- Philip G D Matthews: Ecology and Evolutionary Biology, School of Earth and Environmental Sciences, Darling Building, University of Adelaide, Adelaide, SA 5005, Australia. philip.matthews@adelaide.edu.au
Unlike all other diving insects, backswimmers of the genus Anisops can exploit the pelagic zone by temporarily achieving near-neutral buoyancy during the course of a dive. They begin a dive positively buoyant due to the large volume of air carried in their ventral air-stores, but rapidly enter a protracted period of near-neutral buoyancy before becoming negatively buoyant. This dive profile is due to haemoglobin found in large tracheated cells in the abdomen. Fibre optic Oxygen probes placed in the air-stores of submerged bugs revealed that Oxygen partial pressure (P(O(2))) dropped in a sigmoid curve, where a linear decline preceded a plateau between 5.1 and 2.0 kPa, before a final drop. Buoyancy measurements made by attaching backswimmers to a sensitive electronic balance showed the same three phases. Inactivating the haemoglobin by fumigating backswimmers with 15% CO eliminated both buoyancy and P(O(2)) plateaus. Oxygen unloaded from the haemoglobin stabilises the air-store during the neutrally buoyant phase after a decrease in volume of between 16% and 19%. Using measurements of air-store P(O(2)) and volume, it was calculated that during a dive the haemoglobin and air-store contribute 0.25 and 0.26 microl of Oxygen, respectively.