Demetrias Bonelli, 1810
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ADEPHAGA Clairville, 1806
LEBIINI Bonelli, 1810
D. atricapillus (Linnaeus, 1758)
D. imperialis (Germar, 1823)
D. monostigma Leach in Samouelle, 1819
This Palaearctic genus includes nine species of small and very distinctive ground beetles included in two subgenera, Demetrias s.str. (7 spp.) and Aetophorus Schmidt-Gobel, 1846 (2 spp.). Three species occur in Europe and a fourth, the Asian D. (A.) muchei Jedlicka, 1967, which extends west into South European Russia, may well become more widespread with climate change etc. and spread further west. Of the five remaining species, D. (D.) amurensis Motschulsky, occurs in the Amur basin, North China and (probably) Japan, D. (D.) longicollis Chaudoir, 1877 and D. (D.) longicornis Chaudoir, 1846 are known from a small area (Primorsky) of the Pacific coast of Russia, D. (D.) marginicollis Bates, 1883 is widespread in Korea and Japan, and D. (D.) nigricornis Chaudoir, 1877 is recorded from China. Of the three European species all are widespread and extend into the UK. The species are easily distinguished from similar genera such as Dromius and Paradromius by the broadly bilobed fourth segment on all tarsi. They are elongate, slender and flattened and characteristically coloured; pale with the head black and various dark markings on the elytra. The head is at least as wide as the pronotum, with long converging temples and cheeks and a smooth surface, at most pubescent only towards the margins, with two supra-orbital punctures and weakly developed frontal furrows. Antennae long, filiform and pubescent from the fourth segment. Pronotum quadrate or nearly so, broadest in front of the middle and narrowed to rounded anterior angles and protrucing posterior angles, apical and basal margins more or less straight, surface very finely punctured and evenly convex but for depressions about the posterior angles. Elytra parallel-sided or only weakly dilated from rounded shoulders to an obliquely-truncate apical margin which leaved the abdomen partly exposed, striae narrow, weakly impressed and finely punctured, interstices flat and with or without scattered large and shallow punctures. Femora and tibiae finely pubescent, front tibiae notched and with several larger setae before the apex, middle and hind tibiae with very fine apical spurs. Front tarsi only slightly dilated in males. Our UK species are easily keyed as follows (adapted from Joy, 1932):
1.
Thorax without basal foveae; elytra with a dark mark near middle of suture, another on margin of each. Claws simple. Head glabrous behind the eyes. (Subgenus Aetophorus) L. 5-5.5mm. Eastern England, rare, on reeds in fen districts.
-D. imperialis (Germar, 1824)
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Thorax with basal foveae close to hind angles; elytra without or with dark mark broadest near apex of suture, without one at sides. Claws toothed.
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2.
Elytra with an abrupt black mark near apex of suture, covering at least 3 interstices of each; claws with one small tooth on each. Head glabrous behind the eyes. 4-4.5mm.
-D. monostigma Leach in Samouelle, 1819
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Dark mark on elytra (if present) gradually broadening to apex of suture, and not covering more than 2 interstices; claws with three small teeth on each. Head finely pubescent behind the eyes. L. 4.5-5.5mm.
-D. atricapillus (Linnaeus, 1758)
Demetrias atricapillus (Linnaeus, 1758)
This is a generally common species throughout Europe from the Mediterranean, including North Africa, to southern Scandinavia and the UK and from Portugal east through Asia Minor and Russia to Siberia although across much of this range, including the UK it has declined drastically in recent decades. Here it is locally common in southern England and the midlands, more scattered and rare further north and apparently absent from Scotland, in the west and in Ireland it is more coastal, occurring in wetland areas and on coastal dunes where it occurs among tall grasses etc. near water, more generally it occurs in parkland, wasteland, gardens and arable land where both adults and larvae are important predators of aphids on cereal crops. Adults occur year-round; they overwinter under debris or in tussocks of Elymus repens (L.) (couch grass), Dactylus glomeratus L. (cocksfoot) or Holcus lunatus L. etc., among litter beneath nettle-beds on field margins or hedgerows, they become active early in the spring when they aggregate on nettle foliage etc. before migrating to open grassland or crops. Breeding occurs in the spring among grass or crops and the adults remain common into June or July, new-generation adults appear from August or September and large populations may accumulate by late September or October before they migrate to marginal overwintering sites. Adults may be swept from suitable vegetation by day but they are mostly nocturnal, climbing plant stems in search of aphids, they fly well and may move among crops if necessary and are sometimes attracted to light. From mid- to late spring, when they disperse, adults might be swept from vegetation almost anywhere but they seem to have a strong preference for nettle-beds and, at least in the southeast, long grass in fairly dry situations.
