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On the regulation of insect discontinuous gas exchange cycles: the role of hemolymph oxygen and carbon dioxide on ventilatory rhythms in Madagascar hissing cockroaches Rowe, Tormod T.C.

Abstract

Many evolutionarily distant species of insect display an episodic pattern of breathing termed a discontinuous gas exchange cycle (DGC), defined by bursts of ventilation interspersed with long apnoeic periods. Internal O₂ and CO₂ are not tightly regulated during DGCs as PO₂ and PCO₂ fluctuate significantly. It is as of yet unknown what mechanisms drive the emergence of DGCs, although one hypothesis states that DGCs arise from an unstable ventilatory control system that is unable to quickly respond to fluctuations in internal PO₂ and PCO₂ resulting in alternating cycles of ventilation and apnoea. Essentially, this hypothesis suggests that a temporal lag present between chemoreception of CO₂ and a ventilatory response results in CO₂ levels oscillating around a ventilatory CO₂ threshold as ventilation is turned on and off. This hypothesis is tested in this thesis by implanting PO₂ and PCO₂ optodes into the hemocoel of Madagascar hissing cockroaches, to measure hemolymph PO₂ and PCO₂ fluctuations in vivo during periods of continuous and discontinuous ventilation. Additionally, rates of CO₂ release were measured using a flow-through respirometry setup, and ventilatory frequency was measured using an infrared phototransistor. The stable hemolymph PCO₂’s measured during continuous ventilation were assumed to represent the CO₂ threshold stimulating gas exchange, and these levels were compared with the CO₂ fluctuations during DGCs elicited in decapitated cockroaches. Cockroaches were also exposed to hypoxia (low O₂) and hypercapnia (high CO₂) in order to artificially manipulate hemolymph PO₂ and PCO₂. Decapitated Madagascar hissing cockroaches were observed maintaining DGCs with internal O₂ and CO₂ levels outside of the assumed threshold values. Results suggest that the DGCs displayed by Madagascar hissing cockroaches are not the result of PO₂ and PCO₂ oscillating around fixed ventilatory thresholds. However, it was observed that patterns of DGCs, such as ventilatory burst duration, interburst duration, and ventilation pattern were altered by exposure to hypoxia and hypercapnia.

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