Description

Byrrhids are small to medium-sized beetles, approximately 1-15mm in length and generally very convex dorsally, moderately so ventrally, and short- to elongate-oval in outline. Most are drab coloured; grey, brown or black although some are metallic bronze or green e.g. Lamprobyrrhulus nitidus (Schaller, 1783) or the common U.K. species Cytilus sericeus (Forster, 1771), and some have a distinct mottled pattern e.g. some Byrrhus species, while some are glabrous e.g. Carpathobyrrhulus the majority are to some extent pubescent or have a layer of dense recumbent setae or scale-like setae e.g. Morychus aeneus (Fabricius, 1775), and some have outstanding bristles or setae which may be simple, flattened, clavate or truncate e.g. Chaetophora spinosa (Rossi, 1794). The head is usually hypognathous so that in life it is generally only narrowly visible from above, although it is readily manipulated for setting, narrower than the pronotum though often as wide as the anterior margin, with the vertex and frons convex, the eyes weakly to moderately convex, and oval or sometimes weakly emarginate behind the antennal insertions; in life they are often hidden under the pronotum. The eyes and antennae are placed laterally; the antennae are 11-segmented and often have a small but distinct scape, and vary from simply filiform e.g. Simplocaria, to gradually expanded e.g. Porcinolus, or clubbed e.g. Curimopsis .The clypeus is reduced to almost absent, the labrum articulated and usually emarginate, the maxillary palps 3-segmented, labial palps 2-segmented, with the terminal segment pyriform or, in Amphicyrta, securiform. Mandibles generally inconspicuous; variously toothed apically and deeply notched internally about the middle. The pronotum is convex, broadest at the base and narrowed to obsolete or rounded anterior angles, the posterior angles are generally acute and to some extent produced, lateral margins finely to strongly bordered and the basal margin sinuate, often strongly so, and sometimes finely crenulate. Prosternal process well-developed; broadly rounded to acute, truncate or T-shaped apically, the pro-coxae are transverse with the cavities open posteriorly. The mesosternum is short and deeply recessed to receive the prosternal process; the meso-coxae are only slightly transverse, globular to flat and usually widely separated.  The metasternum is broad and long with a distinct longitudinal suture and often only indistinctly divided from the mesosternum and generally deeply emarginate around the meso-coxae. The meta-coxae are widely transverse, almost reaching the elytral epipleura, and nearly touching at the centre. The scutellum is very small, sometimes hardly visible, and triangular to oval. Elytra completely covering the abdomen, convex and continuously or separately rounded apically and with variously developed epipleura which are sometimes complete to the apex. Explanate margin, if present, narrow and only present towards the apex, the surface varies from glabrous to completely and densely pubescent, in either case there may also be modified setae, scales or bristles, smooth to striate or with longitudinal impressions or series of punctures, the background punctation from fine and diffuse or sparse to strong and very dense. Abdomen with 5 ventrites; the first and second connate. Legs generally relatively long and slender and often flattened; trocanters triangular, the femora usually clavate and unadorned, tibiae slender to stout, and usually flattened, often characteristically curved, angled or toothed, and  grooved externally to receive the tarsi, the surface often pubescent and with stiff setae or spines. Tarsi 5-5-5, (Byrrhinae), or 4-4-4, (Syncalyptinae), the third segment is sometimes lobed, and in some species all the tarsomeres are transverse and have pubescent pads ventrally. The claws are usually simple.

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Byrrhids seem never to have been popular with coleopterists in the U.K.; the fauna is limited and some of these are very rare, and of the 13 species on our list 2 have been added in recent decades, Byrrhus arietinus Steffahny, 1842 in 1966 and Curimopsis nigrita (Palm, 1934) in 1978. In general their behaviour makes byrrhids difficult to find and because they are adapted to fully retract the appendages into grooves on the ventral surface of the body they are difficult to manipulate and set. Adults remain still and compact when disturbed, resembling seeds or small animal droppings, and so can be very difficult to find without-or even with-experience. On the other hand they may be found when sampling tussocks or litter from around the base of plants or moss, or from moss etc. on the surface of old logs. Both adults and larvae of most species are obligate moss-feeders but some have been recorded from lichens and liverworts, and some from higher plants e.g. species of Amphicyrta feed upon the leaves and stems of various herbaceous plants and deciduous shrubs and occasionally occur in large numbers among cultivated turf. Adults of bryophagous species graze on the surface while the larvae burrow into gametophyte layers and feed from the burrow entrance by extending the body to graze on nearby mosses. The larvae of some species of e.g. the Holarctic Cytilus or the Nearctic Arctobyrrhus are surface feeders and some have been recorded feeding upon clover, grasses and conifer seedlings. Adult byrrhids are generally crepuscular or nocturnal although some e.g. species of Byrrhus and Cytilus may be seen wandering paths or on bare soil among grass etc. in bright sun, most tend to remain within litter or among moss on the ground or on logs and stones but are readily seen by torchlight with careful searching. Several species occur generally in the U.K. e.g. Byrrhus pilula (Linnaeus, 1758) and Cytilus sericeus (Forster, 1771), but many will only be found in specific situations e.g. we have found Chaetophora spinosa (Rossi, 1794) in great abundance on calcareous grassland, Morychus aeneus (Fabricius, 1775) occurs near coastal streams, Porcinolus minimus (Fabricius, 1794) is a very rare heathland species, and Curimopsis maritima (Marsham, 1802) occurs on open calcareous or sandy soils near the coast. In the Nearctic zone byrrhids are most diverse and abundant in northern mountainous regions and around the Great Lakes, while some genera e.g. Lioon Casey, 1912, Lioligus Casey, 1912 and Listemus Casey, 1912 are restricted to northwest conifer forests. Sierraclava is restricted to California, and some species occur only in extensively wooded areas where numbers are known to increase dramatically after disturbance e.g logging or timber salvage that follows forest fires.