Contacyphon coarctatus Paykull, 1799
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Although widespread and generally common this species has a rather patchy European distribution; it occurs from Spain and Italy east to Rumania and the Caucasus but is apparently absent from many areas e.g. Bulgaria, Slovenia and Hungary, in the north it extends to the Arctic circle in Sweden and Russia (Karelia), and is generally common throughout the UK north to Orkney. Here it is our most common member of the family; adults occur from April or May until August in wetland marginal situations, usually beside shaded stagnant water although in the UK generally not from acid peat-bogs, and are often present in swept samples from suitable habitats, during the spring they are generally common on Buckthorn and Hawthorne blossom and later on may occur far from wetlands as they disperse. Adults may be very common on permanently damp woodland and marshland margins with varied and lush vegetation, and will often occur with other members of the genus. Oviposition probably occurs in early summer as gravid females have been observed in June, and larvae develop among marginal soil and organic detritus through the summer; they pass through 9-12 instars and the number of antennal segments increases as they grow. Unlike most Scirtids the larvae overwinter and continue to feed and grow in the spring, the final instar is distinguished by a sharp tooth at the mandibular apex which is used to build a spherical pupal chamber beneath the surface of moist soil near the water edge. Both adults and larvae feed on fungal spores and organic detritus and it seems they are not predatory as previously thought.
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In general species of Cyphon Paykull, 1799 can be very difficult to identify but the present species is an exception; the combination of large size, longitudinal ridges on the elytra and the pale and short third antennomere are distinctive. The only other UK species with elytral ridges, C. palustris Thomson, C.G., 1855, has the third antennomere equal in length to the second and is often infuscated. The aedeagus of coarctatus is quite distinctive with a large and rounded basal margin to the ventral plate and paired long inwardly curved and hook-like apical processes.
3.5-4.5mm. Entire body shiny black or dark brown or sometimes with diffusely paler margins to the pronotum and elytra or with the forebody and elytra contrasting, dorsal surface with rather dense overlapping pale pubescence, appendages pale except the antennae are variously darker from the fourth segment. Head transverse with weakly convex eyes, anteriorly –produced frons and clypeus and prominent curved mandibles. Third antennomere distinctly narrower and shorter than the second, the remainder elongate and filiform. Pronotum widely transverse and broadest at distinct posterior angles; basal margin sinuate and distinctly bordered, lateral margins strongly narrowed to a rather rounded anterior margin, surface punctation diffuse and much finer than that on the elytra. Scutellum large, triangular and punctured as the pronotum. Elytra with prominent and rounded shoulders, weakly curved lateral margins and continuously rounded apically. Surface without striae, randomly and quite strongly punctured and with 3 longitudinal ridges which may be very weakly defined but soon become obvious with experience. Legs slender and relatively short; tarsi 5-5-5 with all segments obvious, the fourth is deeply bilobed but the long fifth segment always extends beyond the apex of the lobes.