Megasternum Mulsant, 1844
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POLYPHAGA Emery, 1886
HYDROPHILOIDEA Latreille, 1802
SPHAERIDIINAE Latreille, 1802
M. concinnum (Marsham, 1802)
M. immaculatum (Stephens, 1829)
This is a small Holarctic genus of about ten species; two are native to the Nearctic region and the rest are Palaearctic and/or Oriental although one of these has become established through introductions into North America. Until recently only two European species were recognized, the amazingly-named M. promethius Shatrovskiy, 1990, which is endemic to the Caucasus, and another which was variously referred to as M. obscurum (Marsham, 1802) or M. boletophagum (Marsham, 1802), but this is now accepted to comprise two distinct species; M. concinnum (Marsham, 1802) and, originally described by Stephens in 1829, M. immaculatum. Older records need to be verified and so the European distributions are uncertain but M. concinnum is certainly widespread and so far this also seems to be the case with M. immaculatum, and both seem to have a very wide distribution in Asia. Regarding our UK species, the differences are subtle and there are no external sexual characters and so specimens will need to be examined very carefully. Various features are sometimes quoted as being diagnostic e.g. the form of the scutellum, the strength of the elytral microsculpture and the extent of the last row of punctures but these are variable and no diagnostic value although in series they may become so. There are differences in the genitalia; in M. immaculatum the parameres are less dilated and angled towards the apex and they extend beyond the median lobe, in M. concinnum they are broader and more strongly angled towards the apex and do not extend beyond the median lobe. There are subtle differences in the form of the elytral apex, in M. immaculatum they are more smoothly rounded while in M. concinnum they taper and are more acuminate, but this becomes obvious only by comparison. The best distinguishing feature is the overall colour; the body and antennal clubs in M. immaculatum being black or very dark, while in M. concinnum they are brown although the head is usually darker. They are small, 1.5-2.5mm, glabrous and convex above and below, and among our UK fauna may be distinguished by the long maxillary palpi and strongly excavate front tibiae. All species are associated with decaying organic matter.
M. concinnum (Marsham, 1802)
This species is generally common throughout most of Europe from Portugal to Greece and north to the UK and above the Arctic Circle in Scandinavia, it is widespread across Asia Minor and into Western Russia and is known from Siberia and China, it is absent from most of the Mediterranean islands but is widespread in north western Africa and occurs locally on the Channel Islands. Although the precise date of introduction is uncertain, it has long been known from North America and is now established and widespread in the United States and Canada. In the UK it is generally common and often abundant throughout England, Wales and Southern Scotland, including the islands, and rather less so further north to Shetland, including the Western Isles. In Ireland it is common across the north and west but much more local and sporadic in the south. Adults are present year-round; they are generally common from May until October or November and peak in abundance from July until September. Typical habitats are decaying organic matter of all kinds although only occasionally in carrion, usually among compost, dung, hay, straw and matted vegetation but also recorded at sap and from ant and subterranean mammal nests, among seashore drift and decaying fungi and reed litter, they often occur in abundance in older and drier dung samples and may be very abundant in decaying terrestrial bracket fungi in the autumn. Reproduction probably occurs mainly in the spring as both larvae and pupae have been found during May and teneral adults are common from June. Adults may be sampled by sieving all kinds of decaying matter, they often occur in flight-interception traps and at UV light, and through the winter are frequently recorded from flood refuse. During the summer large numbers often occur among compost in domestic gardens and they may be quick to colonize freshly piled grass cuttings etc. Because of their small size and because they often occur along Cryptopleurum or tiny species of Cercyon, they will need to be looked for very carefully.
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1.7-2.2mm. Continuous in outline, broadly-oval, widest about the middle and smoothly curved laterally. Head transverse, broadest across weakly convex eyes that are continuous with the outline and smoothly convex above, surface very finely and quite densely punctured and with only a hint of a frontoclypeal suture, anterior clypeal margin straight or only weakly sinuate. Maxillary palpi as long as the antennae, basal segment dilated from the base and terminal segment long and narrowly pyriform. Antennal scape constricted about the centre, club three-segmented, elongate and truncate apically. Pronotum transverse, broadest across the base and narrowed to almost perpendicular anterior angles, lateral margins finely bordered, surface evenly convex and finely punctured, a little less densely so than the head. Scutellum triangular, elongate and finely punctured. Prosternum with a raised hexagonal plate in front of the coxae which has a strongly-raised antero-lateral border and a finely raised basal ridge (but is not carinate medially) and so appears concave. Mesoventral plate triangular, slightly transverse and densely punctured. Metasternum with at most only traces of femoral lines towards the anteriolateral angles, basal margin slightly curved medially where it meets the mesosternum, surface strongly and moderately densely punctured throughout. Elytra with well-developed and complete punctured striae except for the tenth which ends about the middle, interstices flat, very finely punctured and with fine, often obscure, microsculpture, especially around the scutellum and towards the apex. Pro-tibiae strongly emarginate externally at the apex (unique among our Hydrophilidae), meso- and meta tibiae narrower and truncate apically, all tibiae with stout spines along the external margins. All tarsi with five simple segments in both sexes.
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M. immaculatum (Stephens, 1829)
Megasternum immaculatum (if it turns out to be a distinct species; more work is needed here, especially on the DNA) is broadly similar (see above). It appears in the same habitats and is also widespread and common, although at the moment it appears to be the less frequent of the two species. The wider Palaearctic distribution seems to be similar as it has also been recorded from across Europe and into Siberia (it was first recorded from Russia in 2017), it is not yet known from North America but this may be due to insufficient recording.