Around the World

Staphylinidae is the largest family in the U.K. as well as in many other European countries, and it is also, with more than 4000 species, the largest in North America. Some idea of the extent of the family, as recently listed, may be appreciated by considering the following numbers of species from various regions: Europe >4000 of which The U.K. >1000, Central Europe about 2000, Scandinavia >1200, Australia about 2500, New Zealand about 1000, and even relatively recently colonized islands tend to have a diverse fauna e.g. there are more than 100 species in Hawaii.  And these figures are likely to greatly increase. Frankly there is little point in discussing the world distribution of the family as it is such a bewilderingly wide subject, but some consideration of this aspect will be discussed briefly under the subfamily headings given below and more extensive information will be given in the pages devoted to particular groups or species. Many fossil specimens have been collected and some of these are of modern species so they can shed light upon their modern distribution. Pleistocene fossils are known from Europe and North America while Oligocene fossils are more widespread; from Dominican and Baltic amber and from shales in Europe and North America. Cretaceous and lower Jurassic deposits have also produced Staphylinid fossils. The most ancient of such fossils, from Virginia (U.S.), is from the upper Triassic, more than 200 million years old. Most of these fossils resemble modern groups.

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Description

The vast majority of species are instantly recognizable as staphs; they are generally elongate and slender with short elytra and an exposed and flexible abdomen. Some subfamilies are atypical e.g. Scaphidiinae or Omaliinae, but with a little experience these also become obvious. Staphylinids range in size from less than a millimetre to about 40 mm although for the purpose of identification size is often a poor guide as many species are able to ‘telescope’ the abdomen and so may appear longer or shorter than usual, this telescoping often occurs as set specimens are drying. The majority of species are small, <6mm, and some subfamilies are composed entirely of small species e.g. Micropeplinae or Proteininae, or mostly so e.g. Aleocharinae. Given the huge diversity it might be surprising that there are no larger species, but size is a reflection of the lifestyle of most species; the form and flexibility of the body enables staphs to live in physically constrained environments that would not be available to other beetles, and so it follows that an increase in size or rigidity would not be adaptive. By no means do all groups have the short elytra typical of so many, and in general, beyond the elongate and parallel to fusiform habitus, most staphylinid features vary so much that a more detailed description of the species is better given at the subfamily level e.g. the most developed eyes are seen in Steninae or some Quedius (etc.), but beyond this they vary in size and convexity, and many cavernicoles and myrmecophilous have rudimentary eyes or lack them altogether. Some groups e.g. Omaliinae also have ocelli. Similarly while many species possess well-developed hind wings and disperse by flight, many specialized species living on seashores, caves or in the soil have reduced or lack them entirely. Tarsal development is also very variable with all combinations from 5-5-5 to 3-3-3 being represented. Those species with short elytra are generally very agile and flexible, but in many cases this advantage is traded for a susceptibility to desiccation, and in many such species the abdomen has evolved to (partially) counter this e.g. in some Paederinae, Osoriinae, Euasthetinae and, particularly, Steninae, the abdominal tergites, sternites and paratergites are fused into complete rings. Typically the antennae are 11-segmented and filiform, tending towards a moderate club in some groups and distinctly clubbed in some Steninae. Most species are drab and dark but some groups include bright yellow to red colours e.g. Tachyporinae or Oxyporinae, and many are strongly metallic e.g. in Steninae and Paederinae, but some truly strikingly coloured species are to be found in the Staphylininae. The only group of insects likely to be confused with the staphs are the Dermaptera (earwigs) but these have opposable abdominal appendages and many more antennal segments.

