Respiratory function of the plastron in the aquatic bug Aphelocheirus aestivalis (Hemiptera, Aphelocheiridae)

The river bug Aphelocheirus aestivalis is a 40 mg aquatic insect that, as an adult, relies totally on an incompressible physical gill to exchange respiratory gases with the water. The gill (called a 'plastron') consists of a stationary layer of air held in place on the body surface by millions of tiny hairs that support a permanent air-water interface, so that the insect never has to renew the gas at the water's surface. The volume of air in the plastron is extremely small (0.14 mm³), under slightly negative pressure and connected to the gas-filled tracheal system through spiracles on the cuticle. Here, we measure Po₂ of the water and within the plastron gas with O₂-sensing fibre optics to understand the effectiveness and limitations of the gas exchanger. The difference in Po₂ is highest in stagnant water and decreases with increasing convection over the surface. Respiration of bugs in water-filled vials varies between 33 and 296 pmol O₂ s⁻¹, depending on swimming activity. The effective thickness of the boundary layer around the plastron was calculated from respiration rate, Po₂ difference and plastron surface area, according to the Fick diffusion equation and verified by direct measurements with the fibre-optic probes. In stagnant water, the boundary layer is approximately 500 μm thick, which nevertheless can satisfy the demands of resting bugs, even if the Po₂ of the free water decreases to half that of air saturation. Active bugs require thinner boundary layers (∼ 100 μm), which are achieved by living in moving water or by swimming.Roger S. Seymour, Karl K. Jones and Stefan K. Het