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

摘要

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 mm(3)), under slightly negative pressure and connected to the gas-filled tracheal system through spiracles on the cuticle. Here, we measure P-O2 of the water and within the plastron gas with O-2-sensing fibre optics to understand the effectiveness and limitations of the gas exchanger. The difference in P-O2 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 O-2 s(-1), depending on swimming activity. The effective thickness of the boundary layer around the plastron was calculated from respiration rate, P-O2 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 mu m thick, which nevertheless can satisfy the demands of resting bugs, even if the P-O2 of the free water decreases to half that of air saturation. Active bugs require thinner boundary layers (similar to 100 mu m), which are achieved by living in moving water or by swimming.