Claviger Preyssler, 1790
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POLYPHAGA Emery, 1886
STAPHYLINOIDEA Latreille, 1802
CLAVIGERINI Leach, 1815
C. longicornis Müller, P.W.J., 1818
C. testaceus Preyssler, 1790
Claviger longicornis Müller, P.W.J., 1818
With a similar distribution to the previous species this is the only other widespread European member of the genus; it is locally common in southern areas but generally very local and scarce in the north, reaching Sweden and the UK but absent from Norway and much of the Baltic coast, and there has probably been a decline in abundance since the mid twentieth century. In the UK it is a very rare species known from only a few, mostly older, records from Surrey, Oxford and around Cardiff; it is usually associated with the ant Lasius umbratus (Nyl.) under stones in calcareous grassland but has also been recorded from the nest of L. mixtus (Nyl.) On the continent it also occurs with L. umbratus, in one instance in a basement, but individuals have been found among large populations of Claviger testaceus in nests of L. flavus and L. niger.
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Superficially similar to the C. testaceus but easily distinguished by the larger size, 2.5-2.7mm, and the form of the head and antennae. The head is elongate and distinctly narrowed towards the base although this may be difficult to appreciate due to the dense lateral pubescence, and the antennae are very different, segments 3-5 are elongate, the third being about twice the length of the fifth, and the terminal segment is more elongate and rounded apically.
Claviger testaceus Preyssler, 1790
This widespread species occurs throughout Europe from the Mediterranean north to the UK and southern provinces of Fennoscandia, it extends east into Ukraine, Western Russia, Turkey and Iran, and is generally very local and sporadic in occurrence but because of its lifestyle probably largely under-recorded. Here it is very local and generally rare; in the west and north east it is mostly coastal while in the south it is more widespread with sporadic inland records north to Nottingham. Members of this genus are highly specialized myrmecophiles that depend on their hosts throughout the whole life-cycle, the main host of the present species is Lasius flavus (Fab.) but it has also been recorded from nests of L. niger (L.) and L. alienus (Förster) and single specimens from nests of various species of Myrmica Lat., adults have been recorded from nests throughout the year and are sometimes found clinging to males and sexual females suggesting that they may be present at the start of a new colony. During the summer they may occur in large numbers, up to 100 in each nest, and they appear to be tended by the ants which feed them with regurgitated liquid food and carry them underground if the nest is disturbed, but the beetles also consume larval excreta and secretions, feed on dead insects in the nest and may suck the contents from ant eggs. For the most part the beetles are ignored by the ants who will occasionally stop and lick at their trichomes-tufts of flexible hairs surrounding exocrine glands at the base of the abdomen-but they will occasionally carry the beetles about the nest and beetles will sometimes be seen clinging to ants as they forage inside the nest, the beetles seem to display no defensive behaviour towards the ants and it is supposed that chemicals produced by the beetles modify the ants behaviour. Ants obviously value the beetles because they stop frequently to lick them and when disturbed they will often carry the beetles to safety in their mandibles in preference to their own larvae. The life cycle and developmental stages of the beetle remain unknown which, given that adults are among the best studied myrmecophilous beetles, might suggest that development occurs outside the nest. Ant nests will need to be examined to find these beetles and the best time seems to be early summer, host nests usually occur under stones or debris in open situations or gardens and they may also be recognized as mounds of soil overgrown with herbage, but once their runs are exposed the ants will quickly grab the beetles and take them underground and so some careful excavation and sieving may be needed.
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Adults are tiny, 2.0-2.3mm and very distinctive with the forebody narrow and elongate and the elytra and abdomen broadly oval, entirely pale brown and finely pubescent except for the basal half of the abdomen which is mostly glabrous. Head elongate, sub-parallel and rounded anteriorly, densely pubescent and lacking eyes, antennae broad and 6-segmented, the basal 2 segments small and inconspicuous, 3-5 broadly transverse and the terminal segment elongate and truncate. Maxillary palps greatly reduced and inconspicuous. Pronotum quadrate, sinuate laterally and with rounded angles, surface evenly convex and densely pubescent. Elytra narrow at rounded shoulders then strongly broadened to rounded posterior angles, smoothly convex and without striae, pubescence dense and fine towards the base, becoming longer and sparser towards the apex and, especially, around the posterior angles. Abdomen from above consisting of a single shiny plate made up of fused tergites 4, 5 & 6, this is convex and pubescent in the apical half but glabrous and broadly depressed towards the base; the lateral margins of the depression are delimited by an arcuate furrow lined with stiff setae along its anterior margin and these are the sites the ants find most interesting. Legs long and narrow with femora and tibiae narrowed at the base but otherwise almost parallel-sided, tarsi 3-segmented but appearing as a single long segment, the basal tarsomeres tiny and hidden, each with a single minute claw. Middle tibia of male with a short but stout ventral tooth.