Like many other small species of Water scavenger beetles Chaetarthria may crawl belly up, right underneath the water surface, and in this way bring its whole under side in contact with the air above the water. This can be seen on the left picture above. In this way it replenish the air supply that is hold on the belly by a layer of water repellent hairs, visible on the picture above right. You can imagine the surface tension is much more noticeable for an insect of only 1.5 mm tan it is to us.
On the picture below left the beetle has got a bit clumsy hold of a stem, under side visible. On the picture below right one of the club shaped antenna is stretched out. It has a grey appearance caused by a layer of hairs. All water scavenger beetles use their antennae to get air form the surface.
Puzzling name Chaetarthria: breaks in chaet-: hair, bristle and arthria which may mean artery or joint. Bristled joints seems plausible, but this beetle has a hollow space with hairs on the under side, covered by a membrane: so maybe these are seen as "haired arteries"? Please mail me if YOU know!