Cryptopleurum Mulsant, 1844
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POLYPHAGA Emery, 1886
HYDROPHILOIDEA Latreille, 1802
SPHAERIDIINAE Latreille, 1802
C. crenatum (Kugelann, 1794)
C. minutum (Fabricius, 1775)
C. subtile Sharp, 1884
This is a small genus of terrestrial hydrophilids associated with decaying organic matter. There are twenty-four described species of which only three occur in Europe, including the UK, six species are known from the Nearctic region although it is thought that only one of these, C. americanum Horn, 1890 is native, seven are known from the Oriental region, eight are Afrotropical, three occur in the Palaearctic and Australasia regions and a single species is known from the Pacific region. All but two are native to particular regions; C. subtile Sharp, 1884 is the most widespread species, being Holarctic and extending widely into the Oriental region, and C. minutum (Fabricius, 177) is Holarctic. All species are small, <2.5 mm, they are broadly oval and continuous in outline, very convex above and more weakly so below, finely pubescent and drab coloured, most are black or dark brown although many have paler areas to the pronotum or elytra. Among the UK genera they may be distinguished by the small size, dorsal pubescence and entire front tibiae (c.f. Megasternum). Head transverse, with weakly convex eyes and converging temples, vertex and frons smoothly convex and extensively punctured, antennae 9-segmented with a long and curved scape and abrupt or gradual but always distinct compact club. Maxillary palps almost as long as the antennae, basal segment dilated, penultimate segment long and usually broadened towards the apex and the terminal segment long and narrow. Pronotum broadest across the base and rather strongly narrowed to a rounded (from above) apical margin, the surface smoothly convex and punctured throughout. Scutellum small but distinct, triangular and usually punctured as the pronotum. Elytra each with ten punctured and usually well-impressed striae, interstices weakly to strongly convex and randomly punctured throughout. Ventral surface glabrous, shiny and extensively punctured. Prosternal disc pentagonal and flat, the basal margin strongly notched to receive the produced margin of the transverse mesosternum. Tarsi 5-segmented. The biology is largely unknown but adults are generally active over a long season, peaking in late spring and again in late summer and so it is likely that reproduction occurs in the spring and larvae develop among decaying organic matter during the summer. Adults are thought to be saprophagous while larvae, typical of much of the family, may be predatory. In the field they may be distinguished from most other similar species by the long palps, the most likely confusion is with smaller Cercyon Leach, 1817 or Megasternum Mulsant, 1844, both of which are glabrous. Our species are distinct enough to be identified without the need for dissection but they should always be examined carefully as they often occur together in samples.
The European species may be identified from the following key:
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1.
-Head black, pronotum contrasting brown, often with the disc and the basal margin darker. Elytra brown, usually paler towards the apex. Pronotum distinctly microsculptured.
----C. subtile Sharp, 1884
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-Body mostly black, often with dark reddish elytral apices and shoulders. Pronotum smooth or only very finely and indistinctly microsculptured.
----2
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2.
-Elytral striae strongly impressed and interstices distinctly convex, especially below the shoulder. Pronotal disc more strongly punctured than the interstices across the elytral base.
----C. crenatum (Kugelann, 1794)
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-Elytral striae less strongly impressed and interstices only weakly convex. Elytral interstices as strongly punctured across the base as the pronotal base.
----C. minutum (Fabricius, 1775)
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C. subtile Sharp, 1884
Originally described from Japan this species is widespread and generally common across the eastern Palaearctic region including China and Japan, it was introduced into North America during the early 20th century and Europe during the 1950s. The European distribution is rather patchy but it is locally common across Central Europe and reaches the Mediterranean in France and Italy although it seems to be absent from the islands and the Balkan Peninsula, to the north it extends to the UK and Denmark, and while it is absent from some of the southern Baltic countries it occurs throughout Fennoscandia, extending beyond the Arctic Circle. In the UK it is locally common and often abundant throughout England and Wales including the islands and more sporadic and local through Scotland and Ireland. Adults occur year-round, they overwinter in tussocks or among leaf-litter and are active from late May until November, peaking in abundance in June and again from September. They typically occur among warm compost and decaying dung and straw mixture but adults have been recorded from decaying fungi and in old bird nests, they usually occur in numbers and sometimes swarm during warm summer evenings. They may be sampled at any time by sieving suitable material or taking samples for extraction, they often come to light and sometimes appear in flight-interception traps.
