Stenagostus rhombeus (Olivier, 1790)
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POLYPHAGA Emery, 1886
ELATEROIDEA Leach, 1815
DENTICOLLINAE Stein & Weise, 1877
DENTICOLLINI Stein & Weise, 1877
Stenagostus Thomson, C.G., 1859
Stenagostus rhombeus (Olivier, 1790) is widespread and locally common throughout Europe and east to the Caucasus. In the U.K. it occurs in Southern England and Wales north to West Yorkshire. The typical habitat is old woodland, orchards and wooded parkland with a supply of decaying trees. Adults are active from June until August or September depending on the season and are crepuscular or nocturnal; they fly well and sometimes come to light. By day they hide under bark or among litter etc. but we have swept them from vegetation and observed them on umbel flowers by day as well. Larvae inhabit decaying wood in stumps and trunks of a range of deciduous trees but beech (Fagus sylvatica) seems to be preferred; they generally occur among the frass under loose bark. They feed on other insects and are noted for predating cerambycid larvae but they are also, at least in part, saprophagous. Two or possibly more summers are spent in the larval stage. The species has also been recorded from conifers. Pupation occurs in the spring, generally in the harder parts of the wood and sometimes in the burrows of cerambycid larvae.
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This large species is unmistakable among the U.K. fauna. 16-21mm. Entirely brown or with the elytra a little darker, and with pale appendages. Head, pronotum and elytra punctured and pubescent throughout. Antennae pale; inserted beneath the anterior clypeal angles, the first segment a little shorter than the distance between the insertions. Segment two very small and almost quadrate. Serrate in both sexes and with a shiny, raised ridge obvious along the centre of segments five to eleven, the terminal segment (11) constricted before the apex. Pronotum elongate with the anterior margin bisinuate, front angles produced. Hind angles strongly produced and with a small ridge parallel to the lateral margin. Scutellum densely punctured and with a longitudinal keel. Elytral pubescence pale, that behind the middle darkly patterned; to the naked eye forming a rhombus among the lighter pubescence. Side margins explanate towards the apex. Tarsal segments one and five elongate, two to four lobed below.
Stenagostus Thomson, C.G., 1859
This is a small genus of about ten Palaearctic species with more occurring in the Oriental region. Three species occur in Europe of which one, Stenagostus rhombeus (Olivier, 1790), also occurs in the U.K. They are medium sized elaterids, although among the U.K. fauna it is just about the largest, resembling Athous etc. in general appearance but differing in the form of the antennae which, in Stenagostus, are sharply serrate from the third segment*. Several species have the oblique subapical areas of dark pubescence. The European species S. rufus (DeGeer, 1774) is widespread and common across Europe and occurs throughout France but is absent from the U.K. It differs from S. rhombeus in that the scutellum is flat i.e. without a keel, and only very sparsely punctured. S. sardineus (Reitter, 1914) is endemic to Sardinia and perhaps also Corsica. The widest diversity occurs in the Far East and Oriental regions. The species are saproxylic and prey on other insects and their larvae.
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*This form of the antennae is also present in Hemicrepidius but in that genus the terminal antennal segment is normal i.e. not constricted, and the remaining segments lack the shiny line which runs through each segment in Stenagostus. In Stenagostus the second, third and fourth tarsal segments are lobed below whereas in Hemicrepidius only the second and third are lobed.