Megarthrus denticollis (Beck, 1817)
This species is generally common throughout Europe, reaching north to the UK and beyond the Arctic Circle in Fennoscandia, to the south it is more sporadic and absent from North Africa and many northern Mediterranean areas although it is widespread in the south east, occurring in Syria and Turkey and extending through Russia and Ukraine to the far east of Siberia and Japan. Here it is common across Wales and England north to Nottingham although generally absent from the West Country, more scarce and sporadic further north to the Scottish Highlands and there are isolated records from Anglesey, Man and Northern Ireland. Adults are eurytopic, occurring year-round among decaying organic matter in a wide variety of habitats; they may be sieved from leaf-litter, decaying wood, dung and carrion, they often occur among flood refuse and have been recorded at sap during the summer. In spring and early summer large numbers may be found in compost, especially where this is fermenting and warm, and in dung although usually among older and drier dung that has lain undisturbed and formed a crust, and in the autumn they occur in fungi and may be particularly abundant among decaying terrestrial bracket fungi that has become waterlogged, turned black and spread onto the ground; throwing such material onto a tray with a trowel may produce an astonishing number of small staphs along with numbers of the present species, and when this material lies undisturbed around the base of trees etc the beetles remain until the spring when it begins to dry out. Adults are believed to be mycophagous or saprophagous but very little is known of the life-cycle.
1.5-2.5mm. Distinguished among our fauna by the clear red basal antennomere and the form of the pronotal base. Dull brown with paler margins to the pronotum and (usually) abdominal tergites, legs pale brown and antennae extensively darkened but always with the first segment pale, dorsal surface finely punctured and very finely pubescent throughout. Head transverse and broadest across convex and protruding eyes, vertex and frons impressed beside the eyes, sometimes giving the impression of a broad median longitudinal ridge, but otherwise without sculpture, temples very short and usually not visible from above, clypeus extended and rounded anteriorly. Pronotum transverse, broadest at or near the base and more-or-less evenly narrowed to rounded anterior angles, basal margin sinuate, with a small projecting tooth inside strongly excised posterior angles, surface weakly convex and with a complete longitudinal median impression. Elytra with rounded shoulders and gradually dilated to broadly-rounded posterior angles, surface evenly and rather densely punctured throughout and lacking striae, lateral margins narrowly raised. Abdomen broad and usually appearing rounded apically, basal tergites with broadly raised lateral margins, punctation fine and dense throughout, finer than the elytra. Males can be distinguished by the form of the posterior tibiae which are produced and twisted to form an apical tooth.