Backswimmers in the pool are a common sight in Australia, according to Philip Matthews from the University of Adelaide. It was only when Matthews started investigating these curious little insects in more detail that he began to appreciate how unique they are. For example, most diving insects are positively buoyant so bob around at the surface or actively swim to remain submerged. But backswimmers can achieve neutral buoyancy and float at depths of up to 0.5 m, which made Matthews wonder how backswimmers maintain neutral buoyancy when other insects have to struggle to plumb the depths. Knowing that diving insects, including backswimmers, always take a bubble of air down with them for oxygen when they become submerged and that backswimmers have extraordinarily high levels of oxygen-carrying haemoglobin, Matthews and his supervisor Roger Seymour were curious to find out how these remarkable diving bugs combine the two to manage their buoyancy().
But working with the rice grain-sized insects was quite challenging. The team needed state of the art oxygen probe technology to monitor the oxygen levels in the microscopically small diving bubbles, and when Matthews pierced the minute bubble with the probe he was in for a surprise. Instead of falling continually as the insect breathed, the oxygen levels fell for a few minutes,before plateauing at 4 kPa. Matthews admits that he had expected to see a constant fall in the oxygen level as the insect consumed its SCUBA store.
Wondering whether the insect's unusually high haemoglobin level may have something to do with this strange effect, Matthews fumigated another insect with carbon monoxide, to prevent the haemoglobin from recharging the bubble's oxygen supply. Then he submerged the bug and remeasured the bubble's oxygen level. This time the plateau vanished and the oxygen level fell continuously. Matthews realised that the haemoglobin was supplementing the bubble's oxygen level once it had fallen below 5 kPa. The bubble's oxygen level only began falling again when the haemoglobin store was empty, which the insect took as its cue to return to the surface.
Having found that bubble's oxygen level was stabilised by the haemoglobin store, Matthews and Seymour wondered how the insect's buoyancy varied during the course of a dive. Submerging an insect and attaching it to a precise electronic balance, Matthews found that the insect's buoyancy closely matched the oxygen trace. As the bubble's oxygen level initially fell the insect's buoyancy declined, until the oxygen level plateaued and the insect achieved neutral buoyancy. When the oxygen level began falling again, the insect became negatively buoyant and would have sunk if it had not been attached to the balance.
But why was the bubble's buoyancy changing? Calculating the bubble's volume from its buoyancy, Matthews realised that the volume decreased by 17% until the oxygen level stabilised. Matthews explains that the bubble's volume decreased because the inhaled oxygen was replaced by exhaled carbon dioxide,which is lost in turn because it dissolves readily in water. This reduces the bubble's volume until its buoyancy matches the backswimmer's negative buoyancy and the insect becomes neutrally buoyant.
So backswimmers keep their buoyancy bubble topped up with oxygen from their haemoglobin stores to maintain neutral buoyancy while plumbing the depths.
