Rybaxis longicornis (Leach, 1817)
This widespread and locally common species occurs throughout Europe and across northern Africa, extending north into the UK and northern Fennoscandia and east through Asia Minor and Russia into central Asia; in the UK it is generally common across England and Wales north to Nottingham and more sporadic and rare further north to Cumbria, it is also present on the Isle of Man and generally common in the west of Northern Island. Adults are present year-round but are most common in spring and autumn, typical habitats are wetlands and permanently damp woodland and grassland where they occur in moss or leaf-litter, under stones and debris or in tussocks etc. Sampling usually involves sieving or taking samples for extraction but from March to June adults climb stems in hot weather and so may be swept from grass and herbage on wetland margins; thus we have swept them locally from reed beds and grass stems along the river Colne at Watford, and during July 2011we extracted a single specimen from a sample of waterlogged fungus taken from a log floating in a pond at Bricket Wood Common. Adults are predatory and thought to feed primarily upon small mites and springtails but otherwise very little are known of the species biology.
1.5-2.2mm. Recognized among our fauna by the broad habitus and the form of the antennae; the insertions are separated by a little more than the length of the basal segment, in the male all segments are elongate and only segments 9-11 are broadened, the ninth parallel-sided and the tenth gradually expanded from the base and so there is only a very indistinct club, in the female the antennae are shorter and the ninth and tenth segments are transverse so that they appear a little more distinctly clubbed. Entirely dark shiny brown, usually with the base of the elytra extensively red and the tarsi pale brown or yellow, dorsal surface finely pubescent throughout, more densely so towards the elytral apices and on the abdomen. Colour varies widely, completely black and extensively red specimens occur, the extreme colour forms have been named but pointlessly so as all intermediate forms occur. Head elongate and parallel-sided with convex and coarsely-faceted eyes (each with about 30 ommatidia)
and broadly-rounded temples before a truncate basal margin, frons with a small deep fovea each side and clypeus widely depressed anteriorly, maxillary palpi as long as the four basal antennomeres, terminal segment long-oval and acutely pointed. Pronotum transverse, broadest before the middle and narrowed to obtuse posterior angles and indistinct anterior angles, surface with three large fovea linked by a transverse furrow, the median fovea situated towards the base, the median fovea towards the lateral margins. Elytra much broader than the pronotum, with rounded shoulders and curved lateral margins, broadest behind the middle and bisinuate across the apex, each with a complete sutural stria and a narrow sinuate impression extending across the disc from the base to the apical quarter. Abdominal tergites weakly raised laterally, the basal tergite with a very fine oblique impression either side of the base but otherwise without sculpture. Legs long and robust, femora and tibiae without teeth or obvious terminal spurs, tarsi 3-segmented but appearing 2-segmented as the basal segment is tiny and inconspicuous.
In many texts another species, R. laminata (Motschulsky, 1836) is described and the two have variously been considered as conspecific or as separate species, differences in the form of the metasternum and aedeagus have been used to separate the pair but recent molecular data have shown them to be conspecific.