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With a little experience this species is easily recognized but there are several superficially-similar carabids e.g. species of Philorhizus Hope, 1838 and Paradromius Fowler, 1887 but in all cases the bilobed fourth tarsal segment of the present species is diagnostic. Small cantharids might also be confused with Demetrias but they lack the pro-tibial antennal-cleaning notch.
4.5-6.0mm. An elongate, narrow and flat species with a small pronotum and relatively large head. Head shiny black with testaceous mouthparts and antennae, long, tapering temples and large, prominent eyes, each with two supra-orbital setiferous punctures. Pronotum orange; cordate and strongly sinuate before sharp and protruding posterior angles, anterior angles rounded and lateral margins distinctly bordered, basal margin straight. Disc finely transversely wrinkled and with scattered very fine punctures. Elytra sub-parallel with rounded shoulders and obliquely truncate and weakly sinuate apical margin which leaves at least the last abdominal tergite exposed. Each with seven fine striae and an eighth represented by a series of punctures within the lateral border and at least some of the interstices with a single row of larger punctures although these often join the striae. Colour distinctive; testaceous with the suture and a triangular mark around the scutellum dark. Legs entirely pale, all tibiae with small apical spurs and the anterior tibiae with an internal antenna-cleaning notch. Tarsi 5-segmented, the fourth strongly bilobed, each claw with at least three teeth internally. Male with basal pro-tarsal segments weakly dilated.
Demetrias imperialis (Germar, 1824)
This western Palaearctic species occurs throughout Europe from the Mediterranean north to the UK and a few southern provinces of Fennoscandia where it seems to be rapidly expanding its range, it is also widespread across North Africa and extends east through Asia Minor and Russia into eastern Siberia, it is more or less confined to lowland areas and has been recorded up to 600 m in central Europe. In the UK it is locally common across southeast England from Hampshire to the Wash and there are a few records further north into Lincolnshire and Nottinghamshire, and west into Somerset, Devon and Cornwall. Habitats include wetland margins, especially on heavy soils at the banks of eutrophic waters, and the beetles are invariably associated with extensive growths of Common Reed (Phragmites australis (Cav.)), Bulrush (Typha latifolia L.) and various sedges (Carex L.) Adults are present year-round, they overwinter inside dead stems of Bulrush etc. and become active early in the year, they are mostly diurnal and spend much of their time climbing among vegetation where they predate springtails and, to a much lesser extent, aphids and other small insects, but they will often appear in numbers when searching through reed litter near the water margin. The species is wing-dimorphic; in northern Europe the majority of specimens have reduced wings and lack developed flight muscles but UK specimens are invariably fully-winged – a state that can be readily appreciated as the wings are visible through the translucent elytra, and both in the UK and in Europe specimens have been found in flight-interception traps. Breeding begins in May and continues into the summer, little is known of the life-history but adults are most common from March until June and there is a smaller peak in abundance in October and November. Adults may be sampled by searching among reed-litter or by sweeping foliage; they are common among litter and matted vegetation through the season but in the winter they invariably occur inside stems, presumably to avoid flooding, they can be very agile and when alarmed will either drop to the ground or move quickly into leaf axils.
