Silvanus Latreille, 1804
POLYPHAGA Emery, 1886
CUCUJOIDEA Latreille, 1802
SILVANINAE Kirby, 1837
S. bidentatus (Fabricius, 1792)
S. unidentatus (Olivier, 1790)
This small cosmopolitan genus includes about 25 species, they are most diverse in tropical regions and most temperate areas are poor in endemic species. The lifestyles are varied and many have become pests of stored products and established beyond their native areas e.g. S. lateritius (Broun, 1880) occurs under eucalyptus bark and is endemic to Australia but has become established in South Africa, Hawaii and Madeira. Several species have become established in Europe; S. recticollis Reitter, 1876 occurs among decaying vegetation in warmer parts of Eurasia and Africa but now occurs among warm compost in northern Europe, the African S. proximus Grouvelle, 1904 now occurs in tropical regions worldwide and under artificial conditions e.g. in warehouses in Europe, and the Oriental S. lewisi Reitter, 1876 was introduced into Europe among imported cereals and now occurs in food storage facilities. Of the nine species established in North America, 6 are adventive, and even in the UK seven species have been recorded but have failed to become established. In northern regions native species tend to be saproxylic whereas introductions are associated with foodstuffs and agricultural materials; two saproxylic species are widespread in Europe and extend to the UK although they have also become established elsewhere. S. unidentatus (Olivier, 1790) occurs commonly throughout Europe, extending north to the UK and the Arctic Circle in Fennoscandia and east through most of Asia; it has also become established in North America and Chile. S. bidentatus (Fabricius, 1792) has a more limited European distribution and is generally much more local and sporadic, extending north to the UK and southern parts of Fennoscandia, it is also widespread in Asia and has become established in North America, India and Hawaii, it is also (probably) less strictly saproxylic than the previous species, having been recorded from stored products.
All members of the genus are small, 1.8-3.5mm, elongate and flattened, with relatively long legs and antennae, the colour varies from dull pale brown, almost orange, to very dark brown and the dorsal surface is clothed with very fine recumbent golden pubescence. The head is transverse to quadrate with moderately large convex eyes and very short acutely angled temples, punctation varies from strong and dense to very fine and sparse, and in some the vertex is depressed inside the eyes. Antennae 11-segmented with a broad basal segment and 2-8 variously elongate but not strongly so, and 9-11 form a distinct club. Pronotum quadrate to moderately elongate, broadest about the middle and narrowed to a straight basal margin, lateral margins finely denticulate and pubescent, anterior angle produced into a sharp protruding tooth. Surface weakly convex, usually punctured as the head and with a pair of variously-developed longitudinal impressions which may extend to the basal and apical margins. Elytra elongate, with broadly rounded shoulders, sub-parallel laterally and smoothly rounded to a continuously-curved apical margin, each with nine punctured striae which are complete to the apex, alternate striae with rows of fine setae, interstices narrow and variously convex, lateral margins not or only narrowly explanate, epipleurs broad towards the base and tapering to the apex. Legs long and slender, the femora unmodified and substantially visible in normal setting, tibiae narrow and straight or weakly curved or angled externally; male hind tibia with a row of small setae along the posterior margin, tarsi 5-segmented with the basal segments variously lobed, the third simple, the fourth very small and the terminal segment long and curved. Claws long, curved and simple or only weakly toothed or angled at the base. The following characters will distinguish the genus among our fauna: 2.4-3.4mm. Antennae moderately long and distinctly clubbed, temples short and acute, pronotum densely punctured, finely but distinctly denticulate laterally and with the anterior angles produced into a sharp tooth, elytra parallel-sided and at least twice as long as wide. Our two species may be separated as follows:
Smaller, 2.4-2.8mm. Head with distinctly smaller temples. Head and elytra more shiny than the pronotum. Pronotum with at most only traces of longitudinal depressions, anterior angles only weakly acute and not strongly produced laterally.
Larger, 2.8-3.4mm. Head with distinctly larger temples. Head, pronotum and elytra equally dull. Pronotum with a distinct longitudinal impression either side of the disc, anterior angles strongly acute and produced laterally.
S. unidentatus (Olivier, 1790)
S. unidentatus is locally common across south and central England north to Nottingham; it is generally absent from Wales and the West Country and very scarce further north. Typical habitats are woodland and wooded parkland with plenty of fallen and decaying timber but they also occur on isolated trees in gardens and hedgerows etc. Adults occur year-round, peaking in April and May and again in September and October, they are associated with a range of broadleaf trees but especially oaks, poplars, beech and elms. They occur under damp and close-fitting bark and seem to move on when this becomes loose and begins to pack with debris, they generally occur in numbers and often among populations of other saproxylic beetles e.g. Bitoma or Rhizophagus and occasionally Silvanus bidentatus. Larvae occur under bark and are thought to predate other insect larvae.
S. bidentatus (Fabricius, 1792)
S. bidentatus has a similar south-eastern distribution to the previous species but is much more local and scattered, although in our experience it has become much more common (at least in South Herts.) over the last decade or so. This species also occurs under damp bark on standing and fallen timber of a wide range of broadleaf species, perhaps more so on oak, beech and hornbeam and also occasionally on various conifers, on the continent it has also been found among moss on old trees, among leaf litter and in rotting wood in buildings, especially in damp basements. Adults occur year-round and seem to be most common in early spring and late summer.