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1.6-2.4 mm. Pale to dark reddish-brown with the head, basal pronotal margin and ventral surface except for the apical abdominal sternites dark reddish-black. Dorsal surface with very fine pale pubescence throughout. Head, pronotum and metasternal disc with a very fine but always distinct wavy longitudinal microsculpture. Head smoothly convex to a shallowly impressed and often interrupted fronto-clypeal suture, clypeus narrowed from the base to a widely emarginate anterior margin, palps pale or with the terminal segment darkened, antennae pale with the clubs darker. Punctures on head and pronotum fine and well-separated except sometimes across the pronotal base where they may form a dense transverse series. Elytra broadest in the basal half and smoothly narrowed to a continuous apical margin, striae distinctly but not strongly impressed throughout, strongly punctured near the base but these fade and from the middle are often obsolete, the seventh and eighth striae more or less fused but the punctures are usually distinct. Legs pale brown, contrasting with the generally dark ventral surface. Tibiae gradually expanded from the base and with fine spines along the external margins, the middle and hind tibiae truncate and the front tibiae rounded apically, all with fine but distinct pairs of apical spurs.
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C. minutum (Fabricius, 1775)
Native throughout the Palaearctic region and widely established following introductions in North America, this species is generally common from lowlands to above the tree line in mountain regions throughout Europe from the Mediterranean north to the UK and far above the Arctic Circle in Fennoscandia, it occurs through the northern Balkan countries and the Near East to the Black Sea but is generally absent from North Africa and the islands. In the UK it is generally common throughout England and Wales and less so but widespread in Scotland and Ireland. Adults are present year-round, they overwinter in tussocks and moss or among old dry dung and are active over a long season from early spring into the autumn, peaking in abundance from April until June. They occur among all kinds of decaying vegetable matter and are often abundant in older and drier herbivore dung and garden compost, they often appear in numbers in winter flood refuse samples and in Europe they occur in old bird nests. Sieving likely samples will often produce large numbers of adults during spring and summer and warm compost may produce them at any time of the year.
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1.6-2.3 mm. Body finely pubescent, very dark brown or black with pale subhumeral and apical areas to the elytra, occasionally the elytra are paler across the base and laterally and entirely dark reddish-brown specimens (teneral?) sometimes occur, ventral surface shiny black, palps pale brown or obscurely darkened apically, antennae pale with dark clubs, legs pale with the coxae and femora to some extent darkened. Head evenly convex with moderately convex eyes and converging temples, finely and rather densely punctured above, fronto-clypeal suture shallow and interrupted medially, clypeus narrowed from the base and weakly emarginate anteriorly. Pronotum widest across the base and narrowed to a rounded (from above) apical margin, surface evenly convex, without microsculpture across the disc and finely and almost densely punctured throughout. Elytra striae shallow at the base, becoming deeper towards the apex, the 7th and 8th almost fused, interstices finely and randomly punctured throughout, mostly about as strong as those on the pronotum but finer towards the apex. Legs darkened at the base but with trocanters usually contrastingly paler. Middle and hind femora smooth below, front femora strongly punctured below, all tibiae gradually widened from the base and spinose externally, middle and hind tibiae truncate and front tibiae rounded apically, all with a pair of tiny apical spurs.
C. crenatum Sharp, 1884
This Western Palaearctic species is almost entirely confined to Europe and is generally local and rare; to the east it extends through northern parts of the Balkan peninsula to the Black Sea and extends as far as Latvia in the southern Baltic region, it occurs across Central Europe from the Pyrenees to Poland and Romania but is absent from many regions, and to the north extends to the UK and above the Arctic Circle in Fennoscandia. In the UK it is widespread though very local across South East England from South Hampshire to The Wash and there are a few scattered records from the Midlands, South Wales and Cumbria, it is generally rare but adults usually occur in numbers. More eurytopic than our other species, it occur in herbivore dung and decaying vegetation, but also moss or among leaf-litter in open wetland situations exposed to the sun, and in Europe they has been recorded from carrion and old bird nests.
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2.0-2.2 mm. Entirely black or very dark reddish-brown, sometimes with pale subhumeral and apical areas to the elytra, palps and legs pale brown but often with darker areas, antennae pale brown with darker clubs. Head evenly convex and finely punctured, frontoclypeal suture broadly and deeply impressed and clypeus weakly emarginate across the apex. Pronotum sinuate across the base, widest between acute posterior margins and narrowed to a rounded (from above) apical margin, surface evenly convex and moderately strongly and densely punctured throughout, smooth and without microsculpture across the disc but often finely wrinkled towards the base. Elytra with deep and strongly punctured striae from the base, the 4th and 5th usually broader and more strongly punctured below the shoulders and the 7th and 8th fused and often indistinct, interstices strongly convex; below the shoulders sometimes forming distinct ridges, finely and randomly punctured throughout, across the base more finely so than across the pronotal disc. Ventral surface, including the abdomen, substantially darkened. Coxae, trocanters and femora extensively dark, tibiae pale or in places a little darker. Tibiae only weakly broadened from the base, external margin with fine spines and paired apical spurs, front tibiae rounded apically, middle and hind tibiae truncate.