Stilbus oblongus (Erichson, 1845)
POLYPHAGA Emery, 1886
CUCUJOIDEA Latreille, 1802
Stilbus Seidlitz, 1872
Locally common across the southern Palaearctic region from Spain to eastern Siberia, this species occurs across Europe north to the south of Fennoscandia and the UK, it is present on most of the Mediterranean islands and across North Africa and the Near East, it is a mostly lowland species although it is sometimes found at higher altitudes in the Alps. In the UK it is locally common in eastern England and the midlands north to Yorkshire and there are scattered records from the West Country and the south coast of Wales. Adults occur year-round, they overwinter in stems or among litter or moss and are active between April and October with the generations overlapping, in the UK they occur in wetland marginal situations where they are associated with common bulrush, Typha latifolia L., and possibly other rushes, and reeds, Phragmites australis (Cav.) but on the continent they occur more generally on flowering grasses in meadows and wooded margins as well as wetlands. Little is known of the biology but adults peak in abundance in the spring and again in August and they often swarm on low vegetation and shrubs near wetland margins. (Presumably the larvae develop in flower spikes, feeding on smut fungi, and pupate in the ground during the summer, which is usual for the family, but larvae of another species of Stilbus have been reported feeding on aphids.) We have a very large population on reed beds near Watford town centre and here we have recorded teneral adults each month from June to September. Adults fly well and tend to disperse from reeds to surrounding vegetation and flowers where they feed on pollen during warm weather and so are easily sampled by sweeping, occasionally large numbers occur on umbels and other flowers but we have yet to find them mating.
Stilbus oblongus 1
Stilbus oblongus 2
1.7-2.1 mm. Members of this genus may be identified by the single elytral stria and the form of the terminal maxillary palpomere which is expanded internally and truncate, the present species is distinguished by its colour and the form of its microsculpture. Head black, pronotum black but with variable yellow margins, elytra dark at the base and becoming gradually paler brown towards the apex, sometimes entirely reddish-brown and often extensively black or dark brown with the apex abruptly pale. The dorsal microsculpture consists of isodiametric meshes which are rarely fused (X50), in our other members of the genus the microsculpture consists, at least in the greater part, of long transverse lines. Head with large weakly convex eyes and long straight temples that are usually hidden within the thorax, smooth and rather flat above and without structure, antennae inserted on the front margin beside the eyes, 11-segmented with a loose 3-segmented club, the terminal segment symmetrical. Pronotum broadest across acute posterior angles and narrowed to obtuse anterior angles, smooth and evenly convex, prosternal process broadened and truncate with four or five tiny curved setae across the apex. Elytra broadest across rounded shoulders and smoothly narrowed to a continuous apical margin, surface smooth and shiny with a well-impressed sutural stria in the apical half and sometimes traces of incomplete longitudinal impressed lines on the disc. Legs short and slender, the femora not visible in normal setting, tibiae parallel-sided and truncate or weakly rounded apically, each with two fine apical spurs. Tarsi 5-segmented but the tiny fourth segment is hidden within the strongly-bilobed third segment. Claws smooth and toothed at the base.