Velleius dilatatus (Fabricius, 1787)
Hornet rove beetle
The genus Velleius Leach, 1819 includes 8 species and is Palaearctic in distribution, V. dilatatus is very widespread though sporadic and generally rare throughout Europe and east through northern Russia to China, and is the only member of the genus to occur in the UK. Here the species has historically been associated with ancient woodland in Hampshire, Berkshire and Gloucester but recent decades have seen an expansion in the range to Devon, Cambridgeshire, Norfolk and Nottingham (etc.) which seems to have followed a similar expansion by its host, Vespa crabro Linnaeus, 1758 (The European Hornet), in whose nests the larvae develop. Adult beetles spend most of their lives within the nest, usually in tree-hollows but also recorded from houses, feeding upon diptera larvae etc. among accumulated debris, but they also leave the nest on warm evenings to feed on other insects or at sap-runs on oak and beech. They have a very good olfactory sense and it is thought they detect sap and find their way back to the nest following chemical signals, they fly well and usually arrive at sap about an hour after dark when the temperature exceeds 16oC, several may be present at a single sap-run although they keep well apart and are usually inactive when feeding, they generally remain for most of the night and will regularly find and consume other insects arriving at the sap. The larvae develop among nest detritus, consuming other insects and possibly dead and dying hornets as well. Nests have been found to host up to 10 adults and larvae, and while they do not predate hornets or their larvae these must be present or the beetles will not survive.
Among the largest of our rove beetles at 10-25mm, entirely black or dark brown and distinguished by the rounded and laterally flattened pronotum. Head large and slightly transverse, with weakly convex eyes and long and curved temples, the surface dull due to strong microsculpture. Mandibles large and asymmetrical; the right with a single prominent tooth and the left with 3 smaller teeth internally. Antennae 11-segmented with segments 4-10 serrate. Pronotum rounded to produced anterior angles, widely explanate laterally and strongly microsculptured, sometimes appearing iridescent with low-angle lighting, the margins with various setiferous punctures and the disc with a row of 2 or 3 either side of the middle in the anterior half. Elytra quadrate and dull, with sparse recumbent pubescence and scattered longer setae, especially laterally. Abdomen evenly tapered and with strongly raised lateral borders, strongly microsculptured and weakly iridescent. Legs long and robust; tibiae straight and broadened towards the apex , each with a pair of long spurs at the inner apical angle. Pro-tarsi strongly dilated in both sexes, a little more so in the male.