4.5-5.8 mm. A very flattened species which is readily recognized by its shape and colour; the black head is proportionally large compared with the orange pronotum, the pale brown elytra are dilated towards the apex and have three dark maculae which are usually joined; a long diamond-shaped sutural mark which is broadest towards the apex where it is usually joined to a lateral spot which does not extend to the apical margin, there may also be a triangular scutellary spot but this tends to be diffuse and ill-defined. Appendages entirely pale. Head quadrate, broadest across weakly convex eyes and strongly narrowed to the anterior and basal margins, temples long and only weakly curved, frons with two setiferous punctures beside each eye, surface smooth or with fine transverse wrinkles but lacking punctures behind the eyes. Terminal maxillary palpomere well-developed, and antennae pubescent from the fourth segment. Pronotum slightly elongate, broadest in front of the middle and strongly narrowed to a weakly-curved basal margin, lateral margins very finely bordered, surface transversely wrinkled and very finely punctured but without basal fovea. Elytra with rounded shoulders and truncate apices which leave the abdomen partly exposed, with weakly impressed and indistinctly-punctured striae and flat interstices, the third with four or five setiferous punctures. Legs long and slender, front tibiae with a deep antenna-cleaning notch, fourth tarsomere widely bilobed. Claws smooth.
Demetrias monostigma Leach in Samouelle, 1819
This Palaearctic species extends from Europe to China and the far east of Russia, it is widespread in Asia Minor and reaches into Iran but is otherwise absent from North Africa, in Europe it is widespread and sporadically common from France to Ukraine and the Balkans and extends north into the UK, Denmark and some southern provinces of Sweden; in general it tends to be local and rare in the north and locally common in central and eastern areas. In the UK it is locally common around the coasts of South Wales and eastern England from Norfolk to the Humber estuary but otherwise very local and rare in Wales and England north to North Yorkshire. Adults are present year-round; they overwinter among thick leaf-litter or in tussocks and are active from April until September, peaking in abundance during May and June (earlier on the continent). Typical habitats are damp and well-vegetated dune slacks, reed beds and areas of dense sedges or tall grasses on flood plains etc., but in Europe it also occurs in dry sandy biotopes with dense patches of Lyme grass (Leymus arenarius (L.) and Marram grass (Ammophila arenaria (L.) etc, it is essentially a lowland species, occurring up to about 700 m in Northern Europe, but probably occurs at higher elevations e.g. in Switzerland where it is widespread and locally common. Adults are diurnal, they are adept at climbing stems and foliage where they spend much of their time hunting aphids and other small insects etc., but otherwise remain among litter or tussocks. Breeding occurs in the spring, larvae develop through the summer and pupae have been found during August, at least in northern Europe. The species is wing dimorphic through most of the range although it seems to be consistently short-winged in the UK, in Northern Europe it appears that short winged forms predominate in sandy dune systems while both types occur in wetlands. Adults may be sampled by sweeping during warm weather, here they sometimes swarm and might occur in large numbers, otherwise sieving tussocks or litter can be productive at any time.
4.5-5.5 mm. Flattened, elongate and discontinuous in outline, readily identified by the size and colour; head black, pronotum orange or pale reddish, elytra pale reddish with the suture and a single large subapical spot dark grey or black, appendages pale brown. Head with long converging temples and convex eyes that follow the outline, surface glabrous and shiny with narrow and almost parallel frontal furrows. Antennae densely pubescent from the fourth segment. Pronotum broadest behind obtuse anterior angles and narrowed to perpendicular or slightly produced posterior angles, basal margin almost straight, surface weakly convex with distinct lateral borders and elongate basal fovea. Elytra slightly dilated from sloping shoulders to almost truncate apical margins which leave the abdomen partly exposed, striae narrow and weakly-impressed throughout, interstices flat, and the third with four setiferous punctures. Legs long and slender, front tibiae notched internally before the apex, middle and hind tibiae simple and hardly broadened from the base. Fourth tarsomere broadly bilobed on all legs, claws with a single tooth towards the base. Three basal pro-tarsomeres weakly dilated in males.