Stephostethus alternans (Mannerheim, 1844)

This is a western Palaearctic species with a mostly European distribution; it is generally very local and rare although sometimes common in upland deciduous forests of southern and central regions. The distribution extends from the Pyrenees to northern Italy and the Balkan Peninsula in the south, north to the UK, Denmark and across southern Fennoscandia, and the eastern limit is Ukraine and possibly parts of European Russia. The species was first recorded from the UK in 1994, probably from specimens introduced from Europe, and it now occurs locally across the southeast and south Wales and appears to be spreading. Typical habitats are deciduous woodland and wooded parkland with plenty of fallen timber and older trees in various stages of decay. Adults usually occur among mouldy decaying wood or under bark, most often on older beech and oak trees but also on larger fallen branches, occasionally among litter beneath suitable trees and, in northern Europe, they have been found on decaying fence posts and structural timber. Specimens have very occasionally been recorded from damaged or cut conifer timber in mixed woodland. Here they have been recorded from April until August, peaking in abundance during June, but in Europe over a much longer season, extending into November. Adults are nocturnal and rarely found away from trees, they usually occur in small numbers although populations tend to persist over several years, they are fully-winged but it seems they do not come to light and have not been recorded from flight-interception traps. Most specimens have been found by beating foliage or older fungoid branches or by sieving debris and bark from fallen wood. Nothing is known of the biology but, from the phenology, reproduction probably occurs in the summer and, typical of the family, both adults and larvae are mycophagous.

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​2.4-2.8 mm. Elongate with a narrow forebody and broad elytra, glabrous and entirely dark to pale brown although the elytral interstices are sometimes paler and more reddish, giving a striped appearance. Head transverse, rounded (from above) in front of large and convex eyes and with short, parallel temples, surface roughly sculptured and punctured and longitudinally depressed along the centre. Antennae 11-segmented with two basal segments expanded, 3-9 narrow and elongate, and 10 and 11 forming a distinct club. Pronotum quadrate, broadest across widely-rounded anterior angles and constricted medially (this may be filled with a membrane and so not obvious), surface uneven and roughened, with a round fovea towards each lateral margin in the basal third and a longitudinal keel either side of the disc. Elytra broadly curved from rounded shoulders to a continuous apical margin, striae strongly punctured from the base but fading in the apical third, interstices 3,5 and 7 raised, the seventh keel-like and extending over the shoulder. In the apical third there are two rows of punctures (sometimes faint) between the raised seventh interstice and the lateral margin. Legs long and slender with all femora and tibiae similar in size; in females all the tibiae are straight while in males the middle and hind tibiae are slightly curved. Tarsi 3-segmented; the basal segments lobed and narrow and the terminal segment long and curved. Claws smooth and not or only weakly toothed at the base. Males may be distinguished by the curved middle and hind tibiae and further by a row of very fine teeth towards the apex of the front and middle tibiae, these are absent in females.