Ephistemus Stephens, 1829
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POLYPHAGA Emery, 1886
CUCUJOIDEA Latreille, 1802
ATOMARIINAE LeConte, 1861
ATOMARIINI LeConte, 1861
E. globulus (Paykull, 1798)
E. reitteri Casey, 1900
This is a small genus of tiny silken fungus beetles; only six species are known and the distribution includes the Holarctic and Neotropical regions, three species occur in North America and only two are known from Europe, both of which are widespread and extend into the UK. Both European species are common but both are also quite nondescript and have avoided attention from researchers and so the true extent of the distributions may be more extensive. E. globulus (Paykull, 1798) occurs throughout much of the Palaearctic region and is also established in North America and New Zealand, probably from European imports; it is generally common throughout Europe, although less so in the south, and is present on most of the Mediterranean and Atlantic islands, in the UK it is generally common as far north as the Scottish Highlands and throughout most of Ireland. So far as is known E. reitteri Casey, 1900 is widespread in Southern and Central Europe and extends east into Asia Minor, it seems to be more common in the south and generally absent from northern regions, it was first discovered in the UK in 2006 and thus far remains rare and very local in the south east. Both are associated with decaying organic matter of all kinds with the general exception of dung, they occur in most situations but are often common among compost in parks and gardens etc., in leaf-litter, decaying fungi, among sedge and reed litter and, often in great abundance, in hay, straw and farm refuse in stables and farms. They fly well and sometimes swarm in the evening around compost etc., they occur year-round and are active over a long season from early spring until the autumn, peaking during June and July and often remaining active among compost or decaying fungi during the winter. Adults are likely to occur when sieving or extracting samples of suitable material, and they often appear in unusual samples e.g. from old bird nests, and we once found them in numbers in a sample taken from a recently abandoned moorhen nest. Little is known of the biology but both adults and larvae are very probably mycophagous, feeding and developing among hyphae and fruiting bodies or feeding on hyphae and spores among decaying vegetation. Adults are crepuscular and nocturnal and on warm evenings may be seen on the surface of fungoid wood etc., they otherwise remain cryptic and are rarely encountered.
Adults are tiny, 0.9-1.4 mm, elongate-oval, broadest about the middle and continuous in outline, they are sparsely and minutely pubescent, sparsely and very finely punctured and smooth and shiny or with very weak microsculpture, mostly to the forebody and the base of the elytra. The body is black to dark brown with the elytral apices lighter and the appendages are pale brown. The head is usually substantially retracted into the thorax and so hardly visible from above, the vertex is weakly convex and the frons flat or slightly angles between small and weakly convex eyes. Antennae 11-segmented, inserted anteriorly near the base of the eyes and separated by more than the length of the basal segment, first segment broad and long, second segment distinctly narrower and shorter, 3-8 much narrower than the basal segments; 3-5 elongate, 6-8 quadrate or nearly so, and 9-11 form a well-demarked and elongate club. Pronotum transverse, widest across rounded posterior angles and narrowed to perpendicular anterior angles and a curved apical margin, lateral margins finely bordered, surface evenly convex and without structure. Scutellum transverse and curved laterally. Elytra oval and smoothly curved from sloping shoulders to a continuous apical margin, surface without striae; finely and sparsely punctured, about the same as the pronotum, and extremely finely pubescent, lateral margins reflexed and not visible from above, epipleura narrowing from the shoulders and absent or very narrow beyond the basal two-thirds. Legs long and slender with all coxae widely separated. Femora narrow; front femora slightly shorter than the front tibiae, middle and hind femora as long as the corresponding tibiae. Tibiae only slightly broadened from the base, front tibiae rounded apically, middle and hind tibiae truncate and with tiny, hardly noticeable, apical spurs. Tarsi with 5 simple segments. Claws smooth and not toothed at the base.
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Distinguished from other tiny cryptophagids e.g. some Atomaria and Ootypus globosus (Waltl, 1838) by the small size and the form of the frons which is not produced forward between the antennal insertions, but in any case they are usually discontinuous in outline, and in the case of Ootypus the antennae are distinctive. Tiny hydrophilids have distinctive palps. Some corylophids e.g. Rypobius have a similar convex and continuous form but can always be distinguished by the enlarged basal antennomeres. The tiny Cybocephalus is much broader and discontinuous in outline.
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Our two species of Ephistemus may sometimes be distinguished by the general form; E. globulus (Paykull, 1798) is broader and, on average, slightly larger (0.9-1.4 mm.) while E. reitteri Casey, 1900 more slender and on average smaller (0.9-1.1 mm.) but this is not a reliable guide, even when a series is available for comparison. Both are dark with paler elytral apices but globulus tends to be darker, black or very dark brown, while reitteri tends to be paler but both are variable and specimens of both occur with the apical half or even two-thirds of the elytra paler. Males can be distinguished by the form of the aedeagi; in globulus the parameres are broad and open apically while in reitteri they are narrower and the inner margin is closed apically.