Gyrinus marinus Gyllenhal, 1808

Although widespread across England, Wales and Scotland the distribution of this species is discontinuous and patchy; there are groups of modern records from East Sussex and Kent, south Hampshire, Avon and it is well represented across the midlands and East Anglia but north of the east Riding records are mostly western coastal north to Inverness. There are very few west country records, a few from the south coast of Wales and the Dee valley, also Anglesey (older record) and Man but not, apparently, from Wight or any of the Scottish Islands (NBN). This species is common in fresh peaty and brackish waters, often forming large (Holmen (1987) gives 'very large') schools on open stretches of canals, lakes or rivers (Balfour-Browne) and often in company with other Gyrinus species. They usually avoid vegetated areas such as marshes although they occasionally occur in the backwaters of streams (Holmen). We have yet to record the species from Watford area but we are optimistic; we recorded them as abundant in Denham (Sept 04) only 12km to the southwest, and there are a few very close records on the NBN map.

Larvae have been observed in late spring and summer and freshly emerged adults in July. The adults are known to hibernate clinging to submerged plants and are capable of Flight (Holmen). According to Balfour-Browne the beetles are usually recognisable by the broad oval shape, black claws and habit of 'running' in large schools on canals and lakes.

4.5-7.5mm (Friday) Broad oval, widest at or slightly behind middle. Convex, notably so anteriorly, shiny black with bluish or bronzy lustre, scutellum, elytral suture and margins contrasting metallic golden under strong light. Upper surface glabrous although the terminal abdominal segments are clothed with dense long and pale pubescence. Head shiny, cuticle finely wrinkled and with two or three deep impressions beside each eye, eyes convex and protruding, labrum strongly wrinkled, transverse and continuous with outline of head. Pronotum transverse, broadest at base and very convex, with well developed explanate lateral borders. Front and hind margins strongly sinuate, with a row of strong punctures behind front margin from front angles to near middle where they attenuate. Elytral striae punctate and well developed; 3 and 4 terminate about two thirds from base, 1, 2 and 5-11 continue to behind apex, the punctures becoming much stronger in apical third. Transverse 'lens shaped' group of punctures before apex well developed. Interstices becoming flatter posteriorly, in the male with very fine puncturation, barely visible at X20, and some obscure microsculpture, in the female with dense puncturation which is obvious at X20 so the elytra appear a little duller. Explanate marigin widens continuously from shoulder to behind middle before it narrows abruptly before the outer apical angle. Apical margins separately rounded and somewhat flattened. Legs yellow, contrasting against the otherwise dark underside. Mid and hind claws black or a little lighter towards base. Penis distinctive; sides gently sinuate, tapering continuously from base to a sharp point or very narrowly rounded apically.

Description from 2 Denham species at X20