CLAMBIDAE Fischer von Waldheim, 1821
These minute but characteristic beetles will occasionally be found by sweeping, or sometimes in large numbers among decaying vegetation.
POLYPHAGA Emery, 1886
SCIRTOIDEA Fleming, 1821
In general little is known of clambid biology, they generally occur among decaying vegetation of all kinds although they are not particularly associated with decaying fungi, and in Europe Calyptomerus species are often associated with bark; C. dubius, which in the U.K. more generally occurs among straw beneath haystacks etc., on broadleaved species while C. alpestris Redtenbacher, 1849 occurs among bark on mountain conifers. Some species will be found by general sweeping as they sometimes fly low at dusk around the host material but in general the best way to find them is by pulling apart likely samples over a sheet and waiting for the adults to run although all are capable of rolling into a tight ball and will often display thanatosis. Adults are also likely to occur among extraction samples throughout the spring and summer, and they also fly at night, occurring in flight-interception traps and at u.v. light. Both larvae and adults appear to be mycophagous, consuming the spores of Myxomycetes and Ascomycetes. Various species of Clambus have been recorded on slime-moulds including Arcyria stipata (Schweinitz), Stemonitis axifera (Bulliard) and S. fusca Roth as well as on moulds including species of Mucor. Many species are widespread and occur in a range of habitats which may vary on a regional basis e.g. C. pubescens Redtenbacher, 1849 is associated with decaying vegetation in the U.K. whereas in Western Canada it occurs on mature poplar trees, and the widespread C. armadillo (DeGeer, 1774) occurs among decaying vegetation and is often associated with wetland habitats in the U.K. while in Poland and Finland it occurs in pine forests. As with many species members of the Clambidae have been spread extensively by human activity and some continue to expand their range e.g. the Palaearctic C. armadillo and C. pubescens in the Nearctic region, and the recently recorded Australian species C. simsoni Blackburn, 1902 from the U.K.
Around the World
Colloquially known as fringe-winged beetles, clambids are very distinctive, if tiny, species. The family includes about 70 species in 5 genera and 3 subfamilies and is cosmopolitan although only the genus Clambus Fischer von Waldheim, 1823 has such a distribution. Acalyptomerinae Crowson, 1979 is monogeneric with 4 species of Acalyptomerus Crowson, 1979; 2 from the southern United States and Central America and 1 each from Africa and Southern Asia. Calyptomerinae Crowson, 1955 is also monogeneric with 5 species of Calyptomerus Redtenbacher, 1849, 1 from South Africa, 1 from the United States and 2 Palaearctic species, both of which occur in Europe and 1 of which, C. dubius (Marsham, 1802), extends to the U.K. Clambinae Fischer von Waldheim, 1821 includes the majority of the species and is most diverse in tropical regions. Clambidius Fauvel, 1902 includes the single species C. atomus Fauvel, 1902 from New Caledonia. Loricaster Mulsant & Rey, 1862 is Holarctic and extends into the Neotropical region, 8 species are known including a Japanese endemic and 2 species from Europe. Sphaerothorax Erichson, 1959 includes 8 species from the Neotropics, south-eastern Australia and New Zealand. The large genus Clambus includes about 50 species and is well-represented in most temperate regions; the European and North American faunas each include about 10 species although 2 of these are adventive in the United States, 6 occur in New Zealand and there are a large number of endemics e.g. C. complisans Wollaston, 1864 from the Canaries, C. flavescens Enrödy-Younga, 1959 from Java, C. formosana Enrödy-Younga, 1959 from Formosa, C. hisamatsui Enrödy-Younga, 1986 from Japan, C. octobris Enrödy-Younga, 1959 from Hawaii, C. olympiae Enrödy-Younga, 1978 from the Soloman Islands, C. sundanus Enrödy-Younga, 1995 from Borneo, C. tierensis Blackburn, 1902 from Tasmania and C. vinsoni Enrödy-Younga, 1965 from Mauritius etc.
All species are very small, 0.7-2.0mm, broadly oval, convex and continuous in outline although when set the head is abruptly angled with the anterior pronotal margin. Colouration black to testaceous when mature, often with paler margins to the pronotum and elytra, glabrous to very finely and sparsely to densely pubescent, pubescence generally recumbent, rarely erect, and usually short so that individual hairs do not, or only slightly, overlap. Head transverse and angled laterally about the eyes, with long and usually straight temples that narrow posteriorly, flat or weakly convex and without depressions and strongly deflexed in life. Frontoclypeal area expanded and forming a sharp transverse ridge anteriorly which covers, or at least mostly covers, the mouthparts and completely or partially divides the eyes. Anterior margin usually smoothly rounded from a deep impression in front of each eye or, in Sphaerothorax, with additional anterior notches. Eyes partially to completely divided horizontally, variously convex but usually continuous with the outline of the head, not or only slightly emarginate anteriorly, coarsely faceted and glabrous. Mandibles short and broad with a well-defined mola; sharp, rounded or truncate and with 1 to 3 apical teeth. Apical palpomeres cylindrical or fusiform. Antennae widely separated, the insertions visible from above or placed laterally and hidden, 10-segmented (Clambus, Calyptomerus), 9-segmented (Acalyptomerus) or 8-segmented (Loricaster), with 1 or 2 basal segments enlarged and an abrupt 2-segmented club. Pronotum transverse and wider than the head, smoothly rounded and finely bordered laterally, sometimes narrowly explanate, without basal fovea or other depressions and usually broadest at the base which slightly overlaps the base of the elytra. Surface evenly convex, posterior angles acute or perpendicular, anterior margin smoothly rounded and only rarely produced forward e.g. Clambus vulneratus (LeConte, 1879). Prosternum short in front of externally open coxal cavities, prosternal process complete, incomplete or missing. Pro-coxae contiguous or nearly so and conical or only weakly projecting. Meso-coxae narrowly separated, meta-coxae contiguous and large with expanded plates which cover, or mostly cover, the meta-femora and part of the first abdominal ventrite. Elytra completely covering the abdomen; without prominent humeri, smoothly rounded laterally and acuminate apically. Convex, very finely punctured and microsculptured, without striae but in some with up to 12 rows of fine punctures and in Sphaerothorax with an impressed sutural stria before the apex. The suture continues to the apex, the lateral margins are straight or only weakly sinuate and the epipleura are weakly, if at all, developed. The abdomen has 5 or 6 visible ventrites, the first larger than the others, and all sutures are distinct. Legs slender and proportionally long, the trocanter-femoral joint strongly oblique but the femora do not touch the coxae, femora generally broadened, often only weakly so, and usually not visible from above. Tibiae long and slender; not broadened towards the apex and lacking lateral or apical teeth or spurs; often curved or weakly angled. Tarsi 4-segmented; the tarsomeres equal or nearly so and without lobes. Claws paired and equal, without lobes, teeth or setae, and sharply pointed. There are no obvious external sexual characters. The larvae are weakly pigmented, elongate and sub-cylindrical and lack urogomphi.
Clambid identification can be very difficult, more especially so as there have been many misidentifications and nomenclatural changes over the years, and the family has never been a popular one with collectors. Most specimens will need to be dissected as morphological features e.g. colour, pubescence and microsculpture, can be very subtle and variable. The U.K. list includes Calyptomerus dubius and 9 species of Clambus. There is no complete guide to their identification but the RES handbook includes guidance on specimen preparation, nomenclatural correlation etc., and is an essential starting point for the family in the U.K.