TRECHINAE Bonelli, 1810
These small ground beetles are very widespread and abundant in almost every habitat. Includes the UK's largest carabid genus Bembidion.
This is a huge cosmopolitan group of very diverse carabids, the scope and classifications have changed greatly over the years and many differing schemes will be found in the literature but in the modern sense four tribes including more than 5400 species are included, this includes the Pogonini Laporte, 1834 and the Zolini Sharp, 1886, which together include about 150 species, and the Trechini Bonelli, 1810 and Bembidiini Stephens, 1827 which include the rest in roughly equal numbers. Zolini includes 10 genera and about 60 species in three subtribes and is a predominantly southern hemisphere group; Zolina Sharp, 1886 includes about 50 species in 8 genera, one from South America and the rest from the Australasian region with New Zealand being particularly rich in species. Sinozolina Deuve, 1997 includes 6 species of a single genus from China, and Chalteniina Roig-Juñent & Ciccino, 2001includes a single species from Argentina. Pogonini includes about 90 species in 12 genera and occurs in all major geographic regions but is most diverse in the Palaearctic region; 6 species of 3 genera occur in North America while the 56 species are listed from the Palaearctic region and the European fauna includes 29 species in 4 genera, 19 of these belong to the genus Pogonus Dejean, 1821 and of these 3 extend to the UK. All species of this tribe display a tolerance to saline conditions and all are either coastal or live on river margins with varying salinity.
This subfamily includes several well-defined and distinct tribes which may not immediately appear to form a natural group but all members share features of both adult and larval morphology as well as ribosomal sequence data and (so far as is known), in males, features of chromosomal structural rearrangement during meiosis that are not seen in other carabids. Most species within this subfamily are small or very small and the largest reaches about 8mm, in general members of Trechini are drab black or brown while many of the Bembidiini, which are mostly diurnal surface predators, are colourful and/or metallic, or at least have some colour or pattern. All have paired supra-orbital setae and a lateral mandibular seta and all, even where they are adapted for a subterranean or cave-dwelling existence, have unmodified front tibiae, they have the internal subapical antenna-cleaning notch but are not fossorial. The typical shape is elongate
and rather flat with the forebody narrow compared with the elongate elytra i.e. very typically carabid, but many cavernicoles (etc.) are atypical, often with very elongate body sections, long sensory setae, legs and antennae, reduced pigments and small or absent eyes. No such species have been discovered in the UK despite central and southern Europe being a major centre for such forms. A defining character of the Trechini is the curved frontal furrows that orbit the eyes; these may be faint or even incomplete but are present to some extent in all species. Other features are general for the group but may be seen in certain Bembidiini, most posses a recurved sutural striae, a feature also seen in e.g. Ocys, and in most the terminal maxillary palpomere is normally developed i.e. long, broad at the base and tapering to a point, although in some Trechini e.g. Aepus and Perileptus it is narrowed and variously shorter. Bembidiini are characterized by the reduced maxillary palpomere, they are otherwise a very diverse group which beyond a rather typical habitus are very diverse morphologically; they vary greatly in the form of the frontal furrows on the head, and the form of the pronotum and the elytra which may have regularly punctured striae, reduced or incomplete striae or, in e.g. Asaphidion, lack regular striae altogether. All have long and robust legs with 5-segmented tarsi and in many the front tarsi are sexually dimorphic. Species of Pogonini have the terminal maxillary palpomere well-developed and lack the orbital furrow seen in Trechini, they are thus distinct from our other tribes of Trechinae, the presence of a lateral mandibular seta will distinguish them from superficially similar UK species with the exception of Patrobus Dejean, 1821, from which they are distinct in having a fine border across the base of the elytra, joining the base of the striae. Beyond this there is enormous variety within the subfamily but a familiarity with the UK fauna-the morphological variation of which is very wide-can soon be gained from looking at pictures or examining specimens.
