Hydroglyphus geminus (Fabricius, 1792)

Often referred to in the literature as Hydroglyphus pusillus (Fabricius, 1781) or Guignotus geminus (Fabricius, 1792), this is our only UK member of an otherwise speciose genus; it is almost cosmopolitan with about 70 described species, the greatest diversity is in Africa and four species are known from Europe. This is a very widespread Palaearctic species; it is locally common throughout Europe including most of the Mediterranean islands, north into the UK, central Fennoscandia, Faroe and Iceland and extends to the far east of Russia and China, it has recently been recorded from South Korea and is widespread across North Africa and the Atlantic islands. In the UK it is generally common throughout England and Wales though less so in the west and very local and rare in the far north. Adults occur year-round; they overwinter out of water and become active from early March, peaking in abundance during spring and late summer. Typical habitats are sparsely vegetated small water bodies, often on silty soils or sand but the species is a good flier and quick to colonize new sites so should be expected from most still-water habitats, most of our records are from reed bed margins and small ponds and we have also found them in sphagnum pools, cattle troughs and garden ponds. The biology remains unknown but both adults and larvae are predaceous and the phenology might suggest a spring breeder with larvae developing early in the year and new-generation adults through the summer.

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1.9-2.2 mm. Adults are easily recognized by the small size and very distinctive colouration, they do vary a little with respect to elytral pattern but the pronotal and elytral sculpture is always distinctive and so they cannot be confused with any other species. Elongate-oval and discontinuous in outline; weakly but distinctly constricted where the pronotum meets the elytra, dorsal surface with fine recumbent pubescence, often only obvious on the elytra. Head dark with a pale anterior margin, eyes emarginate anteriorly and weakly convex, only slightly protruding from the outline, vertex smooth, frons finely punctured, antennae dark with 4 or 5 pale basal segments. Pronotum pale brown with dark basal and apical margins, broadest across acute posterior angles and narrowed to protruding anterior angles, surface finely punctured, towards the base uneven and with a deeply-impressed oblique impression either side of the middle. Scutellum not visible. Elytra finely punctured and microsculptured throughout, each with a long sutural stria and a short stria at the base which almost aligns with the short pronotal striae, otherwise without impressions. The typical elytral colouration is pale brown with a dark grey base, suture and sub-median transverse band but this varies so that the apical half may be substantially or entirely dark and the pale basal area reduced to longitudinal streaks. Legs entirely pale. Tarsi apparently 4-4-5 due to the diminutive fourth segment on the front and middle legs.