Enochrus testaceus (Fabricius, 1801)
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POLYPHAGA Emery, 1886
HYDROPHILOIDEA Latreille, 1802
HYDROPHILINAE Latreille, 1802
Enochrus Thomson, C.G., 1859
A generally common species throughout Europe, though less so in the south, from Spain to Russia and Asia Minor and north to the UK and northern parts of Fennoscandia, also known from Kazakhstan, Siberia and China, in the UK it is generally common across lowland England and Ireland, sporadic and very local in Wales and there are a few scattered records from southern Scotland. Adults occur year-round, peaking in abundance during May and August, typical habitats include well-oxygenated ponds, lake and reservoir margins, ditches, fens and marshes with abundant vegetation, they spend most of their time on submerged stems or substrate but they usually occur in numbers and are easy to find among samples of vegetation, they also fly well and are attracted to UV light traps through the spring and summer. Although they lead a mostly aquatic life they breathe via a plastron held ventrally and under the elytra which they replenish by climbing vegetation and breaking the water surface with their antennae which are covered with hydrofuge pubescence. Mating occurs in the spring and females produce silken egg cases soon afterwards which they carry beneath the abdomen prior to attaching them to emergent vegetation or marginal debris. Larvae leave the cases as soon as they emerge and will remain submerged as they develop through the spring and summer, rising to the surface to breathe through spiracles on the apical abdominal segments, they are predaceous and climb among plants or substrate hunting small insects and other invertebrates, they pass through three instars and are fully grown by the summer. Mature larvae enter a brief prepupal stage where they become more active and leave the water to find a suitable place among damp substrate to form a chamber below the surface in which to pupate. New generation adults appear during the summer; they enter the water and remain there, scavenging and feeding mostly on plant remains, until the autumn when they leave to find overwintering quarters among nearby substrate.
At 5.7-7.0mm this is the largest of our UK members of the genus, and this size coupled with the general appearance and distinctive colour should make it unmistakable among our fauna. Elongate-oval and almost continuous in outline, entire dorsal surface finely and quite densely punctured, head black dorsally, paler anteriorly, pronotum and elytra pale to dark brown, sometimes with various faint darker marks, legs black to dark brown with paler tarsi, antennae pale with the club darker, maxillary palps substantially pale with at least the base of the second (first visible) segment dark brown. Head smoothly rounded in front of weakly convex eyes, evenly convex above, antennae much shorter than the maxillary palps, 9-segmented with a loose elongate club, basal maxillary palpomere curved outwards, apical segment shorter than the penultimate segment. Pronotum broadest in front of rounded posterior angles and smoothly narrowed to projecting anterior angles, surface smoothly convex and without basal fovea. Elytra slightly narrower across rounded shoulders than the base of the pronotum, smoothly curved laterally to a continuously rounded apical margin, surface with several longitudinal rows of slightly larger punctures and a deep and well-defined sutural stria in the apical half. Tarsi 5-segmented. Anterior tarsal claws very strongly curved in the male.