Philonthus Stephens, 1829
This is a cosmopolitan genus of more than 1250 species, it is well-represented in most regions e.g. 112 (including 17 adventive species) occur in North America while less than 50 are known from South America (although in such diverse faunas as this the limits of the genus are uncertain and at least some are likely to be included in other genera), but the Palaearctic and Oriental regions, with more than half of all known species seems to be particularly diverse, but this includes some of the best studied faunas and the true world total is likely to much higher. About 70 species are known from Central and Northern Europe, of which 46 occur in the UK. Of the UK species 42 have been recorded from Ireland and one of these, P. furcifer Renkonen, 1937, does not occur in Britain. There have been several attempts to split the genus into subgenera or even separate genera but the group is large and as a consequence naturally diverse, and so a molecular genetic analysis should be applied before any such divisions are made. Regarding our UK fauna, several species have been removed over the years, mostly to the genus Bisnius Stephens, 1829, and the very distinctive P. marginatus (Müller, O.F., 1764) is included in the subgenus Onychophilonthus Neresheimer & Wagner, 1924. Our remaining species are included in Philonthus s.str. P. marginatus is generally common throughout the Palaearctic region while the only other European member of the subgenus, P. lederi Eppelsheim, 1893 is restricted to Fennoscandia. Onychophilonthus differs from Philonthus s. str. in having the front claws as long as the terminal tarsomere, they are also articulated and can be withdrawn into a cavity beneath the last tarsal segment. In Philonthus s.str. the claws are much shorter than the terminal tarsomere and are fixed. For identification purposes this character is irrelevant as P. marginatus is easily recognized by its bicoloured pronotum, but interesting nonetheless. Our species are very typical of the subfamily with filiform antennae inserted dorsally in front of the eyes and within the base of the mandibles, the eyes are of normal size and placed towards the front of the head and both the head and pronotum are smooth, lacking grooves or other sculpture, and glabrous but for various sensory setae.
A very good character for recognizing Philonthus is a longitudinal series of punctures either side of the pronotal disc, these are present in all but three of our species (which are otherwise very distinctive) but various Quediinia Kraatz, 1857 also have similar puncture series e.g. Quedius Stephens, 1829 but here there are three punctures (a number never seen in Philonthus) and they are confined to the anterior half of the pronotum. Some other genera within the Philonthina Kirby, 1837 also have longitudinal puncture series, these may be otherwise very distinctive e.g. Cafius Stephens, 1829 has the pronotum strongly narrowed to the base, Rabigus Mulsant & Rey. 1876 has a very distinctive terminal maxillary palpomere and Gabronthus Tottenham, 1955 is smaller, <3.5mm, but certain Gabrius Stephens, 1829 and, especially, Bisnius Stephens, 1829 can be very similar. Ostensibly both Gabrius and Bisnius may be distinguished by the elongate second segment of the front tarsi, in Philonthus it is always transverse, but this can be difficult to appreciate and some specimens may need to be dissected or identified by comparison, B. puella (Nordmann, 1837) is a good example, being closely similar to P. punctus (Gravenhorst, 1802). But in general, and with the above in mind, the following series of characters will distinguish our UK species: 5.0-13.5 mm, forebody glabrous but for various sensory setae, head without longitudinal furrows, pronotum parallel-sided or tapered anteriorly, lateral border reflexed underneath towards the front, surface impunctate or with two longitudinal series of at least four punctures that extend from the anterior margin into the basal half, elytral suture not overlapping, terminal maxillary palpomere narrowed at the base and at least as long as the penultimate segment, second segment of the front tarsi transverse.
Individual species may vary subtly in shape and there is often pronounced sexual dimorphism in the shape of the head, and the group as a whole displays a wide diversity in general morphology (at least to the critical eye) but important characters with regard to identification include the following. The shape of various antennal segments, especially the subapical segments, is often diagnostic of species or groups of species e.g. in P. addendus Sharp, 1867 segments 9 and 10 are weakly transverse while in the closely similar P. politus (Linnaeus, 1758) and P. succicola Thomson, C.G., 1860 they are more strongly so, this sort of thing can be subtle and will require patience to appreciate but it is a powerful aid to identification. An absolute gift of an antennal feature is seen in P. cognatus Stephens, 1832, here uniquely the basal segment is bicoloured, dark above and pale below, nothing more is required for this ident! The shape of the forebody provides crucial aids to identification, the head varies from transverse and quadrangular to elongate rounded, extreme cases being the transverse form seen in e.g. P. splendens (Fabricius, 1792) and the elongate and almost continuously-rounded form of P. albipes (Gravenhorst, 1802), and the pronotum may near parallel-sided to distinctly tapering anteriorly. Microsculpture is often a good aid to identification, usually on the head and/or pronotum but sometimes on the elytra e.g. P. quisquiliarius (Gyllenhal, 1810) has at most very faint microsculpture on the head and pronotal disc whereas the closely similar P. ventralis (Gravenhorst, 1802) is strongly microsculptured, P. quisquiliarius is an interesting case in point here as specimens may be identified in the field from a distinct coppery reflection they display with sunlight from a certain direction. The number of punctures in the pronotal series, and this should include the anterior most puncture which is often very close to the apical margin, beyond this the pronotum will have various sensory setae towards and around the margins but these are of no use in identification work. With a little familiarity (and probably a lot of field work) of the group these punctures will usually give a good indication of the species, or at least the species group, even in the field and other species e.g. the very common and superficially similar Bisnius fimetarius (Gravenhorst, 1802) (with four punctures in each series) or Gabrius splendidulus (Gravenhorst, 1802) (with five punctures in each series) will soon become