POLYPHAGA Emery, 1886 

CUCUJOIDEA Latreille, 1802

PEDIACUS Shuckard, 1839

P. depressus (Herbst, 1797) 

P. dermestoides (Fabricius, 1792) 

3.5-4.5mm

CUCUJIDAE Latreille, 1802

Flat Bark Beetles

Our two species are widespread and should soon be found when searching standing or fallen timber. P. dermestoides is the more frequently recorded species. 

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Around the World

This is a small family of about 60 species contained in 4 genera, it was formerly much larger but a revised classification, including the promotion of several subfamilies i.e. laemophloeinae, silvaninae and passandrinae to family status has reduced it drastically. With the exception of Africa and Antarctica the family is represented throughout the world with the greatest diversity in the Holarctic region. All species are saproxylic. Adults are often conspicuous and striking while the larvae are large and flattened, rather like Pyrochroid larvae, and obvious when seen under bark but little is known of their biology. There are 6 European species; 2 Cucujus Fabricius, 1775 and 4 Pediacus Shuckard, 1839.

Description

They are small to medium sized beetles, 3.5-25mm, elongate, parallel sided and characterized by the extreme dorsoventral flattening. The larger species are often brightly coloured; brilliant red or yellow or a pattern combination of one of these with black, making them very attractive insects. Species of Pediacus Shuckard, 1839 are smaller and generally drab. Most species are glabrous, otherwise any pubescence tends to be inconspicuous. The head is large, sometimes looking out of proportion with the pronotum, usually triangular and with well-developed temples. The antennae are 11-segmented, filiform or variously clubbed and placed laterally in front of the eyes, the insertions being hidden. The labrum is small and fused to the clypeus, the mandibles robust and large, sometimes sexually dimorphic. The terminal segment of all palps is truncate. Eyes round and convex, varying from almost flat to strongly convex. In most species the pronotum has very uneven lateral margins, dentate or variously sinuate, and varies from transverse to quadrate, the surface usually with well-developed depressions. Scutellum quadrate to pentagonal. The elytra are parallel sided and very flat, the surface very finely punctured and, often, pubescent. They generally lack striae although there are sometimes vague longitudinal raised lines and hints of incomplete striae.  They are entire in all species; rounded at the apex and completely covering the abdomen. Abdomen  with 5 visible  sternites. The 

femora are robust and tend to be pedunculate, the tibiae slender with 2 small and unequal spurs at the apex. Male tarsi 5-5-4, in the female 5-5-5. Male protarsomeres weakly dilated. Claws simple.

UK Species

Our 2 U.K. species are superficially very similar and so the following general description will serve to illustrate both. 3.5-4.5mm, elongate, parallel and dorsoventrally flattened. Head triangular and broadest across the convex and prominent eyes. Temples strongly constricted behind the eyes to a broad neck. Antennae robust; basal segment large and broad, 2, 4 and 5 quadrate, 3 elongate, 6-8 transverse; 7 wider than 6 or 8, and 9-11 transverse forming a loose and elongate club. Pronotum quadrate or transverse, with uneven or dentate lateral margins and 2 depressions either side of the centre. Scutellum transverse. Elytra depressed either side of the suture so appearing concave, rounded apically and completely covering the abdomen. Sinuate (in side view) and narrowly explanate. Variously punctured and very finely pubescent. Sutural stria well impressed from apex, becoming evanescent towards basal quarter and not reaching the basal margin. Femora robust, tibiae and tarsi long and slender. Tarsi 5-5-4 in the male and 5-5-5 in the female.

Both species are widespread. P. depressus occurs locally across England north to South Yorkshire but seems to be absent from the West Country. There is a single Welsh record from the south coast. P. dermestoides is by far the more common species with numerous records across England, including the West Country, and Wales north to South Yorkshire. Both occur in woodland, wooded pasture and parkland where all stages are found under the bark of a wide range of broadleaf and coniferous trees. Adults generally occur in groups and both species may be found together. The larvae are predacious while the adults are thought to be mostly fungivorous. Adults are crepuscular and nocturnal and fly well, they may be found at sap, fungus and mould on standing timber and logs. The species are difficult to distinguish in the field and so samples consisting of groups of adults should be taken (to be returned, of course) to be examined under a microscope.

