Berosus signaticollis (Charpentier, 1825)
POLYPHAGA Emery, 1886
HYDROPHILOIDEA Latreille, 1802
HYDROPHILINAE Latreille, 1802
BEROSUS Leach, 1817
BEROSUS Leach, 1817
This widespread and locally common species occurs in lowland marginal situations throughout Europe, from North Africa to Southern Scandinavia and from Portugal east through Asia Minor to west and central Siberia; in the UK it is generally common through southern and central England and South Wales, becoming scarce further north to southwest Scotland. The typical habitat is margins of shallow, often temporary pools in a range of situations, including brackish environments, but they are often very abundant around stagnant heath and moorland ponds exposed to the sun. Within the Hydrophilidae they are among the most adapted to an aquatic lifestyle; adults respire from air trapped beneath the elytra and this is oxygenated from a plastron held beneath the body, the beetles surfacing occasionally to replenish this by breaking the water surface with their hydrophobic antennal clubs and connecting the plastron to the atmosphere, larvae respire using long tracheal gills on the abdominal segments, a feature which will distinguish them from most other aquatic hydrophilid larvae which respire by hanging upside down beneath the surface and exposing spiracles at the abdominal apex to the air. Adults occur year-round; they become active in early spring and persist into late autumn with peaks in abundance during early spring and autumn, they fly in the evening and at night, occasionally occurring at light traps, and are very effective colonizers appearing in the most temporary of habitats e.g. woodland and moorland puddles and small pools caused by rain and may appear in stagnant garden ponds etc. New generation adults mate either in the autumn or the spring after overwintering in marginal habitats, eggs are deposited on aquatic vegetation in a silk ‘cocoon’ and larvae emerge after a week or two, they develop rapidly and pass through three instars before leaving the water to pupate in a cell among marginal soil and roots etc. Adults are phytophagous and possibly also detritivores, often occurring in numbers among mats of algae, while the larvae are predatory, feeding upon a wide range of small aquatic animals. They are thought to be univoltine in the UK.
The rounded elytral apices and dark parallel marks to the pronotum will identify this species. 4.8-6.0mm. Dorsal surface almost glabrous, with very sparse scattered fine hairs, entirely dark to pale brown with the head, two pronotal and various elytral marks darker. Head dark metallic, finely and quite densely punctured; the clypeus generally more densely so than the vertex, with large convex eyes and a distinctly sinuate frontoclypeal suture. Antennae 7-segmented with a pubescent 3-segmented club, maxillary palps as long as the antennae, the terminal segment as long as the penultimate. Pronotum pale brown with two characteristic longitudinal dark metallic markings that do not extend to the anterior or posterior margins but which are sometimes variously united. Surface evenly convex and moderately strongly punctured, the punctures generally separated by at least a puncture width. Elytra pale brown with various small dark marks; evenly rounded and broadest behind the middle, each with 11 complete narrow and punctured striae and a short scutellary striole, interstices flat or very weakly convex, and finely and irregularly punctured. Ventral surface dark, with dense hydrophobic pubescence. Legs long and slender; femora densely punctured and pubescent at the base but legs otherwise glabrous, all tibiae with a strong spine at the inner apical angle, and tibiae and tarsi with long swimming hairs. Male tarsi 5-5-4, with the two basal protarsomeres strongly dilated, female tarsi 5-segmented.