familiar. Unfortunately it is sometimes the case that one or more punctured are missing from one or both series, this is not uncommon and can be distressing at first but certain aspects of this are soon appreciated e.g. there is usually a gap where the missing puncture is supposed to be and the most numerous puncture series is usually the one to work from. In general most species are black or mostly so, some have a distinct metallic sheen to the forebody or just the elytra but there are also many distinctly coloured species e.g. P. spinipes Sharp, 1874 and P. rubripennis Stephens, 1832 have bright red elytra, and several species e.g. P. sanguinolentus (Gravenhorst, 1802), P. coprophilus Jarrige, 1949 and P. varians (Paykull, 1789) have dark elytra with red markings, sometimes these markings are diffuse and might be missed in the field but they display well under the microscope with good light and often shine when photographed with strong flash, some species vary e.g. P. jurgans Tottenham, 1937 usually has dark elytra but red marked specimens sometimes occur, while P. quisquiliarius occurs in two forms, one with black elytra and one (var. inquinatus Stephens, 1832) that has red elytra that are black across the base. Other useful features include the size of the eyes or the proportion of the lateral margin they occupy, and the form of the fine transverse line across the base of first few visible abdominal tergites, this is usually straight but in a few species e.g. P. corruscus (Gravenhorst, 1802) and P. rectangulus Sharp 1874 it is angled at the middle. A very frustrating character can be found in the hind tarsi, this again requires patience and experience but can be very useful for separating certain groups e.g. P. quisquiliarius and P. ventralis from P. concinnus (Gravenhorst, 1802) and certain others, in the former group the basal segment is thicker than the following segment while in the latter they are more or less equal in thickness. These characters in combination with the size of a specimen will allow most of our UK species to be identified with confidence but certain groups or pairs of species can only be determined by the form of the genitalia, thus P. jurgens, P. varians and P. confinis Strand, A., 1941 will need to be dissected to examine the proportions of the paramere compared to the median lobe, and P. micans (Gravenhorst, 1802) and P. micantoides Benick & Lohse, 1956 will need to be dissected to examine the form and length of the median lobe beyond the parameres. In some species there are small dark spots towards the apex of the paramere and these can be useful for identification. Fortunately Philonthus are easy to dissect, the genitalia are well sclerotized and easy to clean and manipulate and so in this sense they are an easy group to work with, furthermore males may be recognized by a small angled incision on the apical margin of the sixth abdominal sternite. Females are sometimes impossible to assign but many may be confidently named by comparison or by association. These various characters are useful per se but become much more so once a specimen narrowed down according to how many punctures are present in the pronotal series, the usefulness of this can be appreciated from the following which lists species according to the puncture series-it really does make identification easier!
Pronotum with 2/3 punctures
Pronotum with 4 punctures
Pronotum with 5 punctures
Pronotum with 6 punctures
Pronotum with 9-15 punctures
All species are thought to be predatory both as larvae and adults and in most cases adults are active over a long season and are probably present year-round. Regarding the wider Palaearctic fauna there are species that occur only in alpine regions and some have been recorded from caves but in general most are terrestrial and many inhabit a range of biotopes. Many of our UK species may be found in a range of habitats including disturbed areas such as parks and gardens, and open grassland and wasteland are good places to look for a variety of species. Inevitably there are a few specialists e.g. P. atratus occurs among vegetation on sandy riverbanks, P. cervinus occurs in low-nutrient fens, and P. fumarius is most frequently found in nutrient-rich fens, but the key to finding a good variety of species is to search among decaying organic matter that is likely to host large populations of other insects and their larvae, garden compost heaps are often good in this respect. Many species e.g. P. laminatus and P. concinnus prefer open and permanently damp habitats, and wooded margins, moorland and heathland are good places to search for these. Searching under logs and among litter in shaded woodland will produce several species and is the best way to find P. decorus, if decaying fungi are present all the better as this will often host several species, decaying terrestrial sporophores are the best types to search, especially when they are layered and wet. Dung pasture is among the best habitats to find a range of species, they often occur in numbers and several will often be found together, these are mostly generalists that will also be found in litter and compost etc. but some e.g. P. splendens seems to occur almost exclusively in dung, among the generalist dung dwellers are P. addendus, P. marginatus and P. varians. On the other hand many occur in decaying organic matter but are only infrequently encountered in dung, these include e.g. P. rectangulus, P. politus, P. discoideus and P. sanguinolentus. A few of our species are known from only a few records e.g. P. alpinus has been recorded from horse dung while P. dimidiaticornis has been recorded twice from the Suffolk coast. Carrion attracts a wide range of staphs and is often a good source of Philonthus; among those likely to occur are P. cognatus, P. longicornis and P. succicola. Most species are mainly nocturnal and searching suitable habitats at night is usually the best way to find them although many may also be active in bright sun e.g. P. cognatus or P. carbonarius often occur when sweeping vegetation generally and some may be seen running on pathways in the evening. Pitfall trapping will often produce good results from spring until the autumn, and extraction samples from litter or tussocks often produce specimens during the winter, as may flood refuse, but by far the easiest and most productive way of sampling Philonthus is by sieving likely samples over a tray, but be warned they can run very fast and many will fly at the first chance, pootering is as good a way as any to catch them although they usually require a certain amount of chasing, and a good supply of tubes will be needed as it is not a good idea to keep specimens together.