Challenges associated with tidal ventilation in dragonfly nymphs

Insects have reinvaded the aquatic environment multiple times across their evolution, with five orders having evolved predominantly amphibiotic life cycles consisting of water-breathing juveniles and air-breathing adults. Among these water-breathing insects, dragonfly nymphs are unique as they possess an internal rectal gill which is tidally ventilated: water is drawn into and expelled out of the rectal gill through the nymph's anus. Since water is more viscous and dense than air, the cost of aquatic ventilation is innately higher than breathing air, while aquatic tidal ventilation is expected to be particularly costly as water must be accelerated, decelerated, then re-accelerated in the opposite direction. In addition, the mixing of inhaled fresh water and residual stale water in a tidally-ventilated gill is expected to further limit gas exchange by reducing the gradient for oxygen diffusion into the insect. Thus, the predicted high energetic cost and reduced efficiency of aquatic tidal ventilation is used to explain why the vast majority of water-breathing animals are unidirectional ventilators, and only a few select groups have evolved tidal ventilation (e.g. certain freshwater turtles, sea cucumbers, and dragonfly nymphs).
The current study aims to test this prediction by investigating the cost and efficiency of the dragonfly nymphs' rectal gill, by measuring the water-convection requirement and whole gill gas conductance. A custom-designed respiro-spirometer will simultaneously measure the metabolic rate and ventilation frequency/volume to calculate how much water must be ventilated to meet metabolic demand and how permeable the rectal gill is to oxygen and carbon dioxide.