their haemolymph PCO2was forced above or below the putative ∼2 kPa ventilation threshold, demonstrating that the characteristic oscillation between apnoea and gas exchange is not driven by a lag between changing haemolymph PCO2and a PCO2chemoreceptor with a fixed ventilatory threshold. However, it was observed that the gas exchange periods within the DGC were altered to enhance O2 uptake and CO2 release during hypoxia and hypercapnia exposure. This indicates that while respiratory chemoreceptors do modulate ventilatory activity in response to haemolymph gas levels, their role in initiating or terminating the gas exchange periods within the DGC remains unclear.