BEMBIDIINI Stephens, 1827 is a large cosmopolitan group of about 2800 species classified into 6 subtribes although in some cases the tribal placement is not certain. All but a single genus are characterized by the reduced terminal maxillary palpomere, a feature seen in only a very few other carabid groups. Horologionina Jeannel, 1949 includes only the monotypic genus Horologion Valentine, 1932 which is unique among the tribe in having normally developed terminal palpomeres, H. speokites Valentine, 1932 is known only from the Holotype collected from a cave in West Virginia. Lovriciina Giachino, Gueorguiev & Aailati, 2011 includes 4 species of 3 genera of cave dwelling carabids from the Balkans. Anillina Jeannel, 1937 is a large worldwide group of 60 genera and about 400 species of tiny blind and apterous cave and soil dwelling species, they are most diverse in tropical regions but well-represented in northern temperate areas and Europe has a diverse fauna of more than 150 species of 16 genera, these occur mostly in southern regions and none extend to the UK. Xystosomina Erwin, 1994 includes about 130 species of 8 genera, they are mostly small arboreal beetles of Neotropical and Australian rainforests, the largest diversity is in the Amazon basin and only a single species extends north into the USA. Tachyina Motschulsky, 1862 is a worldwide group of about 800 species, they are most diverse in tropical regions but are well-represented in northern temperate areas e.g. the European fauna includes 52 species of 9 genera, of which 6 species of 3 genera (depending on how the subgenera of Tachys Dejean, 1821 are classified) extend to the UK. In temperate regions they tend to live in a variety of terrestrial habitats but in the tropics they are more diverse with many hypogean, myrmecophilous and saproxylic species. Among the subfamily they are morphologically well-defined; they possess the reduced terminal maxillary palpomere and the vast majority have a recurved sutural stria and apicolaterally notched and truncate front tibia. Among our UK fauna species of Ocys Stephens, 1828 also have a recurved sutural striae but here the front tibiae are truncate or gently curved and the elytra possess an abbreviated scutellary striole, a feature absent in all our Tachyina. Bembidiina Stephens, 1827 is a large group of more than 1350 species in about 25 genera and is by far most diverse in temperate regions; it includes most of the larger species of the tribe. The subtribe is dominated by Bembidion Latreille, 1802, which is a morphologically distinct genus but is nonetheless in the process of being divided into smaller genera; there are numerous genera which were formerly included as subgenera and there are many subgenera variously considered to be distinct genera and so the total number of species and groups is difficult to estimate but the species seem to form three large series (clades) centred around the subgenera Ocydromus Clairville, 1806, Odontium LeConte, 1848 and Bembidion s.str. In the widest sense the genus includes more than 1200 species in more than 100 subgenera and species groups, they occur throughout the world with about 75% in northern temperate regions; about 220 are European and more than 250 occur in North America, the UK fauna includes 54 species of 27 subgenera and is a good representation of the genus as a whole. Other genera represented in the UK include Asaphidion Gozis, 1886, Ocys Stephens, 1828, Bracteon Bedel, 1879, Cillenus Leach in Samouelle, 1819 and Sinechostistus Motschulsky, 1864. Asaphidion includes about 40 species, it is Holarctic but mostly Palaearctic with only 3 species, one of which is adventive, occurring in North America, 13 occur in Europe and 4 extend to the UK. Ocys is a Western Palaearctic genus of about 20 species, 17 of which occur in Europe and 3 extend to the UK. Bracteon includes 2 UK species. Cillenus, either as a genus or a subgenus of Bembidion, has variously included up to 20 species with groups distributed in Asia and Australasia but following recent research it will probably be split and include only the Western Palaearctic species C. lateralis Samouelle, 1819. Sinechostictus includes 2 UK species formerly included in subgenera of Bembidion, the genus is restricted to the Western Palaearctic region and includes about 25 species, 14 of which occur in Europe. The classification of the Bembidiina is obviously very volatile and the most recent works will need to be consulted in order to keep up with what is going on, the group is a very popular one and as new species are being added all the time it is very unlikely that any present-day system will remain intact for long.
