Hydrobius fuscipes (Linnaeus, 1758)
The most widespread member of the genus and common throughout much of its range, occurring throughout Europe, Asia including the Himalayas, North Africa, temperate parts of the Middle East and North America. It is the only species to occur in the U.K. and is generally common and often abundant north to Orkney. The species inhabits both fresh and brackish water, preferring to live in small and sunlit pools and ponds with plenty of marginal vegetation, and will often occur in temporary pools. Adults appear during the first warm days of spring, they fly well and come to light, often far from suitable habitat, and will alight on reflective surfaces e.g. car bodywork. They occur year-round with the peak of abundance from July to September, during the winter they often turn up in samples of marginal vegetation, leaf litter and tussocks etc. Adults swim with alternate movements of the legs and replenish their air by surfacing head-first. Mating occurs in the spring and females oviposit and construct egg cocoons which are attached to marginal plants. The larvae appear in May and June about a week after oviposition and soon begin feeding upon small invertebrates etc. when larger they hunt aquatic prey and drag it out of the water for consumption. Pupation occurs from mid-June and new generation adults emerge from July onwards creating a peak of abundance as they add to the previous generation adults.
The size and elytral punctation will readily identify this species. 5-8mm Convex-oval and flat below, entirely black and weakly metallic or with the lateral margins and appendages pale. The entire upper surface is finely punctured. Head with an oblique series of punctures beside each eye. Antennae 9-segmented with a 3-segmented club, inserted in front of the weakly convex eyes. Palps about as long as the antennae. Pronotum transverse and widest at the base; the lateral margins bordered and with a group of large punctures inside the front angles. Elytra completely covering the abdomen, each elytron with 10 punctured striae and an abbreviated scutellary stria; odd-numbered striae have a series of larger punctures, at least in the basal half. Interstices flat, becoming convex towards the apex.
HYDROBIUS Leach, 1815
This is a small genus of about 10 species distributed throughout the Holarctic, The Caribbean and Australasian regions. Two species occur in North America and there are four in northern Europe:
H. arcticus Kuwert, 1890 is an alpine arctic species.
H. rottenbergii Gerhardt, 1872 occurs in coastal rock pools. It was formerly considered to be a variety of H. fuscipes.
H. subrotundatus Stephens, 1829 is a widespread species of shaded cool waters and the vegetated margins of slow-moving waters. Also formerly considered as a variety of H. fuscipes.
H. fuscipes Linnaeus, 1758 is a widespread and common species of stagnant ponds etc.
The genus soon becomes familiar; elongate-oval and convex medium sized beetles, 4.5-9mm, and flat underneath. Head convex with weakly protruding eyes; the frons with an oblique row of setiferous punctures behind the eyes and the clypeus with a transverse row of setiferous punctures either side. Antennae 9-segmented. Maxillary palps as long as the antennae, the terminal segment longer than the penultimate. Pronotum transverse and narrowed anteriorly, laterally with 2 oblique rows of punctures. Elytra with 10 punctured striae and an intercalary stria which is sometimes very weakly impressed. The interstices are flat or, especially towards the apex, weakly convex and with irregular and quite strong punctation; odd-numbered interstices with a row of larger setiferous punctures. The underside is finely shagreened, densely punctured and pubescent. Prosternum raised medially but not ridged. Mesosternum raised posteriorly, often forming a distinct and sharp process. Metasternum with a fine ridge between the mesocoxae which is not continued posteriorly. The abdomen has 5 visible sternites. The legs are long and slender; the femora densely punctured and pubescent towards the base, smooth and sparsely pubescent towards the apex. Tibiae straight with fine spines along the outer margins; pro-tibiae with a pair of small apical spines on the outer margin and a larger, curved spine either side of the tarsal insertion. Meso- and meta-tibiae with 2 large and unequal spines at the inner apical angle. Tarsi 5-5-5; the meso- and meta-tarsi with fine swimming hairs. The basal segment on all the tarsi is small, less than half as long as the second segment. Apical segment of all tarsi very elongate and with well-developed and smooth claws.
Slightly larger (8-10mm).
Only sutural stria deeply impressed towards the apex.
Mesosternum distinctly raised into a central keel.
Considerably rarer, almost exclusively coastal.
© Lech Borowiec