Compressible gas gills of diving insects: Measurements and models

Many diving insects collect a bubble of air from the surface to supply their oxygen requirements while submerged. It has been theorised that these air bubbles may also act as compressible gas gills, as the low oxygen partial pressure (PO2) within the bubble caused by the insect's respiration creates a gradient capable of driving the diffusion of oxygen from the water into the bubble. Under these conditions nitrogen diffuses in the opposite direction, resulting in a situation where the volume of the bubble is continually shrinking while oxygen is obtained. This study measures changes in volume and PO2 within the gas gills held by a tethered water bug, Agraptocorixa eurynome. Both gill volume and PO2 drop rapidly at the beginning of a dive, but eventually the PO2 reaches an apparently stable level while volume continually declines at a slower rate. Active ventilation of the gill is crucial to maintaining oxygen uptake. These measurements are used to calculate oxygen flux into the gas gill and the oxygen consumption rate (V˙O2) of the bug. The effectiveness of a gas gill as a respiratory organ is also demonstrated by determining the critical PO2 of the water bug and comparing this with measured gas gill PO2 and calculated V˙O2.