Hydrophilus piceus (Linnaeus, 1758)
Great Silver Water Beetle
This is the most widely distributed of the European Hydrophilus species, occurring locally throughout the Palaearctic region north to Southern Scandinavia and south to the Mediterranean including North Africa. In the east it extends through Siberia and Northern India although the distribution is discontinuous throughout following local extinctions e.g. in Norway and Luxembourg, and it is now scarce in many regions. They occur in a wide range of wetland habitats; in Greece they inhabit lagoons and estuaries and have been recorded at elevations up to 1000m. The U.K. distribution is now rather restricted following a recent decline; there are modern records from south-western and eastern England and south-western Wales, many of which are coastal or near-coastal. Adults are long-lived, up to 3 years; they become active early in the year and remain so into the winter, their usual habitat is well-vegetated, often shallow, and permanent lowland water bodies; marshes, drains and especially coastal grazing marshes. They are strong fliers and readily come to light, sometimes at considerable distances from suitable habitat. Adults are omnivores but generally favour plant material and detritus etc. Females oviposit from April to June, 50-70 eggs are enclosed in a silken cocoon which is set afloat or left among marginal vegetation. The larvae are predatory, especially upon freshwater snails (Lymnaeidae), drilling holes through the shells to feed upon the soft bodies. They occur from April onwards and develop rapidly; they are full-grown in about 4 weeks, reaching up to 7cm in length, and in late summer leave the water to pupate in a silken cocoon among marginal soil etc. although they may also wander for long distances, they have been observed on pathways up to 200m from the nearest water, moving with an energetic ‘looping’ motion, and when disturbed they can emit a loud squeak.
The large size, 34-50mm, and distinctive shape should make this species impossible to confuse with any other U.K. species. The entire dorsal surface is black, often with a faint blue or green tinge, which is more obvious towards the lateral margins in live specimens. The dorsal surface is smooth and shiny with only very fine punctation apart from series of larger punctures beside the eyes
and towards the lateral pronotal margins. The elytral striae vary in strength but are usually distinct, punctured and complete to the apical margin which bears a short and sharp spine. The mesosternal keel is wide, and proportionally more so in the male, and bears a short longitudinal impression that does not extend to the apex. The metasternal ridge is produced backwards into a long spine which extends over the first abdominal sternite. The mid and hind tarsi are lined with swimming hairs along the inner margins and the terminal segment of the pro-tarsi is widely dilated in the male.
Hydrophilus piceus elytral spines.
HYDROPHILUS Geoffroy, 1762
A genus of about 50 species included in 3 subgenera with a worldwide distribution but despite their large size and distinctive appearance they are still only poorly understood in tropical regions and it seems that many more will eventually be described. All are very large water beetles, among the largest in the world, with the greatest diversity in tropical regions; 3 species occur in Europe and 3 in North America. They are of a characteristic appearance and the majority are black and shiny although some appear greenish or blue, especially when submerged, and some tropical species are deep metallic green. The common name of Silver Water Beetles refers to the appearance of the underside which, when submerged, is covered with a layer of air. All swim with alternating leg movements and carry air beneath the elytra which they renew by surfacing head first. The adults are in general detritivores or vegetarian but will also feed on worms etc. while the larvae are predatory upon aquatic animals. They occur in a wide variety of aquatic habitats but in general inhabit well vegetated still or slow-moving and shallow water; they fly well and come to light or will alight on illuminated surfaces, sometimes far from water. Adults are long-lived, up to 2 or 3 years, and generally leave the water to overwinter among marginal vegetation or in soil etc. but some remain active through the year; in high latitudes some species are active in the winter under ice, and at this time they may congregate in large numbers. Oviposition occurs in the spring; the females use structures similar to arachnid spinnerets to produce egg cocoons which are generally placed among marginal vegetation or left to float. The larvae are aquatic but when fully grown leave the water to pupate among marginal soil etc. and have occasionally be found far from water.
All species of the genus are large, between 40 and 50mm. In outline they are oval with the elytra broadest about, or a little behind, the middle and characteristically narrowed towards the apex. The dorsal surface is usually black and shiny although some tropical species are variously microsculptured; apart from a general very fine punctation there is usually a series of larger punctures beside each eye and towards the lateral margins of the pronotum, and these vary in size and number. The head is proportionally large and, including the eyes, as broad as or very nearly as broad as, the pronotal apex so giving the insects a more or less continuous outline. The palps are much longer than the antennae with all the segments narrow and elongate; the terminal segment is at most as long as the penultimate. The antennae are 9-segmented with a loose, asymmetric 3-segmented club above an apically expanded pedicel. The anterior clypeal margin is widely and weakly emarginate so exposing the membrane supporting the labrum. The pronotum is smoothly narrowed towards the anterior margin and finely bordered laterally, the scutellum is always large. Elytra with 10 rows of punctures which are very variable in strength and striae which are generally more strongly impressed towards the apex, the epipleurs narrow from about the middle and sometimes do not reach the apex. Apical margin variously dentate or smoothly rounded. The prosternum is raised medially and deeply excavate at the base to receive the base of the mesosternal keel. Mesosternum broad and flat medially and variously grooved, generally more strongly so in the male. The metasternal ridge is produced apically to form a long and sharp spine which generally overlaps the first abdominal sternite although in some tropical species this can be very long. Hydrofuge pubescence generally covers the ventral parts of the thorax and at least, although this is variable, the lateral parts of the abdominal sternites. The sternites are variously sculptured; flat or convex laterally or raised medially, sometimes forming a central ridge. Legs relatively long and robust, the tibiae have long apical spines and the meso- and metatarsi are clothed with long swimming hairs along the outer margins. Sexual dimorphism is strong with the males having modified protarsi and ventral sculpture.
Of the 3 European species only one, H. piceus (Linnaeus, 1758), occurs in the U.K. and is distinctive in having a sharp spine at the elytral apex (below). H. aterrimus Eschscholtz, 1822 occurs in eastern and central Europe south to the Mediterranean and east to Siberia, in this species the apical spine is short and blunt. H. pistaceus Laporte, 1840, which lacks the spine altogether, occurs in southwest Europe and North Africa.