Cerylon Latreille, 1802
POLYPHAGA Emery, 1886
CUCUJOIDEA Latreille, 1802
CERYLONINAE Billberg, 1820
C. fagi Brisout de Barneville, 1867
C. ferrugineum Stephens, 1830
C. histeroides (Fabricius, 1792)
About 75 species of these minute bark beetles occur throughout the world with the exception of Australia, diversity is greatest in the Old World tropics and temperate faunas are comparatively poor, e.g. seven species occur in central Europe and only three of these extend to the UK. The New World fauna is also poor with only five species occurring in North America and four recorded from Argentina. The central European fauna includes 6 species, of which 3 extend north into the UK. Among the European fauna they are closely similar to Philothermus Aubé, 1843, of which 3 are local and sporadic but do not extend to the UK, but here they are quite distinctive and should become familiar very quickly in even the field. Some small histerids e.g. Paromalus (Herbst, 1792) are superficially similar but otherwise readily distinguished by the smooth elytra, broad anterior tibiae and geniculate antennae etc. Cerylon are small, 1.8-2.6mm, shiny, drab-coloured, flattened and compact beetles with a distinctive broadly-elongate shape. The head is narrow with convex eyes and is usually partly retracted into the prothorax, the vertex finely punctured, flat or weakly convex and sometimes impressed between the eyes and the antennae are inserted laterally in front of the eyes. Antennae 11-segmented but appearing 10-segmented as the two terminal segments are fused and form a distinct club, basal segment enlarged and usually curved internally, second and third variously elongate and 4-9 transverse to elongate. The pronotum is transverse to slightly elongate, parallel-sided or sinuate laterally and incurved towards the apex to distinct anterior angles, the lateral margins are finely bordered and the basal margin sinuate. Pronotal punctation usually relatively strong and diffuse with the punctures separate and distinct, in some there is a variable depression inside the posterior angles but the surface otherwise lacks structure. Scutellum transverse and rounded or obtusely-angled, coloured as
the surrounding cuticle and smooth or with a few scattered micropunctures. The elytra are very distinctively-shaped; elongate, dilated in the basal two-thirds then narrowed to a continuously rounded apical margin, the basal margin is sinuate and the shoulders are variably produced and sometimes appear toothed from above. Each has 10 punctured striae which usually continue to the apex although in some they fade in the apical third, the interstices are weakly convex or flat, variously punctured and have very fine short and erect setae. The legs are robust and relatively long with curved and unadorned femora visible in normal setting, tibiae long and gradually expanded to oblique apices that are fringed with dense stiff setae but without obvious spurs; pro-tibiae variously produced externally into a sharp apical tooth. Tarsi 4-segmented; basal segments short, apical segment long and curved, longer than the others combined, claws fine, weakly curved and lacking a distinct basal tooth. There are no obvious sexual differences although the pronotum is on average wider in females than in males but they will need to be seen in a series to appreciate this.
Adults and larvae occur under bark of a wide range of broadleaved and coniferous trees in various stages of decay, most species occur on a range of trees and they may be common on old fallen branches. They also occur on or within various fungal fruiting-bodies upon which they are thought to feed; adult gut-contents have been found to contain various spores and hyphae, and where adults or larvae occur in the sub-cortical burrows of other insects they are thought to graze upon mycelia and spores which are usually present. In some works they have been described as predatory, feeding on small larvae etc. under bark but there appears to be no evidence of this. Adults generally occur from March until October, peaking during June and July, although they also overwinter as we have found them under bark in numbers during December. They are active on the surface of wood at night and will often be found in numbers, especially on the ends of recently-cut trunks that are damp with sap and infected with fungi. They also occur at sap-runs and in flight-interception traps during the summer, and two or three species may be present together.
Cerylon fagi Brisout de Barneville, 1867
Of our UK species this has the most restricted European distribution, it is locally common in lowland forests across central Europe from the Pyrenees through northern Italy and Romania, extending north to the southern provinces of Fennoscandia and the UK where it is generally common in the southeast but of local occurrence through the rest of England and Wales to the Scottish borders. The species occurs on both deciduous and coniferous trees in a wide range of habitats including parkland and gardens and the adults usually occur alongside other members of the genus. Adults may be sampled by searching under bark or on the surface of wood by night; they often infest old rotten stumps and fallen timber and logs, and during the winter may occur in samples of leaf-litter and twigs from beneath trees.
2.1-2.6mm. Adults are dark brown to black with pale appendages and so might be mistaken for C. histeroides but the form of the antennae and the habitus are distinctive; here segments 4-7 are transverse and wider when compared with the club, at least segments 6 and 7 are obviously rounded laterally and more than half the width of the club. The pronotum is a little less densely punctured than in our other species and the basal fovea are always more strongly impressed, the elytra are broader, about 1.4-1.4:1, and more strongly narrowed in the apical third, and the striae leas strongly impressed and punctured, the outer striae often evanescent in the apical third.
Cerylon histeroides (Fabricius, 1792)
This is the commonest and most widely distributed European species, it occurs throughout the area from the Mediterranean to the north of Fennoscandia and east through Asia Minor, Russia and Kazakhstan to the Pacific coast of Asia, here it is generally common across England and Wales though less so in the north and very local and sporadic further north to the Scottish Highlands. Adults and larvae inhabit a range of both deciduous and coniferous trees in just about any habitat and both often occur near fungi and in the burrows of subcortical insects, they usually occur in numbers and often among other members of the genus. On the continent they are typical of old beech forests; adults are sometimes found among the decaying wood and debris of saproxylic ant nests including those of Lasius fuliginosus (Lat.), L. brunneus (Lat.) and Formica rufa L., and they often occur in accumulated leaf-litter, twigs and soil beneath trees. Adults are readily sampled by searching the surface of timber at night.
1.8-2.3mm. Adults are dark brown or sometimes dark reddish-brown, superficially similar to C. fagi but overall more slender in appearance and readily distinguished by the form of the antennae; here segments 4-7 are quadrate to slightly elongate, segments 8 and 9 are transverse and widest at, or very close to, the apex, and none are rounded laterally. Compared with C. fagi the pronotum is more strongly punctured and has only weak depressions inside the posterior angles, and the elytra are more elongate, between 1.53 and 1.62 times longer than wide. Elytra strongly punctured and impressed almost to the apex although the outer ones tend to become confused in the apical third, interstices weakly convex and quite strongly punctured, more so than in our other species.
Cerylon ferrugineum Stephens, 1830
This is another common species occurring throughout Europe; it extends to the UK and the far north of Fennoscandia but in southern regions may be local and restricted to mountainous areas, and to the east it reaches Siberia. Here it is our commonest member of the genus, it is generally abundant throughout England and Wales, including the islands, and extends north to the Scottish Highlands. Adults among decaying deciduous and coniferous timber, they usually occur in numbers and often along with other members of the genus, are nocturnal and likely to be found in almost any habitat.
1.7-2.1mm. Distinctive due to the entirely pale brown colour. Antennae much the same as in histeroides, with the middle segments only weakly, if at all transverse and not rounded laterally. More slender and parallel-sided than histeroides, with the pronotal punctation similar but the basal fovea a little more distinct, elytra more elongate, 1.64-1.70, with relatively strongly punctured striae and flat interstices that are only very finely punctured.