Platystethus arenarius (Fourcroy, 1785)


A common and often abundant species throughout England, Wales and Scotland including the Western Isles (Hammond, 1971). The usual host is herbivore dung and the beetle is likely to be found wherever this occurs, irrespective of situation; locally they occur in abundance on Common moor where cattle are grazed through the summer and autumn, on horse grazing pasture throughout our area and around the bridle paths through Whippendell wood etc. Beyond dung habitats they may be found by general sweeping as they fly and obviously travel some distance as we have recorded them at our town centre gardens. Crespuscular and nocturnal sweeping around shrub borders in open situations has often produced them, sometimes in company with other Oxyteline staphs e.g. Anotylus rugosus (Fab.) or Oxytelus laqueatus (Marsham) and sometimes in large numbers. They have also been recorded from piles of rotting grass cuttings, carrion and decaying fungi (Hammond, 1971). Adults are sometimes extracted from dry dung, orthe ground beneath this, during the winter months. The species breeds from spring to autumn and the female displays social behaviour, a detailed account of the facinating life history is given by Hinton (1944) and a brief summary is given here.

The female constructs an oval or spherical egg chamber, generally between 6-10mm across, in dung usually above ground level but sometimes part of this is below the ground. Males are not present in the egg chamber. The female lays eggs in layers within the cell oveer 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 P.arenarius larvae, smaller larvae are accepted by the female. She will also destroy, by biting, any fungal hyphae that enter the cell, these are known to prevent hatching both by physical constriction and also by direct attck on the egg. Incubation takes three to four days eclosion is assisted by an egg burster on the megatergum, this 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, again they will feed upon carrion within the dung but they are not predaceous. As well as feeding, the larvae spend much time repairing and enlarging their chambers. Fully grown larvae cease feeding and wander around to find a suitable site to build 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 from freshly laid egg through three larval instars to adult takes from 29-35 days at 25C. Although larvae are faculative 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 are very active when disturbed eg seived, and superficially resemble other very common Oxyteline species among which they are usually found. The centrally grooved pronotum devoid of other sculpture, which distinguishes the genus from other Oxytelines, is the most obvious hand lens character but specimens will usually need to be stunned and examined critically.

4-5mm although smaller specimens occasionally occur. Distinguished from other Platystethus species by the lack of longitudinal impressions beside the eyes. Antennae inserted outside outer margin of mandibles, base of first segment hidden from above. Black, or pitchy red towards base. Segments 1-3 elongate, 2 about as long as 3, 4-6 quadrate, 7-10 transverse, 9 and 10 strongly so. Palps lighter. Mandibles protruding. Head shining black, strongly raised behind eyes and depressed between longitudinal ridges beside eyes to clypeus, depressed area dull and granulate. With a transverse impression to behind temples and a longitudinal impression through the raised vertex. Moderately strongly punctured, microsculpture just visible at X20, stronger in male. Pronotum shiny black and transverse(L2:W3), rounded and strongly bordered, front angles and centre of anterior margin weakly produced and with a central longitudinal impression. Punctured as head but stronger and longitudinally strigose laterally. Elytra transverse (L1:W3) and without striae, normally shining black to pitchy red but disc sometimes yellowish (Hammond, 1971). Puncturation longitudinally strigose, stronger laterally. With a fine sutural striae which extends for a short distance along basal margin but not apically, sutural angle rounded so elytra strongly diverging apically. Abdomen shiny black, unpunctured (X20), granulate microsculpture obvious at this magnification, with strongly raised side borders. Legs brown, femora darker. Protibiae with strong spines along outer edge and small apical spurs, strongly emarginate subapically, more so in male, apical dilation stronger in male. Meso and metatibiae with smaller spines along outer edge and two small spurs on outer apical margin. Tarsi 3-3-3, terminal segment longer than first two combined. Male temples much longer than female in relation to eye diameter and clypeus with a single central projection

Description from 6 Watford specimens at X40

Hammond, P.M. 1971 Descriptions, figures and key to British species of Platystethus. Ent.Mon.Mag. 107:93-111
Hinton, H.E. 1944. Some general remarks on sub-social beetles with notes on the biology of the staphylinid, Platystethus arenarius Proc.R.Ent.Soc.Lond.(A) 19:115-128