Byrrhus pilula (Linnaeus, 1758)
This generally common species has a native Holarctic distribution, occurring from Portugal to the far east of Asia, Japan and across the Nearctic region; to north it extends above the Arctic Circle in Scandinavia, and in the south it occurs across North Africa and much of the Middle-East to Asia Minor and Kazakhstan, it is generally a lowland and low mountain species, extending to 2000m in Poland. The UK distribution extends throughout England, Wales and Scotland including the Western Isles but not Orkney or Shetland. Adults occur year-round, peaking in April and July and overwintering among moss and tussocks or under stones on open grassland, typical habitats include short-cropped grassland, pathways through forests and agricultural land and sparsely vegetated situations on sandy or light soils but they are also partly synanthropic, occurring in gardens and town-centre situations. They become active in early spring when they may be seen on pavements or low down on walls in sunny situations, on grazed grassland and footpaths they walk on exposed areas and with a little experience are readily spotted although when handled they pull in their appendages and feign death (thanatosis), remaining this way for some time and when placed among grass they resemble rabbit-droppings. At least a proportion of adults are capable of flight but under artificial conditions they seem very reluctant to do so and seem not to be attracted to light or to occur in flight-interception traps. Both adults and larvae feed on various mosses of the Brachytheciaceae and Hypnaceae but little is known of the life history. The species is represented in the UK by the nominate subspecies but several others are known from Europe e.g. B. p. regalis Steffahny, 1843 or B. p. alpinus Newman, 1832 but the distribution and relationships are only poorly understood and in some areas several occur together.
The large size, 7-11mm, and convex, elongate-oval form will distinguish the present genus among our UK byrrhids; in B. pilula the pronotum is shagreened throughout and with the punctures separated by at least their diameter. Entire beetle dark with dense recumbent pale grey to light brown
pubescence unevenly distributed on the dorsal surface although this is commonly rubbed-off in life. Head only narrowly visible from above, in life generally hidden under the pronotum; punctured and usually sparsely pubescent, with large oval and convex eyes and gradually thickened antennae; the last four or five segments forming an indistinct club. Pronotum broadest at acute posterior angles and strongly narrowed to a rounded anterior margin, basal margin strongly curved either side of the middle. Elytra elongate and broadest at or behind the middle, generally a little narrower at the base than the base of the pronotum, and continuously rounded apically. Each with well-impressed striae and up to six rows of velvety black patches of pubescence on odd-numbered interstices, in older specimens this tends to become extensively rubbed in the field. Legs dark; pro-tibiae straight or weakly curved externally; meso- and meta-tibiae strongly rounded externally, all excavate dorsally to receive the reflexed tarsi and with strong spurs at the inner apical angle. Tarsi 5-segmented, without lobed segments; terminal segment long and evenly broadened apically, claws well-developed, curved and smooth.
Of the 4 UK species of Byrrhus (all in subgenus Byrrhus s.str.), B. arietinus Steffahny, 1843 and B. fasciatus (Forster, 1771) have the entire pronotum smooth between the punctures while in B. pustulatus (Forster, 1771) it is shagreened along the anterior margin, although sometimes only indistinctly so; the pronotum is only extensively shagreened, and often entirely so, in the present species.