Helophorus strigifrons Thomson, C.G., 1868
Widely distributed although local and sporadic throughout mainland UK; through England there are more records from eastern areas (NBN) with many from East Anglia and south Yorkshire, in Scotland most records are from the southwest. With the exception of Anglesey there are no records from the islands. Adults typically inhabit the margins of small ponds and streams, often in woodland although avoiding the more shaded sites (Hansen), are vegetarian, feeding on filamentous algae and decomposing vegetation. They may occasionally be abundant in grassy pools (Angus, 1978). Included in our Watford list from a single male extracted from a sample of silt and matted vegetation taken from the edge of a small and stagnant pond below Cassiobury park during October 2008.

Eggs are laid in the spring or sometimes in the autumn but in case they do not hatch until the spring or early summer (Hansen), they are deposited in a silken cocoon constructed by the adult and placed in mud or sometimes among vegetation near the waters edge. These egg cocoons are characteristic of many species (Angus, 1973); they consist of an egg chamber and , at one end, a 'mast' which in the present species is leaf-like and concave in form and which may act as a plastron so helping the eggs to survive short periods of submersion. In laboratory rearing experiments by Angus (1974) the eggs usually after a week or so (20c) and the resulting larvae escape the cocoon by biting through a flat area of silk at the base of the mast. Larvae are not aquatic but live among soil and vegetation beside water, they are carniverous and feed for about two weeks during which time they moult twice.Fully developed larvae, 6-7mm in length, burrow into soft soil to pupate, the pupal phase is brief with adults emerging after a week or so but they remain underground, possibly waiting for floodwater. Overwintered adults become active during March and new generation adults occur from early summer; in general the adults are more abundant in the spring and may be found through to October. After emerging the adults undergo a reproductive diapause, the gonads not maturing for three or four months, this may be adaptive, preventing oviposition in sites which may become unsuitable for the larvae through summer drying.

3.0-4.5mm (Friday) (3.5-4.2mm Hansen) Head black, pronotum black with lateral margins narrowly and obscuely paler. Head and pronotum shiny and strongly metallic green, bronze, purple or a mixture of these. Elytra pale brown, darker in places but with an obvious, if obscurely delimited, backward oblique mark behind the middle from the suture on each, forming an inverted V. Head coarsely granulate, less so towards clypeal front margin, and finely punctate. Eyes strongly protruding, excised on hind margin. Y- shaped groove deeply impressed, the basal stem parallel sided and narrow, a little narrower than the width of the last segment of the maxillary palp (look straight down onto this as the cuticle either side of the stem is convex and can confuse matters.) Surface either side of the stem without a longitudinal impression (diagnostic for H.nanus). Antennae 9 segmented, testaceous with club darker. Palps testaceous, darkened towards tip, last segment asymmetric; inner edge almost straight, outer edge curved. Pronotum strongly arched anteriorly, less so across hind margin. Strongly transverse, widest before middle and with sides evenly curved or almost straight to obtuse hind angles, strongly bordered and appearing crenulate. Front and hind margins sinuate. Surface granulate, barely so on disc, strongly so towards lateral margins. Very convex between central furrow and internal furrow opposite second elytral stria. Scutellum rounded, almost circular. Elytra evenly rounded laterally, the side margin obscured from above by the humeral prominence. Each elytron with ten complete, well impressed and strongly punctured striae, without an abbreviated scutellar stria. Interstices convex and finely punctate. Viwed from below the elytral flanks - which are visible from the side as a flat area between two fine longitudinal lines outside the tenth row of punctures - are clearly visible and very nearly as wide as the epipleurs (the reflexed part of the elytra outside the flank) at the level of the metacoxae. Apical margin of the last visible (7th) abdominal sternite smooth. Legs light brown, last tarsal segment darkened towards apex. Tibiae with rows of fine spines, meso and metatibiae with a strong spine inside at apex. Tarsi 5-5-5 although the basal segment is small so they may appear 4-4-4, without spines, with fine hairs above but these are generally not visible. Claws well developed and smooth, each pair of equal length.

Description from 1 Watford specimen at X40

Angus, R.B. 1973 The habitats, life histories and immature stages of Helophorus. Transactions of the Royal Entomolgical Society of London 125(1):1-26
Angus, R.B. 1978 Key etc to British Helophorus species. Balfour-Browne Club newsletter, 11