Trechus obtusus Erichson, 1837
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ADEPHAGA Clairville, 1806
TRECHINI Bonelli, 1810
Trechus Clairville, 1806
A western Palaearctic species extending east into the Balkans and Ukraine and south into North Africa, to the north it extends into the UK and rather patchily into Fennoscandia; it is absent from Finland and generally rare in Sweden and Norway to about 68°N, also recorded from Faroe and Iceland. First recorded in North America from Seattle in 1925, presumably among nursery stock imported from Europe, it has since become established and spread rapidly along the west coast. It was discovered in Hawaii in 1998 and has subsequently expanded at a rate of 3km each year. The species is generally common in central and parts of northern Europe but in the south it occurs mainly at higher altitudes and is probably boreo-montane throughout. In the UK it is generally common and often abundant throughout, including the islands, although it is more local and mostly coastal or near-coastal in the Irish Republic. Typical habitats are dry grassland, heaths and moors, and the species is often common in disturbed areas such as waste ground, parks and gardens, less often in coastal fields and dunes. In Ireland it occurs in open woodland, wetland margins and marshes and among peat at higher altitudes. In southern Europe the species is more typical of upland mesophilic deciduous woodland and alpine grassland. Adults occur throughout the year; breeding occurs mostly in the autumn and the main overwintering stage is the larva, but surviving adults are long lived and may overwinter to reproduce in the spring. They are mostly nocturnal and are active from April or May until late in the autumn, peaking in numbers towards the end of summer. Specimens from Britain and Ireland are exclusively wingless while in central and northern Europe a low percentage are macropterous (although flight muscle development varies and may be seasonal; fully winged gravid females have been found), and in North Africa they are always fully-winged. Populations in North America and Hawaii are fully-winged. Adults are easily found by searching in open grassland and other well-vegetated habitats, on the continent they come to light and occur in flight-interception traps, and while UK specimens do not fly they will often appear at lights mounted on garden walls or run on sheets in open areas. Large numbers sometimes occur among abundant other small carabids etc. on illuminated pathways or in gardens on hot summer evenings.
3.6-4.1 mm. Body reddish-brown, often with the head and/or pronotum darker and sometimes with the elytra faintly iridescent, appendages pale brown. Head broadest across convex eyes, temples curved and converging to a broad and short neck, surface with dense cellular microsculpture throughout, frontal furrows produced widely back around the eyes to the temples, each side with two supra-orbital punctures, the posterior of which lies very close to the inner margin of the furrow. Terminal maxillary palpomere well-developed. Pronotum transverse, broadest in front of the middle and strongly rounded laterally to slightly projecting anterior angles and rounded and sharply-toothed posterior angles, basal margin almost straight. Pronotal surface weakly convex, explanate laterally and broadly but shallowly impressed each side across the base. Elytra smoothly curved from rounded shoulders to a continuously curved apical margin, each with five or six weakly punctured and impressed striae, the outer of which fade laterally and apically, sutural stria recurved apically, interstices flat, the third with three setiferous punctures, the anterior two usually join the third stria, and the apical puncture situated close to the apical angle. Front tibiae with a strong subapical notch and a single internal spur at the apex. Claws smooth. Male with two front tarsal segments dilated.
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Very similar to T. quadristriatus (Schrank, 1781) but with more rounded and slightly more convex elytra, and less well defined striae. UK specimens are wingless and so the abdominal tergites are usually visible through the elytra with good lighting. The position of the posterior supra-orbital puncture is usually diagnostic; in the present species it is placed close to the frontal groove while in quadristriatus it is placed about midway between the eye and the groove. Males can be distinguished by the form of the median lobe; here it is broader, straight and more-or-less symmetrical when viewed from above, while in quadristriatus it is much narrower and appears twisted from above.