Pronotum quadrate, hind angles with a small blunt tooth. Lateral margin with a large and sharp tooth at centre. Elytra punctured throughout (X20) and almost as shiny as the forebody. Unicolorous or with the contrast between the forebody and the elytra much weaker.

Pronotum strongly transverse, hind angles with a sharp tooth. Lateral margin with a small blunt tooth at the centre, or simply sinuate. Elytra moderately strongly punctu-red at the base, otherwise appearing virtually impunctate at X20. Forebody shiny, elytra much less so, usually appearing dull by comparison. Head and pronotum generally much darker than the elytra.

In general both species are drab, usually with some contrast between the forebody and the elytra, but they overlap; specimens with a darkened pronotal disc are usually dermestoides, but pale specimens of both occur.  Colour alone is a poor guide to the species but it may suggest a mix of the two in the field. The form of the pronotal margins also varies but typical specimens are obvious and a pair of each should be kept for comparison.

Other Genera

  • Palaestes Perty, 1830 includes 8 species of Central and South American saproxylics. They are typical flat and parallel species and all are strikingly bicoloured yellow and black. All are strongly dimorphic with the male possessing hugely enlarged and sickle-shaped mandibles.

  • Platisus Erichson, 1842 includes 4 species of typical cucujids; flat and parallel with the pronotum quadrate or transverse and, perhaps a little unusual in the extent of its development, a very broad head. The genus is known only from Australia and New Zealand. They are either entirely very dark or bicoloured with a dark forebody and red or brown elytra.

  • Cucujus Fabricius, 1775 contains 14 species distributed throughout the Holarctic with some indigenous to North America, Europe and Asia. The widest diversity is in Asia. They vary widely in size and include the largest members of the family. Most species are red or black or a combination of these but some Asian and Japanese species are either entirely blue or have a dark forebody and brilliant metallic blue elytra. Distinguished from other genera in having the head wider than the pronotum. In most cases the biology is poorly understood; all stages occur under bark and are thought to be predacious. The larva of one subspecies of C. clavipes Fabricius, 1777, a brilliant red species from North America, Canada and Alaska, is the subject of much research as they survive extremely cold conditions. It is thought to be beneficial to timber production as both the larvae and adults are known to be predatory beneath bark.

  • Pediacus Shuckard, 1839 includes more than 30 species with a Holarctic distribution which extends south to Australia and, in the New World, at high altitudes to Guatemala. They are small, <7mm, flat and without, or with only very small temples. The pronotum usually has heavily sculpted lateral margins. They have short antennae which may be distinctly or indistinctly clubbed. All stages occur under the bark of a wide range of deciduous and coniferous trees in all stages of decay. Adults and larvae are thought to be predacious and the adults are active nocturnally. There are 2 British species (see above).

Cucujus cinnaberinus (Scopoli, 1763)

This is one of the only two species of Cucujus to occur in Europe; the other species, C. haematodes Erichson, 1845, is very rare and local. Both have suffered drastic declines over recent decades due to habitat manipulation and are extinct in many of their former localities. C. cinnaberinus is native to Europe, including much of Scandinavia, and may still be found to be common in some central countries. It occurs mostly beneath the bark of deciduous trees e.g. oaks, maples and poplars and also, although nowhere near so commonly, on a range of conifers. Typical habitat is forested areas but they have also been recorded in urban situations. Adults feed on rotting wood and have also been observed eating the larvae of other insects. 11-15mm. Body bright red with the pronotal margins, appendages and mandibles black. In C. haematodes the entire pronotum and the mandibles are red.

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