POLYPHAGA Emery, 1886
STAPHYLINOIDEA Latreille, 1802
PTILIIDAE Erichson, 1845
Although many species are common, these tiny beetles are poorly understood and rarely encountered. They are generally only found via specialized collecting techniques.
Around the World
A distinguishing feature of this family is the small size of the species, the majority are between 0.5 and 1.5mm in length although some tropical species of Acrotrichis Motschulsky, 1848 can reach 2mm but this is exceptional. The smallest UK beetle was until recently the ironically-named Nephanes titan (Newman, 1834) at 0.55-0.65mm, while the largest UK member of the family, Acrotrichis intermedia (Gillmeister, 1845) may reach 1.25mm, the smallest European beetle, Baranowskiella ehnstromi Sörensson, 1997 at around 0.45mm has recently been recorded in the UK, and the world’s smallest beetles may be members of the genus Nanosella Motschulsky, 1869 which can measure 0.25mm. They are, very obviously, not popular with coleopterists and the faunas of many areas are very poorly understood, only those of Europe and North America being studied in any depth, in consequence the classification and number of species is likely to change drastically if ever the wider world fauna is understood. Presently there are about 550 described species in 70 genera and at least 2 subfamilies and the distribution is cosmopolitan with temperate regions being generally diverse; about 120 species of 27 genera are listed from the Nearctic region and about 220 species of 34 genera from the Palaearctic region, the European fauna includes about 140 species of which 85 extend to northern Europe and about 75 species of 19 genera are listed from the UK although this may be one of the more intensively studied faunas as only 70 are known from Poland. Because keys to many faunas are either acknowledged to be incomplete or lack the necessary detail to provide accurate identifications the group is often frustrating and unattractive for coleopterists generally and may be a last resort after other groups have been explored but this situation has been substantially remedied in the UK by the recent publication (2012) of volume 1 of Beetles of Britain and Ireland by Andrew Duff; chapter 16, written by Michael Darby, provides well-illustrated keys and much useful advice about how to study our fauna. Even so the group will require the use of some very delicate dissecting and mounting techniques, and it is best to keep specimens in alcohol, which may be sufficiently daunting to deter many workers but on the other hand many species are very abundant and easily collected in large numbers, indeed it is often difficult to avoid them when working extraction
samples or sieving compost etc. The family is divided into 2 distinct subfamilies, Ptiliinae Erichsom, 1845 (with 44 UK species) and Acrotrichinae Reitter, 1909 (with 30 UK species)-which includes the largest genus, Acrotrichis Motschulsky, 1848 with about 150 species- and various tribes or other groups sometimes considered to be subfamilies e.g. Nossidiinae, which is more generally considered to be included within the Ptiliinae, and Cephaloplectinae Sharp, 1883, a group of about 40 species in 5 genera and sometimes referred to as horseshoe crab beetles owing to their characteristic shape, they occur in Australia and the New World and so far as is known all are associated with ants; they are a very distinctive group lacking eyes and hind wings and displaying an unusual combination of characters which makes them thus far the best candidate for a distinct subfamily.
Most species are microphagous hyphae and spore feeders and so at least some may be expected to occur wherever decaying vegetation is left undisturbed but they also occur in a wide variety of habitats and many are specialized e.g. the recently recorded UK species Baranowskiella ehnstromi occurs only in pores of the bracket fungus Phellinus conchatus, itself a specialist of the willow Salix caprea L. More generally they occur among compost, leaf-litter and decaying vegetation or wood, under bark or in tree hollows, among fungi, in bird and mammal nests, dung and seaweed etc. and some occur in marginal wetland environments, along the margins of rivers, ponds and lakes etc. in fact any environment that provides conditions for the growth of fungi on which both adults and larvae develop. Because of their small size and cryptic lifestyles they are rarely seen by most naturalists but they should soon become noticed; adults occur year-round and sampling is straightforward; sieving material over a sheet will usually produce them in numbers, they often occur in pitfall and alcohol or fruit-baited traps, they fly in warmer weather, often swarming and so may be netted or sampled in flight-interception traps and individual specimens will often be found in many situations and can easily be pootered. At least some species fly at night and may be found at light. During the colder months many will be found when working extraction samples of soil, litter or tussocks etc. So far very little is known of their biology, numbers are largest during the warmer months and at least some, and this may apply to most species, are known to be continuously brooded. The life-cycle is short, in the few UK species of Ptinella studied it took 6 to 8 weeks and there were 3 larval instars. Fecundity is low, each female produces a single egg every week or so, these develop in turn within the body and may be proportionally very large; up to half the female body size, and depositing it on foliage or among suitable host material. Larvae are similarly tiny, usually less than 2.0mm long, elongate and narrow in form and weakly-sclerotized, white or creamy in colour and lacking eyespots or epicranial sutures, the abdomen is 10-segmented and has (in most groups) 1-segmented urogomphi and a pair of anal hooks. Parthenogenesis is known from several species, some are known only from females, and it may be that this system of reproduction is widespread, or at least unusually common, among the family.
