Striking a deal with the environment, ectotherms have come up with a low-cost lifestyle compromise by hitching their body temperature to that of their surroundings. However, as climate change takes a grip and the equilibrium begins shifting, this compromise may come unstuck if conditions become uncomfortably hot. Leigh Boardman and John Terblanche from Stellenbosch University, South Africa, explain that as temperatures rise, aquatic ectotherms are unable to meet the oxygen demands of their warmer bodies, but it wasn't clear whether insects suffered the same fate. Knowing that oxygen availability should alter the thermal tolerance of animals if it is the factor limiting their ability to adapt to hotter conditions, Boardman and Terblanche decided to test the impact of altering the oxygen supply on the metabolism and activity of silk moth larvae and sluggish pupae as the mercury rises.

Gently increasing the environmental temperature from 25 to 60°C over 2 h 20 min, the duo measured the metabolic rates and breathing activity of silk moth larvae and pupae in air ranging from hypoxic (with ∼2.5% O2) to normoxic (21% O2) and hyperoxic (40% O2) to find out whether oxygen availability affected the insects’ thermal tolerance. They also tested the effects of changing the density of normoxic air (replacing some of the nitrogen with low density helium) and the air humidity to see whether physical changes to the air altered how hard the insects had to work to breathe in order to alter their thermal tolerance.

Surprisingly, the team found that the effects of oxygen availability depended on the insects’ life stage. While the pupae seemed unaffected by oxygen availability – maintaining a critical thermal maximum of 50°C at all oxygen levels – the larvae's critical thermal maximum fell from 53°C in normoxic air to 51°C in hypoxic air: the larvae were oxygen limited while the pupae, with a lower metabolic rate, were not. And the larvae required more oxygen to sustain metabolism than pupae at the same temperature, as well as having a higher critical thermal maximum (53°C) than pupae (50°C) in normoxic air. Next, the team analysed the impact of humidity and density on the insects’ thermal tolerance, and found that humidified hypoxic air increased the larval critical thermal limit, suggesting that the water vapour may have somehow increased delivery of oxygen through the larvae's tracheal system. However, altering the air density did not seem to affect how hard the insects had to breathe and did not affect their thermal tolerance.

So, different insect life stages respond to thermal stress in different ways, with oxygen supply determining the thermal limits of fifth instar larvae, while it has little impact on the thermal tolerance of metamorphosing pupae. Realising that some insect life stages may therefore be more susceptible to the detrimental effects of climate change than others Boardman and Terblanche say, ‘Further investigation into life stage-related oxygen safety margins is urgently needed’.

J. S.
Oxygen safety margins set thermal limits in an insect model system
J. Exp. Biol.