Cutaneous respiration by diving beetles from underground aquifers of Western Australia (Coleoptera: Dytiscidae).
Insects have a gas-filled respiratory system, which provides a challenge for those that have become aquatic secondarily. Diving beetles (Dytiscidae) use bubbles on the surface of their bodies to supply O2 for their dives and passively gain O2 from the water. However, these bubbles usually require replenishment at the water's surface. A highly diverse assemblage of subterranean dytiscids has evolved in isolated calcrete aquifers of Western Australia with limited/no access to an air-water interface, raising the question of how they are able to respire. We explored the hypothesis that they use cutaneous respiration by studying the mode of respiration in three subterranean dytiscid species from two isolated aquifers. The three beetle species consume O2 directly from the water, but they lack structures on their bodies that could have respiratory function. They also have a lower metabolic rate than other insects. O2 boundary layers surrounding the beetles are present, indicating that O2 diffuses into the surface of their bodies via cutaneous respiration. Cuticle thickness measurements and other experimental results were incorporated into a mathematical model to understand whether cutaneous respiration limits beetle size. The model indicates that the cuticle contributes considerably to resistance in the O2 cascade. As the beetles become larger, their metabolic scope narrows, potentially limiting their ability to allocate energy to mating, foraging and development at sizes above approximately 5 mg. However, the ability of these beetles to utilise cutaneous respiration has enabled the evolution of the largest assemblage of subterranean dytiscids in the world.