This is a charismatically shaped bug with its long head and dainty long legs that is common and widespread in rivers and stream margins where the current is slow and at the edges of a wide variety of of ponds, lakes and ditches especially around tall emergent vegetation. There is only one other species in the UK, Hydrometra gracilenta, however this is very rare, not found in Northants and only found in a handful of sites in South and South east england. The main differentiating features are the size and eye position on the head.

Mating takes place during May and early June and new adults arrive around three weeks afterwards.

Hydrometra are unable to grasp prey in their unspecialized forelegs. They are sensitive to underwater vibrations and reach down with the rostrum to spear water fleas (Cladocera) through the surface film: the serrate mandibles and maxillae enable them to hold prey at the tip of the rostrum. Ostracods, mosquito larvae and other prey are also taken from water and the bug feeds too upon drowned insects at the water's edge (SWL, 1959).

Distribution:

They are very common and widespread across the county being found in the majority of 10k squares surveyed.

Habitat:

Occurring throughout Britain at the reedy and shad margins of rivers and streams where the current is subdued, this bug is equally common at the edges of ponds and ditches of all types save upland peat pools.

Identification:

The only other species to be confused with this is H. gracilenta which is not known from Northants. This is larger at 9.0-11.5 mm as opposed to 7.0-8.5 mm, the hind femur of H. stagnorum is relatively longer reaching almost to the end of the abdomen while in H. gracilenta it is shorter reaching about two thirds of the length. The eyes of H. stagnorum are located about 1/3 of the way along the head (see below) while in H. gracilenta they reach almost half way.