Apart from the small size and mostly loose form the most distinguishing feature, and the one which gives rise to the common name of the family, is the form of the hind wings which consist of a narrow stem, which may have some distinct venation, fringed by one or more series of very fine and long hairs. These vary widely in size and development and are absent in some species but they are a general feature of the family, at rest they are usually completely folded under or covered by the elytra but specimens from traps or among extraction samples often have them extended beyond the elytral apex when they resemble fine feathers. It is unlikely that species can maintain directed flight; rather the wings probably function as passive buoyancy devices, rather like some seeds, and facilitate wind-borne dispersal. The habitus varies widely but the majority are discontinuous in outline, being variously constricted between the head, pronotum and elytra; an exception among the UK fauna being the broadly-oval Nossidium pilosellum (Marsham, 1802). Most are elongate, weakly convex and rather flattened dorsally, all are drab coloured, black to variously brown or yellow and they vary from almost glabrous to densely pubescent. Between genera the shape and proportions of the body parts varies widely but the head is usually proportionally large and substantially visible from above, the eyes vary from small and flat to large and very convex and so occupying most of the lateral margin, temples may or may not be present and the vertex and frons are variously finely impressed or sulcate. The antennae are usually 11-segmented, rarely 10-segmented (some Nanosellini Barber, 1924), and inserted laterally in front of the eyes, the 2 basal segments are broadly expanded compared with the others, the funicular segments fine and often ringed with setae, and while some are almost filiform most have a loose 2- or 3-segmented club. The pronotum may be simply convex and rounded laterally with obtuse angles or broadest at the base and produced backwards at acute posterior angles (etc.), it is usually at least as wide as the elytra and often wider and the surface may be smooth, longitudinally or transversely impressed or keeled or there may be various basal fovea, the anterior margin varies from straight between distinct anterior angles to smoothly rounded and lacking angles, the basal margin is generally straight or only weakly sinuate. The prosternum may have a distinct process between the coxae (e.g. Nossidium or Ptenidium) or the process may be missing and the coxae contiguous (most genera). The mesonotum is transverse and usually short, sometimes with distinct longitudinal sutures and coxae that are usually circular, weakly projecting and closely approximated or, rarely, separated by a narrow process (Microptilium Matthews, A., 1872). Metasternum transverse to quadrate and simply convex, without distinct sutures but often extensively and quite strongly punctured, metacoxae very variable; circular and widely separated in e.g. Micridium Motschulsky, 1869 or transverse and close together e.g. Nossidium. Abdomen usually with 6 visible sternites. The elytra are either long, entirely covering the abdomen, or shortened and leaving various abdominal segments exposed, they vary from long and parallel-sided (e.g. Cylindroselloides Hall, 1999, Nanosellini)to rounded and strongly convex. In most species they are finely and randomly punctured and lack striae, the epipleura may be continuous with the lateral margin or may be delimited by a fine keel, apically truncate, continuously or separately rounded. Legs generally short with exposed and relatively large trocanters, long-oval femora and narrow tibia which may have spines along the outer surface, and 2- or (usually) 3-segmented tarsi. Vestigial morphs occur in many species within the Ptiliini, these usually lack pigment and the eyes and wings may be reduced or absent.
Critical identification often depends on an examination of the spermatheca, this organ is very variable between species but constant within species, the often complex form affording excellent characters, it will be essential in several larger genera where morphology is too variable to offer definite characters e.g. Acrotrichis Motschulsky, 1848 or Ptinella Motschulsky, 1844. Male sexual structures are also very useful in many groups although here the differences may be more subtle. Dissection and preservation are rather specialized areas so far as this family is concerned, some of the more obvious species can be dissected and card mounted as usual but more difficult groups may require examination at very high magnification, beyond that of stereoscopes, nad preserving material on slides or in alcohol, many of these aspects are discussed in Andrew Duff’s book. On the other hand a good deal of progress can be made with a very careful examination of external morphology using the online keys provided by Mike Hackston, these become very involved in later couplets and will not necessarily always provide a confident identification (although spermatheca drawings are given for the difficult genus Acrotrichis) - which is no more than a reflection of how difficult the group is - but they are offer a very good overview of the family morphology and give basic information regarding distribution and habitat preferences for many species and, given that books can be expensive, it is undoubtedly an excellent introduction to the group.