Ptenidium pusillum (Gyllenhal, 1808)
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POLYPHAGA Emery, 1886
STAPHYLINOIDEA Latreille, 1802
PTILIINAE Erichson, 1845
Ptenidium Erichson, 1845
This is a very widespread and generally common Western Palaearctic species, it occurs from lowland to middle mountain altitudes throughout Europe to the far north of Fennoscandia and into Western Russia, it is present on most of the Mediterranean islands, the Canaries, Azores and Gibraltar, and is widespread in Asia Minor and North Africa, it has also become established via trade in north-western North America, New Zealand and Southern Australia. In the UK it is generally common and often abundant across England, Wales and the Isle of Man and more local and scarce in Scotland, including the Western Islas, and Ireland. Adults occur year-round, peaking in abundance during May and June although they tend always to occur in large numbers, and they seem to be active in all but the coldest winter spells. Any kind of decaying organic matter is likely to host the species, they tend to occur in large numbers and populations may persist over many years under stable conditions, they are often abundant throughout the year in compost or well-decayed leaf-litter but may also occur in a wide range of habitats such as carrion, dung, moss, decaying fungi, under all types of bark, in bird and mammal nests and, at least on the continent, they are often associated with ant nests. In general they occur in rather damp conditions but during the summer they may appear almost any situation. Little is known of the biology but ptiliid larvae are usually either mycophagous, feeding on hyphae and spores, or saprophagous, either of which may well apply to the present species. Sieving suitable material over a sheet is the easiest way to find them, and despite their small size they are easily seen on a sheet, but they tend to occur in extraction samples throughout the year.
Identifying these small beetles can be challenging at first as the morphology tends to be very subtle, very good optics that will give X100 are needed and dissection is often necessary, more especially as several common and superficially similar species may occur in a single sample. On the other hand there is usually plenty of material available and so when faced with numerous specimens at least some can be compared with dissected material and named with confidence. The spermatheca is very distinctive (at least to a critical eye, see below) and so a few dissected specimens can reduce having to decide about morphological features, all these are featured in the first volume of BBI. 0.9-1.1 mm. Dorsal surface shiny black or very dark grey , sometimes paler laterally or towards the apex, and with quite dense pale recumbent pubescence, on the elytra also with longer semi-erect hairs directed obliquely backwards, legs pale brown, antennal club and larger basal segments darkened, funiculus pale. Head broadest across relatively large and convex eyes and evenly narrowed to a rounded anterior margin, surface evenly convex and with scattered fine punctures. Penultimate maxillary palpomere enlarged and rather globular, terminal segment narrow, long and pointed. Two basal antennomeres enlarged, 3-8 narrow and elongate, the third segment inserted into the apex of the second but clearly visible, and 9-11 form a long and loose club, the ninth segment distinctly narrower than the tenth. Scutellum triangular, along the base with a fine keel in the middle and a puncture towards each lateral angle, and sometimes with one or more extremely fine punctures between. Pronotum transverse, broadest about the middle and evenly curved to obtuse angles, surface evenly convex and punctured and pubescent as the head, lateral margins very narrowly explanate and basal margin with four well-impressed punctures. Prosternal process convex and narrow throughout, metasternal process broad and truncate between the middle coxae lateral metasternal margins finely wrinkled. Elytra broadest in front of the middle and smoothly curved from sloping shoulders to a continuous apical margin, surface evenly convex, without striae and punctured as the pronotum, lateral margin separated from the reflexed epipleura by a fine ridge which is variable but visible at least towards the base. Apical pygidial margin with (only) two or three small tubercles. Legs short and slender, tarsi 3-segmented. There appear to be no external sexual characters. As stated above, the spermatheca can be distinctive but requires a critical eye to judge the proportions and curvatures of the various parts; several other members of the genus have a broadly similar structure and in P. longicorne Fuss, 1868 they may be almost identical.