The other day I found a Great Silver Beetle Hydrophilus piceus on the footpath outside the college in Preuilly. It was distinctly poorly and I assume had been hit by a car.
Great Silver Beetles are aquatic and out of the water a dark olive green. In the water however, they are enclosed in a bubble of air which makes them look like quicksilver. The beetles are covered in fine hairs which trap the layer of air.
There are two families of large aquatic beetles, the Dytiscidae and the Hydrophilidae. The way you can tell them apart is that Hydrophilidae have a very prominent pointy keel running between their legs.
Their shape is streamlined and their legs feathered, in order to move through the water. Despite their rather predatory appearance they are mostly omnivorous scavengers (although they will tackle water snails).
They are amongst the largest beetles in Europe and this one measured 5 cm long. It's female.