Oxygen limitation is not the cause of death during lethal heat exposure in an insect

Oxygen- and capacity-limited thermal tolerance (OCLTT) is a controversial hypothesis claiming to explain variation in, and mechanistically determine, animal thermal limits. The lack of support from Insecta is typically argued to be a consequence of their high-performance respiratory systems. However, no studies have reported internal body oxygen levels during thermal ramping so it is unclear if changes in ambient gas are partially or fully offset by a compensatory respiratory system. Here we provide such an assessment by simultaneously recording haemolymph oxygen (pO2) levels – as an approximation of tissue oxygenation - while experimentally manipulating ambient oxygen and subjecting organisms to thermal extremes in a series of thermolimit respirometry experiments using pupae of the butterfly Pieris napi. The main results are that while P. napi undergo large changes in haemolymph pO2 that are positively correlated with experimental oxygen levels, haemolymph pO2 is similar pre- and post-...