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Ecology

Many species develop in temporary habitats e.g. decaying plant and animal or fungal matter, dung or by temporary pools, and so, in general, the immature stages tend to grow rapidly, in days or weeks, while the adults tend to be long-lived. Staphylinid eggs are round, elongate or ovoid and while many are smooth, some have characteristic microsculpture etc. so that identification to certain groups is possible. The larvae are generally elongate and flattened with well-developed legs; they often resemble carabid larvae but differ in the form of the legs i.e. those of carabids are six segmented and often have two claws while those of staphs are five segmented and always, so far as is known, have a single claw. In staphs the urogomphi are almost always articulated and one or two segmented, any with immovably attached urogomphi are very distinct from carabid larvae; in carabids the urogomphi are not segmented and are immovably fixed to the ninth segment, in some there are articulated urogomphi but in such cases they always have more than two segments. In some groups, more especially Paederinae and Staphylininae, the head is heavily sclerotized and separated from the thorax by a distinct neck. Pupae are obtect (with appendages mostly inside the body), dark and sclerotized in the Staphylininae but in all other groups they are exarate (with the appendages mostly on the surface) and soft or only lightly sclerotized. The prepupal stage in many groups e.g. Steninae, some Paederinae and most Aleocharinae spin a silken cocoon in which to pupate. Staphs occur in such a wide range of habitats that it seems futile to make a list; the majority are probably scavengers while a significant number, especially of the larger species, are active predators of other insects and their larvae, and a few are parasites; parasitic species occur e.g. in the Aleocharine genera Aleochara, Baryodoma and Coprochare, they are external parasites of Diptera larvae enclosed within the puparium, and to facilitate this mode of life the females often oviposit in the soil beneath infected plants. Most attack subterranean pupae but other lifestyles have been documented e.g. Aleochara (Mesochara) valida ( which at 7-13mm is the largest of the genus and probably the largest of all Aleocharinae) is a parasite of the syrphrid Volucella marginata which develops in decaying cacti leaves and stems. Many of these parasites pupate outside the host puparium but sometimes, as in the Nearctic Barydoma bimaculata Gravenhorst, development is completed entirely within, the adults emerging from the puparium. Many immature stages predate other insects and larvae, particularly those of Diptera, these are typified by species of Philonthus, Quedius and Platydracus etc. which generally occupy the same habitats as the prey e.g. dung or carrion, and they are generally very aggressive and active. While staphs occupy just about all habitats many favour moist environments e.g. marginal situations and reed beds, and many will quickly appear at temporary pools in dunes and woodland but none are properly aquatic; many species are adapted to hunt or disperse on the water surface e.g. some Steninae, Paederinae and Oxytelinae are able to walk back to the margin when they accidentally end up on the water surface. Many hundreds of species, in several subfamilies, live only on the seashore and these, even in the U.K., often appear in great abundance. Sweeping vegetation, especially grassland, will always produce staphs, more especially Tachyporinae, Omaliinae, Aleocharinae and Oxytelinae, and some of these will be found in abundance on flowerheads. Decaying leaf-litter, especially on woodland margins, among grass tussocks and in marginal situations can be rich in species, and in fact any decaying organic matter is worth sampling; fallen fruit, dung, carrion, mammal and bird nests and fungi - especially large terrestrial fruiting bodies - can host many thousands of specimens, especially in the autumn. Decaying wood and debris collected from under bark can produce a wide diversity of species, and searching through debris from recently deserted nests of social insects is often rewarding. Many species can also be found in caves and among subterranean fungi and mammal dens. Searching tree-trunks and fungi at night will produce many active specimens and a few are even associated with terrestrial molluscs; staphs have not evolved to prey on molluscs in the same way that some carabids e.g. Cychrus or silphids e.g. Phosphuga have, although many larger species e.g. Ocypus regularly predate them, and some Palaearctic Aleocharinae are parasitic on snails e.g. Zyras sagax Cameron, 1941 has been observed entering the mantle of some pulmonate molluscs and has been assumed to feed on the faeces or mucus within. Apart from sweeping and nocturnal searching some of the best ways to find staphs are by Berlese extraction, tapping likely samples over a sheet or, in the case of dung, immersing it in water, lying down and examining marginal habitats, especially in bright sun when many species will be active, and here it is useful to take a container so that exposed areas of silt etc. can be flooded thus forcing specimens to surface, and trapping. Many species will come to light; properly sited an illuminated sheet may produce thousands of Oxytelinae and Aleocharinae, but many other traps will be productive; carrion traps baited with fish or chicken, pan-traps situated beside dung or carrion, bottle traps baited with just about anything from mushrooms to dry grass, these can be sited in tree hollows or reed-beds etc., fermenting fruit traps can be excellent, and many species can be found at sap runs on damaged trees. In general there is no end to the list of ways to obtain specimens but, apart from a minority of species, most will need to be looked for in moist habitats e.g. we often find Paederus littoralis Gravenhorst, 1802 in abundance on sun-exposed calcareous hillsides in the Chilterns, they are active on grass stems etc. at night but during the day they remain in the soil or under stones where the substrate is always damp.

