Paederidus Mulsant & Rey, 1878
This is a very widespread Old-World genus of about 45 species; 13 are known from the Palaearctic region and of these 4 occur in Europe although only 2 are widespread. P. punctiventris Scheerpeltz, 1957 occurs in south-west Russia, around the Black Sea, while P. algiricus Motschulsky, 1858 occurs in Spain and north-west Africa. The 2 remaining species, P. rubrothoracicus (Goeze, 1777) and P. ruficollis (Fabricius, 1777), are widespread in Europe and both are included in the UK list from specimens found in southern coastal localities of England and Wales in the early nineteenth century, neither has been seen since and both are either long extinct or were only recorded from occasional vagrant specimens.
The following general description applies to both species. Head quadrate or nearly so, with transverse and weakly-convex eyes and long converging temples, vertex depressed near the anterior margin of each eye, vertex and frons sparsely punctured, more densely and strongly so beside the eyes. Antennae inserted laterally outside the base of the mandibles, all segments elongate; the second shorter than the others, palps long and slender, the penultimate segment gradually widened to a broad apex, the terminal segment tiny. Mandibles large and curved, each with 2 sharp internal teeth. Pronotum quadrate to elongate, rounded and unbordered laterally, glabrous and shiny orange or red, with scattered very fine punctures and several strong punctures bearing stiff setae along the lateral margins. Mesonotum distinct between the pronotum and elytra. Elytra near parallel-sided, with broadly rounded shoulders and obliquely-truncate apical margins, densely and roughly punctured and with fine pale pubescence. Abdominal tergites finely punctured and pubescent, strongly bordered and more densely pubescent laterally. Legs long and slender, front and middle tibiae with several stout setae along the inner margin and a strong apical seta at the inner apical angle. Front tarsi weakly dilated, middle and hind tarsi long, especially the basal segments, fourth tarsomere lobed on all legs. Among our UK fauna they are not likely to be confused with any other; they are closest in appearance to species of Paederus but with the abdomen and appendages entirely dark metallic. They are closely similar in appearance and both are very variable; in southern and central Europe there are 5 subspecies of P. ruficollis and 9 of rubrothoracicus, but they may be distinguished from the following key:
Smaller, 6.5-8.0mm. Head and abdomen black with a faint metallic-blue reflection, elytra dark metallic-blue, pronotum pale red or orange. Labrum with a distinct longitudinal and wrinkled tubercle. Elytra quadrate or slightly elongate. Pubescence on the elytra and abdomen longer and paler; on tergites 3 to 6 arranged almost uniformly diagonal to the centre.
Larger, 8.0-9.5mm. Head, elytra and abdomen dark metallic-blue, pronotum bright red. Labrum with at most a small and indistinct tubercle. Elytra distinctly elongate. Pubescence on the elytra and abdomen shorter and generally less conspicuous; on tergites 3 to 6 less uniformly arranged, in places virtually horizontal.
Paederidus ruficollis (Fabricius, 1781)
A generally common and often abundant species of north-west Africa and south-west and central Europe, becoming more sporadic and rare further east and north although recorded from as far east as Turkey and Iran. In many lowland areas confined to river valleys and margins of fast streams, it is generally more common on rocky or sandy substrates beside fast flowing streams in mountain areas and has been recorded above 2000m in Turkey. In the north it is often confined to upland areas although it also occurs on sandy substrates along various English Channel and Baltic coastal areas. Adults are active from early spring until the autumn, they usually occur in numbers and are active in bright sun, moving rapidly as they search for small prey animals among marginal substrates, when disturbed they can quickly take flight or move rapidly across moving water, submerging the abdomen as they go, in rather the same way as riparian species of Stenus. Larvae have been observed during July and August, they live and hunt among marginal substrates and pass through 2 instars before pupating in the soil but it is not known whether any immature stages overwinter along with the adults.
Paederidus rubrothoracicus (Goeze, 1777)
This species is more widespread than ruficollis; it occurs from North West Africa to the south of Fennoscandia and east to Asia Minor and western Russia. Inhabiting the same waterside environments as ruficollis but in the south it seems to be more restricted to mountain river valleys while further north it is generally the more common species and is often abundant along sandy river margins and estuaries. The life cycle is similar to the preceeding species with adults active over a long season and larvae present from July or August, both adults and larvae are diurnal predators of small insects etc in marginal environments.