TRECHINI Bonelli, 1810 includes more than 2600 species in almost 200 genera and a growing number (usually accepted as four) of subtribes, it has a worldwide distribution but is most diverse in temperate regions. All species are small, <7mm, and many are tiny, the group includes many cave-dwelling (Trechodina and Trechina) and hypogean species, the majority of which occur in northern temperate regions with southern Europe and China being hotspots. The majority of Trechini are drab-coloured and most are lacking or have reduced eyes, the majority are of typical carabid form but some are highly developed and adapted to a troglodyte or subterranean lifestyle. Trechodina Jeanell, 1926 includes 18 genera and has a mostly southern hemisphere distribution with only a relatively few species extending north into southeast Asia and the Palaearctic regions, the European fauna includes 10 species of 3 genera, only one of which, Thalassophilus longicornis (Sturm, 1825), extends to the UK. Another UK genus Perileptus Schaum, 1860, generally included in the following subtribe, has recently been demonstrated by DNA analysis to belong to the Trechodina. The group is absent from the Nearctic region and is represented through much of Asia by T. longicornis. Aepina Fowler, 1886 includes up to 10 genera depending on how the species are classified, it is a small group with genera from New Zealand, Chile, Falkland and Campbell Islands and the Atlantic coast from Morocco to the UK, it is represented in the Palaearctic region by 4 species of 2 genera, all of which are restricted to western Europe. A single species of each genus occurs in the UK. Perileptina Sloane, 1903 includes only 3 genera, it is represented in the Palaearctic region by the 8 species of Neoblemus Jeannel, 1923 and about 30 species of Perileptus Schaum, 1860 although only 2 species of Perileptus occur in Europe and the UK fauna includes only P. areolatus (Creutzer, 1799), the genus otherwise includes about 50 species and occurs in tropical regions worldwide. A third genus, Apoplotrechus Alluaud, 1915 includes 2 species from Madagascar. Trechina Bonelli, 1810 includes the vast majority of the species, it is most diverse in northern temperate regions and dominated by several large genera e.g. Trechus Clairville, 1806 is a widespread genus of about 900 species and numerous subspecies in 8 subgenera, more than 90% occur in the Palaearctic region and the European fauna includes about 200 species, of which only 7 extend to the UK. The Nearctic Trechini fauna includes more than 225 species in 9 genera and all are classified in the Trechina, the largest genus, Pseudanophthalmus Jeannel, 1920 includes about 200 species from caves in ten of the eastern states. The European fauna includes about 850 species and numerous subspecies in 40 genera, most genera are small and of very limited distribution e.g. 15 are monotypic and another 14 include less than 5 species but there are several large genera; the largest is Duvalius Delarouzée, 1859, a Eurasian genus of about 350 species, almost 300 of which occur in Europe but none have been recorded from the UK, it includes many cavernicoles with exaggerated morphology and species have been found to depths of 2140 meters in the world’s deepest cave in the Caucasus.
Pogonini is represented in the UK by 3 species of Pogonus. All are coastal, living in salt marshes or on sandy or shingle beeches, only one species, P. chalceus (Marsham, 1802) is widespread and common, the others being very local and restricted to the south and south east coasts, but all may be abundant where they occur, especially beside pools etc. in spring and early summer. Our species are medium sized beetles, 5.5-8.5mm, bright metallic and very distinctive as they are active in bright sun. They lack the circular furrows seen in Trechini but have straight and deep longitudinal furrows on the frons, the pronotum has distinct posterior angles and shallow basal fovea and the striate elytra have 3 setiferous punctures on the third interval.
