POLYPHAGA Emery, 1886
HYDROPHILOIDEA Latreille, 1802
HYDROCHIDAE Thomson, C.G., 1859
No Common Name
Of the seven British species only one (H. angustatus) is widespread and locally common in a variety of wetland habitats, while the other six are very rare.
The Hydrochidae contains about 180 species of the single genus Hydrochus Leach, 1817. They are small and elongate beetles, somewhat reminiscent of Helophorus species but lacking the longitudinal pronotal furrows, formerly included in the Hydrophilidae. The group is represented in all the zoogeographical regions with the European fauna being relatively poor; about 7 species occur in central Europe, the same number as occur in the U.K, and 4 or 5 in Scandinavia whereas 26 occur in North America and at least 25 in Australia where many species are common throughout their ranges. All inhabit well-vegetated marginal lentic habitats where both the larvae and adults are aquatic but do not swim; they crawl on vegetation where the adults are herbivores and detritivores. The larval feeding habits are not known. Adults are generally slow moving and cryptic and will often remain motionless for some time in the water net and so may be overlooked. Adults vary only in detail and so a familiarity with the U.K. fauna will allow all members of the genus to be recognized.
The group varies widely in size, from about 1mm to about 7mm, the U.K. species from 2.5 to 4.7mm. They are elongate beetles with the head and pronotum about equal in width and narrower than the elytra. Head variously punctured and microsculptured; strongly constricted behind very convex and prominent eyes and with a Y-shaped impression on the frons, the anterior transverse suture generally well developed. Clypeus produced in front of the eyes and weakly rounded or truncate anteriorly. Antennae 7-segmented; the scape long and curved, as long as the next 2 segments and the cupule combined, and segments 5-7 form a distinct and pubescent club. Maxillary palps at least as long as the antennae, with the terminal segment thicker and slightly longer than the penultimate. Pronotum quadrate to slightly transverse or elongate, and parallel or narrowed towards the base. The disc is variously punctured and microsculptured and has wide, shallow impressions, generally in 2 transverse series; across the middle with three impressions and across the base with four impressions.
Scutellum triangular and elongate; small but usually distinct. Elytra elongate and usually rather narrow, sometimes parallel-sided, often dilated behind the middle or smoothly, but not strongly, rounded. Each elytron with 10 strongly punctured striae, the interstices vary from flat and finely punctured to convex or, often only on alternate interstices, carinate for some of their length, or with raised tubercles. Without a scutellary stria. The apical margins generally have a series of 3 or 4 punctures which are larger and distinct from those of the striae and these are sometimes useful in identifying the species. The underside is generally very uneven with ridges, furrows, deep punctures or shallow round pits, and entirely covered with dense, fine hydrofuge pubescence that gives the surface a velvet appearance. Legs long and slender. Tarsi 5-5-5 but the basal segment is tiny and so they may appear to be 4-4-4. Most species are drab, black to various shades of brown, but some are distinctly metallic.
All the U.K. species are very local and most are rare; at least some may be recorded throughout the year but most records are from the spring and autumn. Identification can be very difficult on external morphology and many specimens will need to be dissected. Of our U.K. species 5 display sharply defined ridges on alternate elytral interstices while 2, H. angustatus Germar, 1824 and H. nitidicollis Mulsant, 1844 have all the interstices smoothly convex.