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The feeding habits of staphs are similarly diverse but in a very general sense the four groups outlined above tend to specialize in various ways; fungi, and especially decaying fruiting bodies, will host abundant specimens but many of these, especially the larger ones, feed on the equally abundant insect larvae, particularly those of various Diptera, that are invariably present, and true fungus-feeding (mycophagous) species have mostly evolved in the Tachyporinae, Aleocharinae, Scaphidiinae and Oxyporinae; if carefully observed in the field species of Scaphidium Olivier, 1790 can sometimes be seen ‘grazing’ on tiny fruiting bodies under logs etc. in damp woodland. Many species have evolved to feed on plants and some are known to be significant pollinators of specific hosts; many Omaliinae occur on flowers where they feed on pollen as well as structural parts; some Carpelimus Leach, 1819 species have been reported damaging cucumbers, Osorius Guérin-Méneville, 1829 (Osoriinae) are known to cause damage to turf, and some Aleocharinae are known to be phytophagous e.g. Himalusa thailondensis Pace, Klimaszewski & Center, 2010 has been considered as a possible biocontrol agent of the invasive Skunk Vine. Both the adults and larvae of some Bledius Leach, 1819 (Oxytelinae) feed on diatoms in marginal substrates. Beating spring blossom or the flowers of gorse or broom will invariably produce Aleocharinae. Sometimes the relationship with plants is not straightforward e.g. Central and South American species of the Aleocharine genus Charoxus Sharp are associated with figs; adults are attracted to the flowers and fruits and enter tiny holes made by fig-wasps, the beetle oviposits in the fruit and the larvae develop feeding upon wasp larvae and adults. Most staphs are facultative predators, appearing opportunistically wherever suitable decaying substrate is likely to host Diptera larvae etc., this method of feeding is common throughout most subfamilies but especially Tachyporinae, Pselaphidae, Steninae, Paederinae and Staphylininae. Some species are (of course) specialized e.g. members of the Aleocharine genus Oligota Mannerheim, 1830, and the majority of the Scydmaeninae predate mites, and some members of the Staphylinine genus Erichsonius Fauvel, 1874 specialize in consuming soil-borne nematodes. Some are highly specialized e.g. the tropical Odontolamus fasciatus Sharp, Belonuchus cephalotes (Sharp) and some Platydracus Thomson, C.G., 1858 predate aquatic diptera larvae in water-filled Heliconid bracts, and a species of Hesperus Fauvel, 1874 (a pantropical genus of >200 species which develop mostly in old trees) predates the same insects in bamboo stems. Members of the tropical genus Eulissus Mannerheim, 1830 (Staphylininae, Xantholinini) prey on adult dung beetles.

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Social behaviour is known in a few groups where adults construct chambers within the host medium for the protection of developing early stages; some Platystethus Mannerheim, 1830 in dung, and some Bledius in silty or muddy marginal habitats, and the Aleocharine Eumicrota deposits eggs in fungi and remains to guard the developing larvae. Many species, especially in Aleocharinae but also other subfamilies, are inquilines in social insect nests e.g. ants, termites, bees and wasps, these generally have behavioural, chemical or structural adaptations, and sometimes mimic the host e.g. Emus has been reported invading hymenopteran nests in Europe. Neotropical members of the Aleocharine genus Euvira develop among communities of lepidopteran larvae while Nearctic members of the genus develop in oak galls. Adults of Neotropical species of Amblyopinus Solsky, 1875 (Staphylinini, Amblyopinina) occur in the fur of rodents feeding upon fleas etc. and they are transported by adults between nests where the larvae develop, consuming other insects. Many species are adapted in other ways e.g. many inquilines in social insect nests produce chemicals that fool the host into accepting them, thus the Aleocharine Zyras is able to live amongst the ant Lasius fuliginosus, and many marginal Stenus species have abdominal glands which produce the surfactant stenusin, this reduces the surface tension of the water behind the beetle thus propelling it forwards. Stenus also have extrusible mouthparts, the labium and palps being produced forward by internal pressure so enabling them to capture small and illusive prey such as springtails. Adults of many subfamilies e.g. Steninae, Oxyporinae, Paederinae and Staphylininae secrete oral enzymes etc. which allow then to partially digest their food while it is being manipulated by the forelegs and mouthparts; in such species the diet is largely liquid. Adults of the tropical Leistrotrophus versicolor (Gravenhorst, 1806) ‘ambush’ prey, mostly flies, by luring them onto vegetation with a volatile secretion from the abdomen. Many species produce defensive secretions from various parts of the body, Ocypus olens is notorious for releasing fluids from the mouth and abdominal apex, and many Omaliinae and Proteininae possess defensive glands between the seventh and eighth sternites. The powerful toxin pederan is present in the haemolymph of Paederus Fabricius, 1775 and its close allies, it is produced by endosymbionts in some females and passed to the eggs and so is present in all developmental stages, it is a powerful protection against arachnids, although much less effective against insects, and is a well known and powerful human irritant. As many of the larger staphs are voracious insectivores some have been tried as biological control agents in many areas e.g. Tachyporus against aphids in crops, or Aleocharines against mites and horn flies in the United States and root-feeding larvae in Europe but in general without success.

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Sexual dimorphism is common among staphs and well-developed in many species; in many Oxytelinae the males possess thoracic or cephalic horns or other modifications, and in many groups the mandibles are more developed in males. In general the proportions of the head and pronotum differ between the sexes and males often have modified pro-tarsi.

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Staphylinid identification is still very difficult, even in the U.K., although in many European countries the situation is much better and some foreign works cover much of the U.K. fauna but an ability to translate is essential, particularly of German. Here the situation was greatly improved by the publication of two modern R.E.S. Handbooks, and for some of the subfamilies not covered by these Joy’s 1932 Handbook and an old R.E.S. Handbook remain useful but the Aleocharinae remain problematic and will require a great deal of literature searching before any real progress can be made. The ability to dissect specimens, even of larger species within the Staphylininae, is essential.