Elmis aenea (Müller, P.W.J., 1806)
This is generally our commonest member of the family; it occurs in running water throughout mainland UK and Ireland as well as the islands with the exception of Man, the Outer Hebrides and Shetland. On the continent it is widespread and locally common from the northern Mediterranean borders to beyond the Arctic Circle in Scandinavia and is generally most frequent in cold streams and small rivers in upland and mountainous areas. Adults occur year-round and have a long season; they appear from early spring but are most abundant in May and again in late summer and autumn. The typical habitat is well-aerated streams among submerged moss and algae or under submerged or marginal stones, they cannot swim but cling to substrates and may remain submerged for extended periods relying on plastron respiration but they can also respire normally and may leave the water to disperse by flight. Mating occurs in the spring and eggs are laid in small batches among aquatic vegetation in spring and early summer. Larvae are fully aquatic, respiring via anal gills, and they also occur year-round, in the spring mostly under submerged rocks and later in the summer among aquatic algae etc. with generations overlapping, the new generation appears from June and develops through the summer; they pass through 4 instars, overwintering as third or fourth instars, and complete their growth in the spring when they leave the water and pupate among marginal substrate. Freshly eclosed adults are soft-bodied and pale and remain in place for a while until their cuticle hardens; now they may return directly to the water or it is thought they may disperse by flight at this stage. Dispersal also occurs in both the adult and larval stages by drifting with the current and this is mostly nocturnal. Adults may be sampled by general sweeping or by disturbing substrate and netting them as they are dislodged and float downstream.
This small but very distinctive species might only be confused with other members of the family, although without experience Georissus is superficially similar but differs in having very short antennae and lacks the long tarsi of the present species.
1.9-2.2mm. Elongate and broadly oval with the elytra wider than the forebody, and entirely drab brown although most specimens are shiny and to some extent metallic, usually with the head and pronotum darker. In live specimens the head is usually sharply declined and mostly concealed from above. Head roughly sculptured and sparsely pubescent with large convex eyes that follow the outline and parallel and moderately long temples. Antennae pale but usually darkened towards the apex; 11-segmented with all the segments elongate, insertions lateral in front of the eyes. Pronotum transverse and broadest across the base; lateral margins finely crenulate and constricted before acute posterior angles, basal margin strongly bisinuate. Surface very finely punctured and sparsely pubescent, towards the base and lateral margins micro-tuberculate, disc with a long U-shaped furrow which almost reaches the base and extends to the anterior margin. Scutellum small, oval and acutely pointed. Elytra with sloping shoulders and widened to beyond the middle then narrowed and constricted to a continuously rounded apex, each with 5 deeply impressed and strongly punctured striae which extend beyond the middle but fade in the apical third. Interstices roughly sculptured, finely punctured and sparsely pubescent. Lateral margins finely crenulate. Legs robust and proportionally very long, femora and tibiae unarmed, pubescent but without long swimming hairs. Tarsi 5-segmented, the last segment long and broadened to the apex. Claws usually conspicuously yellow against the darker tarsi; large, smooth and without a basal tooth.