Oxygen-sensitive flight metabolism in the dragonfly erythemis simplicicollis

Insect flight metabolism is completely aerobic, and insect resting metabolism is quite insensitive to atmospheric oxygen level, suggesting a large safety margin in the capacity of the tracheal system to deliver oxygen during flight. We tested the sensitivity of flight initiation and metabolism to atmospheric oxygen level in the libellulid dragonfly Erythemis (Mesothemis) simplicicollis using flow-through respirometric measurements of the rate of CO₂ emission (̇(CO2)). Flight initiations were unimpaired in atmospheric oxygen levels as low as 10 %. However, flight metabolic rate was affected by ambient oxygen level. Flight ̇(CO2) decreased in hypoxic mixtures (5kPa or 10kPa oxygen) and increased in hyperoxic atmospheres (30 kPa or 50kPa oxygen), suggesting that ambient oxygen level influences flight muscle oxygen partial pressure (PO₂) and the vigour of flight. These are the first data to show oxygen-limitation of flight metabolism in a free-flying insect. A low safety margin for oxygen delivery during dragonfly flight is consistent with a previous hypothesis that atmospheric hyperoxia facilitated gigantism in Paleozoic protodonates. However, allometric studies of tracheal morphology, and mechanisms and capacity of gas exchange in extant insects are necessary in order to test the hypothesis that the oxygen-sensitivity of aerobic metabolism increases with body size in insects.