Why do water boatmen swim upside down?

Water beetles and boatmen are aquatic insects, yet still obtain oxygen by breathing air via holes (tracheae) along the sides of their bodies, rather than directly from the water using gills.

They maintain a slim bubble of air against their bodies, trapped by water-repelling hairs that prevent saturation. This bubble, called the plastron, is replenished each time the insects come up for air.

Oddly, while beetles and lesser water boatmen generally break the surface back-first, true water boatmen (Notonecta), also called back-swimmers, swim upside-down and break the surface belly-up. The evolutionary origin of this difference is lost, but in Notonecta this unusual orientation is controlled by the antennae.
A bubble of air trapped around these appendages bends them up and away from the head.

If the insect becomes inverted the bubble pulls the antennae towards the head, which the insect detects and ‘rights’ itself. If the bubble is moved experimentally, with a fine needle, the back-swimmer can be tricked into front-swimming.

Main image: Water boatman (Notonecta) © Clouds Hill Imaging Ltd/Getty Images