All four subtribes of the Trechini are represented in the UK. Our only member of the Perileptina, Perileptus areolatus (Creutzer, 1799) is a widespread but very local riparian species. Distinctive among our species by its small size, <3mm, lack of a recurrent sutural stria and entirely pubescent dorsal surface, including the eyes. Aepina includes 2 UK species. Both are widespread coastal species occurring in rock crevices in the inter-tidal zone. Both are pale brown and tiny beetles, <3mm, with a recurved sutural stria and long sensory setae to the head pronotum and elytra. Aepus marinus (Ström, 1783) is distinguished by its tiny flat eyes and truncate elytral apices. Aepopsis robinii (Laboulbéne, 1849) has larger protruding eyes and separately rounded and lobed elytral apices. Trechodina includes only Thalassophilus longicornis (Sturm, 1825), a widespread but generally rare species occurring among sand and fine shingle on river and gravel pit margins, it is distinguished among our fauna by the size, 3.5-4.0mm, glabrous dorsal surface, recurved sutural stria and finely bordered basal margin to the elytra. Trechina includes 9 species of 3 genera, all formerly included in Trechus. All have the sutural stria recurved at the apex. Blemus discus (Fabricius, 1792) and Trechoblemus micros (Herbst, 1784) are distinguished by the finely pubescent elytra and large size; B. discus is larger, 4.5-5.5mm and has a smooth, unpunctured pronotum whereas T. micros is smaller, 3.8-4.5mm, and has the base of the pronotum finely punctured and pubescent. Both are widespread though local across Britain in damp habitats and both may be associated with mammal burrows. Our 7 species of Trechus are all large, >3mm and glabrous beetles, the elytra lack a basal margin and have the sutural stria recurved. All are widespread but only two species are common; T. obtusus Erichson, 1837 and T. quadristriatus (Schrank, 1781) occur in a variety of open habitats, they are sometimes common on agricultural fields and grassland and often come to light, sometimes in large numbers. Identification is straightforward except for species of Trechus which will need to be examined very carefully and will involve occasional dissections.
Bembidiini is the largest of our tribes and includes some of our most widespread and common species, they occur in all habitats and at least some should be found with even the most casual recording techniques. Tachyina is represented by 6 species of 2 genera, all of which are very local and scarce in damp or wet habitats. Among our fauna they are distinguished by the small size, <2.6mm, diminutive apical maxillary palpomere and recurved sutural stria. Our 2 species of Tachyura Motschulsky, 1862 are distinguished by the punctured transverse fovea on the base of the pronotum and the single puncture inside the recurved elytral stria. Species of Tachys have the pronotum impunctate and two punctures inside the recurved stria. They may be sampled by sieving damp vegetation over a sheet but they are very small and move slowly and so will need to be looked for very carefully, two species occur only in coastal situations. Identification can be very difficult without reference material and good keys will need to be used with great care. Bembidiina includes mostly small and very characteristic species with reduced apical maxillary palpomeres and striate elytra that lack a recurved sutural stria but there are 2 exceptions among our fauna. Ocys have the sutural stria recurved but are otherwise rather typical, atypically among the subtribe our 3 species are saproxylic, O. harpaloides (Audinet-Serville, 1821) is common and should soon be found by searching tree trunks at night although the recently added O. tachysoides (Antoine, 1933) is very similar and so far its status is only poorly understood. The rare O. quinquestriatus (Gyllenhal, 1810) also occurs on trunks but may be found by beating ivy on walls and has been recorded in cellars and in rocky coastal habitats. Our 4 species of Asaphidion lack elytral striae but are otherwise typical of the group, they have very large eyes and an overall metallic appearance and superficially resemble small versions of Elaphrus. All are widespread though local in a variety of habitats, usually on damp soil although the 2 common species, A. curtum (Heyden, 1870) and A. stierlini (Heyden, 1880) are more eurytopic, often occurring on agricultural land or in gardens. Bracteon includes 2 UK species. Our only member of the genus, Cillenus lateralis Samouelle, 1819 is a widespread though local coastal species, formerly included in Bembidion it is distinct in having 4 setiferous punctures in the third elytral interstice. Our 2 species of Bracteon are distinguished by the elytral sculpture, both are very local and rare. Sinechostictus Motschulsky, 1864 includes 2 species formerly assigned to separate subgenera of Bembidion. S. inustus (Jacquelin du Val, 1857), which is characterized by a longitudinal pronotal depression deepened strongly towards the base, has only very rarely been recorded from the UK. S. stomoides (Dejean, 1831) is widespread in the north and west though generally scarce, it is characterized by the abbreviated and deepened eighth elytral stria which is present only towards the apex. Both occur among gravel in riparian habitats.