Meloe proscarabaeus Linnaeus, 1758
Black Oil Beetle
POLYPHAGA Emery, 1886
TENEBRIONOIDEA Latreille, 1802
MELOINAE Gyllenhal, 1810
MELOINI Gyllenhal, 1810
MELOE Linnaeus, 1758
This is among the most widespread and (locally) common member of the genus; it occurs in lowland and low mountain elevations throughout the Palaearctic region from Portugal to the far east of Russia, extending north into the UK and southern provinces of Fennoscandia, it is present across North Africa and the Near East and on most Mediterranean islands. It is generally abundant in warmer southern latitudes and more local and sporadic in the north where it has suffered a general decline over recent decades along with various host species of solitary bee. Here it is locally common throughout Wales and England north to Nottingham, the majority of records being coastal or near-coastal, especially in the west, and more sporadic and scarce north to the Scottish Highlands, it also occurs on Anglesey, I.O.W. and various western islands in Scotland. Adults are active from early in the year, February or March and persist into May or early June, typical habitats are unimproved grassland and coastal cliff tops rich in wild flowers but they occasionally occur in open woodland; they may be found in warm weather among long grass or on various yellow flowers, usually lesser celandine (Ficaria verna Huds.1762) but also various buttercups (Ranunculus L.) and dandelion (Taraxacum officinale Wigg, F.H.) where they feed on foliage and flowers. Mating occurs from March and pregnant females excavate burrows in loose dry soil in which to oviposit, usually near to host nesting sites, this generally takes a couple of hours to complete and occurs a few weeks before the hosts become active, up to 1000 bright yellow eggs are laid in a single mass at the base of the burrow which is then back-filled with the excavated earth. Oviposition is a frantic business at this time and several females may be observed excavating in a small area, sometimes even unearthing other burrows made by other females. Eggs hatch after a couple of weeks and the first triungulin larvae emerge from the soil to coincide with the emergence of their host bees but the beetles continue to oviposit into April or May and triungulins may be found into June, they emerge from the soil together in large numbers and are immediately very active, running on the surface and climbing plant stems to find flowers and at this time flowers may be found covered in hundreds of the tiny larvae. Triungulins will attach to any visiting insect and be carried away, usually in numbers, but some
will soon become attached to the correct host, which includes various species of Andrena Fab., Colletes Lat., Halictus Lat., Osmia Panz. and Eucera (etc.), and be transported back to the nest. Once in the nest the triungulins leave the host and invade the larval cells where they feed voraciously on honey and consume any host eggs present before entering an inactive phase, they then moult and appear as a scarabaeiform larvae which will complete its development within the cell, or sometimes invade adjacent cells as well, and overwinter as a specialized stage before a final scarabaeiod form emerges early in the year to construct a pupal chamber or continue feeding for a while before doing so. In at least some species this inactive overwintering stage may survive over several seasons in response to environmental conditions but it is not known whether the present species can do this. With a little experience adults may be identified in the field and so they are best recorded by simple observation rather than sweeping or any other invasive or potentially damaging method.
Oil beetles are very distinctive among the UK fauna and should not be mistaken for any other group; the large but very variable size (freshly emerged adults are short and broad but the abdomen becomes greatly distended as they gorge on foliage etc.), in the present species 13 to 32mm, overlapping elytra which expose much of the abdomen and black or bluish black colouration are quite unique. M. proscarabaeus is very similar to our other widespread species, M. violaceus, but distinguished by the form of the pronotum which is quadrate or slightly elongate, is smooth (without a transverse furrow) in front of a straight basal margin and has a small indistinct rounded tooth at the posterior angles. Males have modified antennae; segments 3-5 are dilated and flattened and the joint between segments 6 and 7 is angled, giving a distinctly ‘kinked’ appearance, female antennae may also be kinked but to a much lesser degree and the segments are unmodified. Other distinctive features of the present species are the strongly punctured and flattened pronotum and the roughly sculptured elytra, most specimens are black with a blue or violet reflection but completely black specimens are not uncommon and in some the forebody is more strongly coloured than the elytra and abdomen. Other species on our UK list which might be encountered, although very local and rare, are M. brevicollis Panzer, 1793, M. rugosus Marsham, 1802 and M. mediterreneus Müller, J., 1925, all are readily separated from the present species by having a strongly transverse pronotum. Triungulin larvae are mostly pale yellow and so are readily separated from those of M. violaceus which are mostly black.