Philonthus Stephens, 1829

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.

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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!