Paederus Fabricius, 1775
This is by far the largest genus of the subtribe Paederina Fleming, 1821, one of 12 subtribes of the Paederini Fleming, 1821, and a group of 15 genera and more than 625 species. Paederus includes about 600 species in more than 12 subgenera based mostly on differences in the structure of the aedeagus but, although always having well-developed parameres, this varies widely and more than 30% of species remain unassigned and so more subgeneric groupings are likely to follow. Adults of many species are known from only a few specimens and many are rare and endemic to certain areas. Several distinct species are known from fossils. The group is cosmopolitan with by far the greatest diversity in tropical regions; 40 species occur in the Western Palaearctic region, about 50 in China and 15 in the Nearctic region of which only one, P. riparius (Linnaeus, 1758), is Holarctic, and only a single species, P. littorarius (Gravenhorst, 1806), occurs in Canada. Conversely, more than 100 species occur in Central America, with Mexico especially diverse, 88 species are listed from South America of which 23 have been recorded from Brazil, suggesting that many more remain to be described from this region, from the Old World, tropical Africa is particularly diverse.
Paederus is one of the most striking and recognizable groups of staphylinids, with all species following a basic pattern of colour, shape and size. All are medium sized beetles, 6-12mm, and most species have fully developed wings and fly well although many are brachypterous or apterous; many of the Central American species are apterous with weakly developed shoulders and the majority are arboreal. The colour follows a basic pattern of a dark head, bright and often metallic elytra, and dark terminal abdominal segments. The appendages are generally pale orange but variously darkened especially the femora and tibiae. Head quadrate or slightly transverse with large convex eyes and long, curved temples; the vertex and frons are sparsely pubescent, the temples more densely so, and there is usually a longitudinal impression between the eyes. Labrum transverse, mandibles curved and sharp with a bicuspid tooth or sometimes two distinct but joined teeth near the middle of
the inner margin. Antennae inserted on the side of the head outside the base of the mandibles; 11-segmented and filiform with the apical segment constricted before the apex. Maxillary palpi with the terminal segment tiny and the penultimate segment broadly dilated towards the apex. Mentum smooth; without carinae, tubercles or teeth, and the gular sutures diverge apically. Pronotum convex and quadrate to elongate, parallel-sided to rather strongly rounded, anterior and posterior angles rounded and the side border is deflexed under the anterior lateral margin. Surface with fine punctures and sparse, fine pubescence, and also with some much larger sensory setae. Prosternum not, or only feebly, carinate, and the pro-coxal cavities are membranous above. Elytra quadrate to slightly elongate, parallel sided to broadened towards the apex and usually with distinct shoulders, the surface with double pubescence and strong, backwardly produced punctures. Apically rather truncate, and separately rounded. Abdominal tergites 3-6 pale and with strongly raised borders, variously pubescent but generally with a subapical transverse row of strong setae and the apical and basal margins glabrous. Tergites 7-8 dark and strongly narrowed apically, lacking a border and variously punctured. The basal sternite is strongly carinate between the posterior coxae, and in the male the eighth sternite is excised to varying degrees. Pro-tarsal segments 1-3 dilated, meso- and meta-tarsi with only the fourth segment bilobed. The terminal segment is long and slender on all the tarsi.
Species occur in a wide range of habitats and some e.g. the Palaearctic P. littoralis Gravenhorst, 1802 are eurytopic, but the majority inhabit wetland habitats, including maritime pools and marshes, permanently damp floodplain meadows and river margins etc both as larvae and adults. Searching peat bogs, standing water margins, drying river beds and damp woodland by sweeping low vegetation or sieving leaf litter will usually produce a few species, they tend to be common where they occur and huge populations sometimes build up; the widespread and common P. fuscipes Curtis, 1826 is often super abundant during the growing season in rice fields in Malaysia but disperses during harvesting. Temperate species typically breed in spring and summer, oviposit in the ground early in the year, and develop through two instars as larvae through the spring and summer to produce summer adults that will overwinter, and both adults and larvae of all species are thought to be primarily predaceous. Most species can fly, they often climb plant stems and foliage and are primarily diurnal but many are also active at night and are common at light traps. About 20 species are known to have toxic chemicals (non-protein based amides with ring structure groups, named Pederin, Pseudopederin and Pederone, produced by symbiotic bacteria passed from adults to larvae via the chorion) in their haemolymph which cause contact dermatitis, and so far as is known this is the only genus within the family to do so, skin reactions are usually mild but can be severe in some people, causing extensive inflammation and peeling skin, it is usually caused by crushing beetles against the skin but cases in temperate regions are very rare. Extracts from crushed beetles have long been used in Chinese medicine to treat nasal polyps, ringworm, skin eruptions and boils. Worldwide many beetles and spiders mimic Paederus but only the Aleocharine species Zyras collaris (Märkel, 1842) is known from the UK.
Four species representing four subgenera occur in the U.K. Two of our species are widespread and common while two are more restricted and local; the common species have almost entirely pale hind tibiae and so are easily recognized in the field. Typical specimens are readily identified from the following key:
Larger, >7.5mm. Hind tibiae entirely pale or darkened only narrowly at the base.
Smaller, <7mm. Hind tibiae more extensively darkened, at least the basal third.
Mandibles black, a character unique among the U.K. species.
Eyes smaller and less convex. Elytra slightly longer and wider than the pronotum.
Eyes larger and more convex. Elytra longer and much wider than the pronotum.