Colydium elongatum (Fabricius, 1787)
POLYPHAGA Emery, 1886
TENEBRIONOIDEA Latreille, 1802
COLYDIINAE Billberg, 1820
COLYDIINI Billberg, 1820
Colydium Fabricius, 1792
This is a widely distributed but local and generally uncommon European species extending north to southern Scandinavia and south to Italy, the eastern distribution extends into the Near East, Greece, Turkey, Ukraine and western Russia. In the U.K. it is very local in South Wales and central southern England although adults are nocturnal and need to be looked for very carefully and so they may be under recorded and more widespread than e.g. the NBN map would suggest (Joy gives Hants; vr.). The typical habitat is woodland and wooded parkland where the larvae develop under the bark of a range of broadleaf and coniferous trees e.g. oak, beech, spruce and fir etc. Both adults and larvae are known to be predatory upon the early stages of other saproxylic beetles, especially scolytids; adults have been observed preying on other insects both under bark and on the surface at night while the larvae have been found in scolytid galleries but it is thought they feed mainly on fungi and organic matter, being facultative predators as the opportunity arises. In our south Herts area the species is locally common and may readily be observed on trunks etc. at night, they generally occur in pairs or as single specimens but nonetheless on warm nights from May to August we usually record many specimens. We have recorded the adults from January to November, and they are not unusual in extraction samples of wood and detritus from dead beech and oak stumps.
4-6mm. Body entirely dark but for the anterior margin of the frons and sometimes the shoulders which are red. Head convex with moderately dense elongate punctures and fine granular microsculpture, the eyes are large, convex and weakly emarginate anteriorly, and the frons is obliquely impressed beside the lateral genae. Palps small with the terminal segment truncate, mandibles robust; bidentate and toothed internally. Antennae inserted laterally in front of the eyes, the insertions hidden below genae; basal segments elongate, 5 and 6 quadrate, 7 and 8 transverse, and 9 to 11 form a distinct club with the terminal segment rounded. Pronotum with 3 longitudinal impressions, the lateral ones sometimes very weak, surface punctation as that on the head
Colydium elongatum 1
Colydium elongatum 2
Colydium elongatum 3
Colydium elongatum 4
but the microsculpture is much weaker, lateral margins smooth and strongly bordered. The anterior angles are almost perpendicular, the posterior angles obtuse and the basal margin straight and strongly bordered. Elytra shiny and only very weakly microsculptured, the striae carinate, very finely punctured and strongly raised towards the apex, the fourth abbreviated, interstices punctured and cross-strigose. Legs entirely pale; femora excavate, or at least flattened, below to receive the tibiae, each with an almost semi-circular subapical lobe above and below; tibiae gradually expanded towards an externally toothed and truncate apex. Tarsi 4-4-4 without lobed segments; basal meso- and metatarsomere as long as 2 and 3 together, the terminal segment of all tarsi very long. Claws smooth and without a basal tooth.
COLYDIUM Fabricius, 1792
This is a small genus of mostly New World species; 5 have been recorded from the United States and about 25 from Central and South America while elsewhere only 2 occur in Europe; C. elongatum (Fabricius, 1787) is widespread including the U.K., and C. filiforme (Fabricius, 1792) which is also widespread and extends to Sweden, Greece, Iran and Ukraine but does not occur in the U.K. All species are small, <10mm and very elongate, parallel-sided and virtually cylindrical, and glabrous dorsally. Most are drab brown to black but in some e.g. C. filiforme the elytra are bicoloured and some Neotropical species are bicoloured; entirely red but for the black elytra. The head is prognathous and proportionally large, with distinct and often longitudinally-oval punctures and microsculpture which renders the cuticle dull, large and weakly convex to strongly protruding eyes and a short, transverse labrum and in most species there is a well-developed preocular fovea. The mandibles are robust, curved and bidentate, the labial palps 3-segmented and the maxillary palps 4-segmented with the terminal segment truncate or weakly rounded apically. The antennae are proportionally short, 11-segmented with a distinct 3-segmented club; the basal segment is usually broad, 2-8 quadrate to weakly elongate and in some Neotropical species segments 3-8 bear lines of very long setae, and inserted laterally in front of the eyes under a gena so that the insertions are not visible from above. The pronotum is elongate with strongly bordered lateral and basal margins and rounded angles, the dorsal punctation is usually strong and diffuse and there are 3 longitudinal furrows or carinae, one median and one towards each lateral margin. The prosternum is long in front of closed coxal cavities and the process is wide and broadened towards the apex. The wings are well-developed, the beetles fly, and most species have been recorded at light. The elytra are long, cylindrical, and parallel-sided and completely cover the abdomen, the apical margins forming a continuous curve. The interstices are distinctly punctured, often with transverse carinae between, and usually continuous to the apex. The striae are variously raised and often more strongly so towards the apex. The basal margin is usually straight although sometimes weakly produced forward laterally, and the humeri are evenly convex. The abdomen has 5 ventrites, the basal ventrite produced acutely between the metacoxae and lacking, or with only very short, postcoxal lines. The last ventrite has a group of long setae either side towards the apex-a character which separates the genus from all others in the family. Legs robust and relatively long, the femora broad and variously excavate behind to accommodate the tibiae at rest, the tibiae sharply ridged externally and gradually widened towards the apex which is produced into an external tooth. Tarsi 4-4-4, without distinctly lobed segments.
All species are saproxylic and fungivores although some are also known to be facultative predators.
The 2 European species are closely similar but easily distinguished by the elytral colouration; in elongatum it is entirely dark or only weakly and indistinctly pale around the base while in filiforme it is distinctly bicoloured; dark brown to black with the basal third gradually and distinctly orange or red.