Limnius volckmari (Panzer, 1793)
This is among the most common of the European members of the family, it occurs from lowland to alpine altitudes across western and northern Europe, extending into northern Russia and reaching the UK, Iceland and beyond the Arctic Circle in Fennoscandia; it is locally common and often abundant throughout the UK, including all the islands north to Shetland, and is widespread in Ireland. Adults occur year round, they peak in abundance during April and May and again in September and October and they appear to be active through the winter in all but the coldest conditions. Typical habitats are among stones and debris in streams and rivers but they occasionally occur under stones on shallow and sparsely-vegetated lake margins, they usually occur in numbers, often in abundance and in warmer weather they sometimes swarm on wet riparian mud and occasionally climb marginal plants. Adults use plastron respiration, they have been observed to fly but it is not known whether dispersal flights occur and it seems that most of the adult life is spent under water. Larvae are present year-round and all instars occur throughout the summer; the adult phenology might suggest a primarily spring or autumn breeder but this is not known for sure. The species has been bred under artificial conditions and females have been observed to lay small batches of eggs on filamentous algae, these hatch within about a month and larval development continues through the year with second and third (of seven) instars overwintering, pupation occurs among marginal substrate from early spring and this stage lasts between one and three weeks. Both larvae and adults are thought to feed on algae and diatoms as well as decaying plant material and both stages often occur together under stones or among open substrate, they are known to disperse by walking or to be carried by currents, mostly at night, but they generally travel only short distances before returning to the substrate. Although common this species can be annoyingly difficult to find, samples of stony substrate can be taken and carefully examined but the beetles are small and easily overlooked, or the substrate can be churned up so that the resulting debris passes through a net held close by, and on rare occasions they may be found on wet marginal substrate. Any reasonably energetic water is worth searching, especially in upland and mountain areas e.g. in Baltic countries they are most common among mosses and stones in the upper and middle reaches of upland and forest streams and much less frequent in lowland areas.
Limnius volckmari 1
© Lech Borowiec http://www.cassidae.uni.wroc.pl/Colpolon/index.htm
Limnius volckmari 2
© U.Schmidt www.kaefer-der-welt.com
2.9-3.2 mm. One of the largest of the UK members of the family, body dark grey or black with sparse and very fine pubescence, antennae brown but darkened towards the apex, legs dark grey with reddish tarsi. Head hypognathous; retracted into the thorax and only narrowly visible from above, smoothly convex and finely punctured, with large round eyes placed laterally and filiform antennae with all segments elongate. Pronotum transverse, raised and curved laterally to slightly acute posterior angles and protruding anterior angles, basal margin bisinuate and apical margin produced, surface convex, very finely punctured and with a curved longitudinal keel either side of the middle. Elytra with rounded shoulders, widest behind the middle and continuously-curved apically, surface finely punctured and rugose throughout and each with six strongly punctured striae complete almost to the apex. Legs long and robust, with massively developed femora and smooth, comparatively-slender tibia. Tarsi 5-segmented; the basal segments simple and the terminal segment greatly elongated and broadened towards the apex. Claws smooth and equal in length, front claws almost straight, middle and hind claws strongly curved. Easily distinguished among our UK species by the discrete longitudinal pronotal keels and absence of keels to the elytra.