Hydrochus angustatus Germar, 1824
This is a very local species in the U.K; widespread in the southeast and with scattered, mostly western records north to southern Scotland. Typically a species of ponds, ditches and gravel-pit margins, especially from heathland habitats. They may be abundant where they occur, adults occur throughout the year and are generally common from April to June and again in September. A population we have been observing near St. Albans in South Hertfordshire has been thriving in a few metres of well vegetated and shallow margins of gravel pit from 2012 to 2016, the substrate is more or less bare gravel and the beetles tend to keep to the areas shaded by overhanging shrubs. Sweeping this small area in May or June will invariably produce a few adults while they seem to be absent from adjacent areas. At this site it would seem that in order to find the species a very extensive and careful survey would be needed and it would be very easy to miss, if this degree of localization is typical then it is probably an under recorded species. In the spring and early summer we have also found them commonly in temporary habitats e.g. flooded tyre ruts in woodland areas of the New Forest.
The relatively large size and the form of the elytral sculpture will identify typical specimens of this species among the U.K. fauna. 3-4mm. Slender and almost parallel-sided, upper surface black or very dark brown with a distinct metallic green or blue lustre; the Herts. specimens are almost all bright metallic green. The head is uneven with large and deep punctures, the cuticle between appearing smooth at X40. Labrum truncate, mandibles curved and sharp with a ridge along the upper surface. Palps brown with the terminal segment dark in the apical third or quarter. Pronotum slightly transverse with 3 wide and circular depressions across the centre and 4 slightly elongate depressions across the base. Elytral striae deeply punctured, entire to the apex and wider than the interstices. The elytral intervals are gently convex and lack ridges although there may occasionally be short and generally weak ones towards the apex, the lateral apical punctures are slightly elongate to circular, or nearly so. Legs pale brown with the femoral/tibial junction and the apex of the claw segment vaguely darkened. Tarsi 5-segmented, the basal segment tiny so that they appear 4-segmented at X40. Claw segment of all tarsi longer than the others combined. Claws smooth, those on the pro-tarsi weakly toothed at the base.