THROSCIDAE Laporte, 1840
Throscid Beetles
These small but distinctive beetles are associated with wooded areas; they disperse by flight and may be attracted to light and so might occur in any situation.
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POLYPHAGA Emery, 1886
ELATEROIDEA Leach, 1815
2
5
1.5-4mm
Around the World
Closely related to the Eucnemidae and Elateridae, the throscids are distinctive among the various superficially similar species; they are distinguished be the small size, 1.5-4mm, the characteristic elongate-oval shape and rather flattened dorsal surface, the pronotum fitting tightly against the elytral base, the deflexed head and the antennae which are distinctly clubbed except in the genus Potergus (Bonvouloir, 1871) in which they are filiform but in any case the genus will be instantly recognized as a member of the Throscidae. The family contains 5 genera and more than 150 species with an almost worldwide distribution; so far none have been recorded from New Zealand. The greatest diversity occurs in subtropical and warmer temperate regions; 30 species in 3 genera occur in the U.S.A. and less than 10 species in 2 genera in central Europe. They occur in a wide range of habitats but in general are insects of lowland and mid-level vegetated woodland, parkland, wooded pasture and scrub etc. and occur frequently in urban situations. They are active from early spring and are diurnal, sometimes occurring on flowers, as well as nocturnal; trapping is a good way to record the adults, either at light or in malaise traps or by flight interception. The adults may be found on tree trunks and logs etc. especially in the evening when they mate, but generally they turn up in the beating tray fairly regularly when working deciduous trees and shrubs. During the winter they occasionally turn up in flood refuse and extraction samples of litter and tussocks etc. from wooded situations. When captured they lie still with the appendages tightly withdrawn, resembling seeds or detritus, and if several are present they will all begin to move simultaneously. Adults have been observed to ‘click’ but not so powerfully as the elaterids and their ability to flip over is poor, the Prosternal process is generally wide and triangular at the apex with no subapical constriction, and the mesosternum is deeply and widely emarginate to receive it, and so there appears to be no elaterid-type jumping mechanism. Larvae have been observed in soil, rotten conifer wood and among grass tussocks, and at least some soil inhabiting species have been observed feeding upon ectomycorrhizal fungi on tree roots.
The family is comprised mostly of 2 genera; Aulonothroscus and Trixagus, the limits of which have yet to be fully defined. For now at least the following genera comprise the family:
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Trixagus Kugelann, 1794. Almost cosmopolitan with about 55 species.
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Pactopus LeConte, 1868. Includes a single species, P. hornii LeConte, 1868 from Western North America.
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Aulonothroscus Horn, 1890. Almost cosmopolitan with about 100 species, most diverse in the tropical and subtropical regions. There are about 16 Nearctic species.
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Potergus (Bonvouloir, 1871). Includes 4 species widespread in the Australasian, Asian and Pacific zones; they occur commonly in rain forest fogging studies. Formerly included within the Eucnemidae.
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Cryptophthalma Cobos, 1982. Includes the single species C. alvarengai Cobos, 1982 from Brazil.
Description
It would be fair to say that with a little experience the family is immediately recognizable against similar sized eucnemids and elaterids; in the field the characteristic oval shape, broadest at the pronotal-elytral base soon becomes obvious. They have the habit of flying onto clothing etc, in the most unlikely places; we have recorded them several times from pub gardens and town centres and occasionally indoors attracted to light.
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Characteristically oblong-oval and very compact, especially with the appendages withdrawn. Drab; brown to black and with fairly dense semi-erect pubescence over the dorsal surface. The head is usually mostly hidden within the thorax; hypognathous with robust mandibles and a movable labrum-a character that will distinguish them from the eucnemids. Maxillary palpi 4-segmented with the terminal segment triangular. Antennae inserted on the frons between the eyes; 11-segmented with an abrupt 3-5 segmented club. Eyes transversely oval, coarsely faceted and finely pubescent, entire to deeply emarginate; sometimes almost completely divided. Pronotum broadest at the base and narrowed towards the apex, fitting closely to the base of the elytra, hind margin sinuate and hind angles produced, lateral margins finely bordered, at least towards the base. Prosternum with deep antennal scrobes and the coxal cavities open posteriorly. Scutellum triangular, small but usually obvious. Hind wings well developed and most species fly well. Elytra completely covering the abdomen, narrowed and rounded apically. Striae punctured but shallow, interstices flat or only very weakly convex, finely to moderately strongly punctured and pubescent. Epipleura broad at the base and usually gradually narrowed to the apex, sometimes strongly narrowed at the level of the hind margin of the metasternum. Meso- and metacoxae excavate posteriorly to receive the femora. Abdomen with 5 visible sternites which are all connate. Legs slender and relatively short, retractable into cavities on the ventral surface of the thorax. Trocanters short and triangular, femora and tibiae flattened, the tibiae ridged along the outer edge and with two fine apical spines. Tarsi 5-5-5, segment 4 lobed below. Claws simple. The larvae are weakly sclerotized and grub-like with a small head and reduced legs. Antennae very short and mandibles rounded, flattened and fused to the head. Legs very short and 5-segmented. Tergite 9 with a pair of tiny urogomphi and segment 10 reduced or missing.
A key to the British species can be found HERE.
UK Species
Trixagus carinifrons
Trixagus leseigneuri
Trixagus meybohmi
Among our UK species of Trixagus, T. dermestoides (Linnaeus, 1767) is the most widespread and probably most frequently recorded, it is a local insect occurring throughout England and Wales and there are a few records scattered through Southern Scotland. The only other frequently recorded species is T. carinifrons (deBonvouloir, 1859) which occurs throughout England and Wales although only sporadically in the north. Our other species are very local and restricted in their ranges: T. gracilis Wollaston, 1854 occurs in early spring in salt marshes on both sides of the Thames estuary and very occasionally on the south coast. T. obtusus (Curtis, 1827) also occurs in salt marshes as well as more generally in Southern and Eastern England but not in the West Country or Wales.
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In the U.K. adult throscids often occur among woodland litter, on trunks etc. and occasionally on vegetation in the evening; they fly readily and come to light. More generally they are likely to occur in many situations, alighting on clothing etc. during the day or evening. Sieving or beating likely material is the best way to find them although they may also be extracted from samples at any time of the year. When disturbed they lie still, usually for some time, with the appendages retracted into the body so that they resemble small seeds and this can make them very difficult to detect among swept or beaten vegetable matter.