Trachys troglodytes Gyllenhal, 1817







POLYPHAGA Emery, 1886



AGRILINAE Laporte, 1835

TRACHEINI Laporte, 1835

Trachys Fabricius, 1801

Widespread and locally common across southern Europe from Spain to the Balkan Peninsula, Greece and Ukraine but becoming more sporadic and scarce further north to the UK, Denmark and Southern Sweden. The UK distribution has become confused since the discovery here of the closely similar T. compressus Abeille de Perrin, 1891 (Levey, 2012), but it appears that the present species is widespread though very local in southeast and southern England while compressus may be much more widespread. Adults are active from April until June although specimens have been recorded much earlier and later than this, suggesting that at least some eclose in the autumn and overwinter. The typical habitat is open and fairly dry heathland and moorland with plenty of herbaceous vegetation but adults sometimes occur in open woodland and wasteland. Adults are usually associated with various Dipsacaceae, usually Field Scabious (Knautia arvensis (L.) Coult) or Small Scabious (Scabiosa columbaria L.), and while the larval host is unknown it is likely to be the former of these species. A relatively wide range of hosts have been recorded from the continent including Field Scabious, Woodland Scabious (Knautia dipsacifolia (Schrank)), Pincushion flower (Scabiosa canescens), Small Scabious, and Devil’s-bit Scabious (Succisa pratensis Moench). Females oviposit into the upper surface of leaves, usually about halfway between the mid-rib and the edge, and seal the oviposition site with a drop of black secretion which soon hardens. Larvae mine the leaves, usually working from the oviposition site towards the edge; they produce a translucent blotch which widens towards the edge of the leaf and contains dark frass granules or thread-like filaments. Fully grown larvae have been recorded during June and July; they are bright green with a transverse dark marking on the dorsal surface of each body segment. Pupation occurs within the mine from July. Adults need to be swept or searched for among host foliage, they are active in warm weather but they do not visit flowers.

Trachys troglodytes 1

Trachys troglodytes 1

© Lech Borowiec

Trachys troglodytes 2

Trachys troglodytes 2

© U.Schmidt

2.8-3.5 mm. Body distinctly bicoloured; head and pronotum metallic coppery, elytra metallic greenish or bluish-green, bronze, dull green or dark violet, appendages dark metallic. Distinguished from our other beetle fauna by the general habitus, and from other members of the genus by the finely incised apical abdominal sternite and the almost continuous body outline; the base of the elytra being at most only very slightly wider than the base of the pronotum and with only weakly developed humeral calli. The elytra have very fine micropunctures which are visible, at least in places, among the sculpture and the larger punctures, these are absent in compressus. Males may be determined by the form of the aedeagi, here the median lobe is widest towards the base and narrowed in a more or less straight line to the apex while the parameres are gradually widened to strongly oblique apices. In compressus the median lobe is smoothly curved from the base to the apex and so broadest about the middle, and the parameres are strongly and often unevenly widened from the base to less strongly oblique apices. Both may be distinguished from our members of the genus by the incised terminal sternite.