Gnorimus variabilis (Linnaeus, 1758)
Widespread though generally very local and rare, this species occurs from France to Italy, Greece and into Asia Minor in the south and north to the UK, Denmark and southern provinces of Sweden, the eastern extent of the range is Ukraine and northern parts of European Russia. The species was formerly much more common but has gradually declined over the 20th century, it is now absent from many of its former sites and remains locally common only in a few areas of central and southern Europe. In the UK it was known from a few sites in Surrey, Kent and Berkshire up until the 1900s and it may still persist at the latter locality (Windsor Forest) but there are modern records from only a very few sites in West Sussex. The typical habitat is dense deciduous woodland, often in lowland regions but also in humid and cold ancient woodland in mountain valleys in the south. Adults are active from May until July or August, they are diurnal and mostly sedentary, spending much of their time among decaying wood or under bark but on the warmest days they bask on trunks and fly to nearby flowers, especially elder (Sambucus L.), chestnut (Castanea Mill.) and various umbels (Apiaceae), where they feed on nectar and pollen. Mating occurs from early in the season soon after the females become active; they release pheromones which quickly attract males and for a short while mating groups may be seen on standing or fallen. Females begin to search out suitable oviposition sites soon afterwards; they search in cavities and burrow deep into moist decaying heartwood where they will lay single spherical pale yellow eggs, each protected with mucus and wood dust, and they may lay up to fifty eggs at suitable sites. Larvae are of typical scarabaeoid form; C-shaped, pale grey becoming darker apically and with a pale brown head, they usually occur in numbers and they feed among decaying wood for up to three years although two years and overwintering only once is the norm, they pupate early in the year and adults eclose in early spring although they remain in situ until the temperature increases in the spring. Under artificial conditions optimum larval growth occurs at 25C in high humidity with wood pulp as a food source. A wide range of deciduous trees have been recorded hosting the species but the usual hosts are oaks (Quercus L.), beeches (Fagus L.) and chestnuts, and in southern France they have been recorded from maritime pines (Pinus pinaster Aiton). Adults can be searched for on trunks or under bark, early in the season they very occasionally swarm or occur on flowers on the warmest days but they tend to be very elusive, often keeping to higher parts of trunks and branches. Many European records are from larvae reared from the field.
Gnorimus variabilis 1
Gnorimus variabilis 2
Gnorimus variabilis 3
17-22 mm. Very distinctive and, despite the specific name, rather invariable, glabrous and entirely shiny black with four (more rarely five) small pale markings on each elytron-hence the synonym G. octopunctatus (Fabricius, 1775). Head smooth between convex and protruding eyes, clypeus produced and emarginate in both sexes, antennae 10-segmented, lamellate and hardly varying between the sexes. Pronotum transverse, broadest towards rounded posterior angles and narrowed to a curved apical margin, basal margin produced medially, surface weakly uneven and densely punctured throughout. Scutellum broadest across the base, triangular and less densely punctured than the pronotum. Elytra slightly transverse, broadest behind the middle and rounded from prominent, almost angled shoulders to separately-curved apical margins, lateral margin not emarginate anteriorly, surface rugose, finely punctured and with vague and incomplete ridges and depressions. Abdomen exposed beyond the elytral apex. Legs long and robust, front tibiae with two (including the apical) lateral teeth, hind tibiae with two apical spurs. Tarsi 5-segmented, claws paired and simple, lacking lateral teeth. Males may be distinguished by their angled middle tibiae and fine brushes of yellow hairs on the underside of the front basal tarsomeres.
Among our UK fauna the only confusion might be with the recently recognized Oxythyrea funesta but this is always much smaller and the dorsal surface has sparse long pubescence.