Nartus grapii (Gyllenhal, 1808)
This widespread European species occurs from France to Ukraine and Western Russia, although it is absent from many of the southern regions bordering the Mediterranean, and extends north to the UK and southern provinces of Fennoscandia, it remains locally common throughout this range but has suffered a decline over recent decades, especially in some northern countries e.g. The Netherlands, due to the disappearance of ponds etc. and land management. The UK distribution is disjunct; it is widespread in Ireland and occurs on Anglesey and Man, in Wales it occurs along the south coast and this forms a larger population extending into Somerset, otherwise it is locally abundant in the southeast, East Anglia, south Hampshire and Dorset and around the east midlands and very local and sporadic further south. Adults occur year-round, they overwinter in soil or under debris away from water and become active early in the spring, they remain active late into the autumn and on warm spring and summer evenings are attracted to light as they disperse. Typical habitats are permanent lowland ponds and lakes with plenty of aquatic and marginal vegetation and a good bottom layer of decaying organic matter; swamps, reed beds, drainage ditches, ponds, and it often occurs among floating mats of Sphagnum on pond margins although only rarely in peat bogs, the species can tolerate slightly acidic or alkaline water but does not occur in brackish water. Breeding occurs in the spring and adults may migrate to temporary pools in fens or marshes to do so, the species appears to be univoltine with larvae developing through the summer and new-generation adults appearing in late summer and overwintering.
Similar in size and appearance to the very common Agabus bipustulatus (Linnaeus, 1767), but lacking the elongate microsculpture of that species, this can be checked with a lens in the field and so many specimens can be ignored when searching for this species but there are other dark Agabus and Ilybius which will need to be examined for the presence of a small comb of setae on the outer margin of the metafemora, a feature absent in Nartus and which, when coupled with the dark colour, will provide a safe identification. 10.0-11.2mm. Elongate and weakly convex, shiny black but for a variable marking on the head and the pronotal and elytral margins red, antennae and palps pale, legs dark brown to black, usually with the apices of the femora and tibiae and the tarsi paler. Head large and rather flat, pronotum curved and bordered laterally to projecting anterior angles and acute posterior angles, basal margin quite strongly sinuate laterally. Elytra each with two or three rows of fine punctures, microsculpture distinct and consisting of tiny irregular meshes. Male basal pro-tarsal segments dilated and with ventral adhesive setae. Male strongly curved at the base, slightly unequal in length and about as long as the last tarsal segment.