Cybister lateralimarginalis

A PERFECT WATER BEETLE. Even more streamlined and flattened than the more common Great diving beetle, and with broad, flat hind legs. The widest part of the body lies behind the centre, which give it a fast droplet shape.
A beautiful beetle: black with a golden stripe over the borders. In clear, direct light the beetle has a green shining, and the black appears to be a very dark green. In the ditch you may mistake it for a Great silver beetle, but the well visible hind legs are moved together in one stroke, not alternating like that beetle does.
Those swimming legs are positioned relatively far to the back, because of the short abdomen. Comparable with diving ducks, like those birds Cybister is also a bad walker on land.
In a dipping net the beetle jumps wild and uncontrolled with its strong hind legs, it's able to jump out of the net if it isn't deep enough! Large thorn like appendages on the hind legs help the beetle when crawling on a surface, I have seen it shuffle forward surprisingly fast on a slope. And it's a very good flyer, so it only needs to crawl a short distance out of the water to reach distant places.

The under side of Cybister is smoother than that of the Great diving beetle, and it's pale yellow and rather transparent. I've seen the beetle clamped itself between a water plant and the bottom to take a rest. Because of its smooth under side he managed that without difficulty. It enables the beetle also to hide very well: under the roots of water plants it's very difficult to trace him back. The speed of the beetle is mostly mentioned as the reason that it's caught seldom in a net, but I'm more under the impression the crawling in hideouts is the cause of that matter. Because the beetle can indeed swim fast, but most of the time has a rather relaxed cruise speed.

The larva of Cybister is caught more often than the beetle. It resembles the larva of the Great diving beetle, but has a number of differences. It's unable to hang below the surface in an "S-shape", but needs support from water plants or the bottom to get the tip of the abdomen to the surface. The larva is a predator who lives from other water insects, which are injected with digestive saliva and sucked out. After a few molts it may grow to 8 cm and than it crawls out of the water to make a "pupal cell" near the water in the mud, below moss. In this cell it pupates (picture at right) and after about 4 weeks the beetle emerges.