Liopterus haemorrhoidalis (Fabricius, 1787)
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This is a generally common species throughout most of Europe; north to southern Scandinavia and the U.K. and south to the Mediterranean, northwest Africa and Asia Minor to Iran. In the U.K. it is locally common in lowland situations throughout England and Wales including the Isle of Wight, Scilly, Anglesey and Man, and there are many records from southwest Scotland. Adults occur throughout the year, they are active from April to September or October and inhabit still or slow-moving water with plenty of marginal vegetation and debris, they may also occur in temporary pools on moors and heaths. They are predaceous and carry air beneath the elytra which they replenish by protruding the abdominal apex through the surface. The larva is predaceous, consuming liquid food which is sucked up through hollow mandibles, and lacks gills, coming to the surface to breathe through modified spiracles on the terminal abdominal segment. Fully grown larvae leave the water to pupate in damp marginal soil.
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Among the U.K. fauna this narrow and medium-sized red species is readily recognized in the field, more specifically it is distinguished among our water beetles by the notched eyes, exposed scutellum and the metacoxal lines which closely approach the middle of the metasternum. 6.3-8.0mm Elongate-oval and continuous in outline with the head rounded ant the elytral apices acuminate. Dorsal aspect red with the head, pronotum and elytra variously darkened, ventral surface is mostly black. Entire beetle faintly microreticulate and punctured. Head with weakly convex eyes that follow the outline, short palps with the terminal segment cylindrical and 11-segmented filiform antennae inserted on the side in front of the eyes, the insertions not visible from above. Pronotum transverse and smooth on the disc, coarsely punctured behind the anterior margin and rugose laterally. Each elytron with several longitudinal series of larger punctures.
COPELATINAE Branden, 1885
This primarily tropical subfamily includes 7 genera and about 700 species the placement of which has undergone some recent changes and the group is likely to remain dynamic in this respect. The majority are included in the pan-tropical genus Copelatus Erichson, 1837 which includes more than 450 species in at least 15 ‘species groups’, some of which are difficult to define, it is only poorly represented in temperate regions; it does not occur in Europe and of the 8 species recorded from the United States most are from the south. It includes many Neotropical species found in water-filled palm-bracts and several cave dwellers which lack eyes and hind-wings and have long sensory setae. Liopterus Dejean, 1833 (formerly included in Copelatus) includes 2 western Palaearctic species of which L. haemorrhoidalis (Fabricius, 1787) occurs in Europe including the U.K. Agapopomorphus Zimmerman, 1921 includes 10 Neotropical species mostly recorded from light-trapping. Aglymbus Sharp, 1881 includes about 20 African and Neotropical species, some of which are recorded from Bromeliads. Exocelina Broun, 1886, originally a subgenus of Copelatus, includes more than 160 Australasian species, about 90 of which occur in New Guinea, some of the Australian species are subterranean. Lacconectes Motschulsky, 1885 includes about 40 eastern- and southeast-Asian species. Medaglymbus Shaverdo & Balke, 2008 includes 10 Madagascan species formerly includes in Aglymbus.
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All species are small and elongate-oval in form, many are drab but the group also includes some brightly-coloured tropical species. The eyes are emarginate behind the antennal insertions and the scutellum is exposed, the elytra vary but many species have distinct striae. The prosternal process is short and level with the prosternum or sometimes slightly raised behind the coxae, the metepisternum extended to border the mesocoxal cavity, the lateral metasternal extension very narrow, and the metacoxal process angled or with an apical notch. The metafemora lack a row of setae behind the lateral posterior angles, a feature present in the Agabinae. Tarsi with 5 distinct segments, in the male the basal pro- and meso-tarsomeres are dilated. Metatarsal claws equal in length.