Platystethus arenarius (Fourcroy, 1785)
This is a widely distributed and generally common species occurring throughout Europe from the Mediterranean to the far north of Scandinavia and east through southern and northern Asia; in the UK it is common and usually abundant on dung pasture throughout England, with the exception of the West Country, and Wales, extending sporadically through Scotland including the Western Isles. It is usually associated with herbivore dung, especially that of cattle, but also occurs among decaying vegetation and fungi in almost any habitat. Adults occur year round and are active from April or May until late in the autumn, they fly and are quick to colonize freshly deposited dung; during hot summer days and evenings they may be netted while flying over dung pasture and may occur in numbers at light. Immersing dung samples in water will often produce them in numbers along with much more numerous oxyteline staphs such as O. laqueatus and A. tetracarinatus. Winter is passed in the adult stage beneath dry dung or in the soil beneath accumulated organic debris and so they will occasionally turn up in extraction samples. Breeding occurs from spring until the autumn and the female displays social behaviour, a detailed account of this is given by Hinton (1944) and a brief summary follows: The female constructs an oval or spherical egg chamber between 6-10mm across in dung above ground level or, rarely, a part of this may be subterranean. Males are not present in the egg chamber. The female lays between 40 and 90 eggs in layers within the cell over a period of a few days and remains with them until the last one hatches, during this time she will repair any damage to the cell and defend the eggs against intruders including other, large conspecific larvae, although smaller larvae are accepted by the female. She will also destroy, by biting, any fungal hyphae that enter the cell as these are known to prevent hatching, both by physical constriction and also by direct attack on the egg. Incubation takes three to four days and hatching is assisted
by an egg burster on the metasternum which is lost at the first ecdysis. Young larvae will not attack intruders, each other, or any remaining eggs but will feed upon the bodies of intruders disabled or killed by the mother. There are three larval instars and the larger first instar larvae actively assist the mother in defending young larvae and eggs against intruders. Larvae begin feeding on dung within minutes of hatching and will not attack each other even when overcrowded. Large first instar larvae leave the egg chamber and construct feeding chambers for themselves, here they feed day and night until fully grown and again they will feed upon carrion within the dung but they are not predaceous. As well as feeding the developing larvae spend much time repairing and enlarging their chambers, when fully grown they cease feeding and disperse to find a suitable site to construct a pupal chamber. After this has been constructed the larva goes into a prepupal phase which lasts a day or two during which time it is motionless, no cocoon is constructed and the pupa, after wriggling free of the larval skin, lies on its back in the chamber. This stage lasts about five days and the whole cycle takes between 29-35 days at 25°C. Although larvae are facultative carrion feeders they normally feed exclusively on dung and those observed to have done so produced normal adults.
Without experience these are tricky to identify in the field; they can be very active when disturbed, running or taking flight, and superficially resemble other oxyteline species; the medially grooved pronotum devoid of other sculpture, which distinguishes the genus from other Oxytelinae, is the most obvious hand lens character but specimens will usually need to be examined critically.
2.5-5mm. Distinguished from other Platystethus species by the lack of longitudinal impressions beside the eyes. Antennae inserted laterally outside the outer margin of the mandibles, black or pitchy red towards base; segments 1-3 elongate, 4-6 quadrate and 7-10 transverse. Head shining black with strong cellular microsculpture on the clypeus; vertex strongly punctured, raised behind the eyes and depressed anteriorly. Distinguished from other UK species of Platystethus by the lack of a longitudinal groove beside each eye. Pronotum shiny black; transverse (L2:W3), rounded and strongly bordered, anterior angles and centre of anterior margin weakly produced, disc with a single longitudinal impression, the punctation a little stronger than on the head, microsculpture elongate. Elytra transverse (L1:W3) and lacking striae, normally shining black to pitchy red but the disc is sometimes yellowish, punctation longitudinally strigose and microsculpture elongate, suture divergent apically and with a fine border that continues for a short distance along the basal margin. Abdomen shiny black and impunctate, with granular microsculpture (X20) and strongly raised lateral margins. Legs pale, usually with the femora darker; protibiae with a single series of strong spines along the outer margin, small apical spurs and strongly emarginate subapically. Meso- and metatibiae with a series of smaller spines along outer margin and two small spurs at the inner apical angle. Tarsi 3-3-3; terminal segment longer than the first two combined. In the male the clypeus has a median projection and the temples are much longer than in the female with relation to the diameter of the eyes.