Callicerus Gravenhorst, 1802
Native to the western Palaearctic region but now much more widespread as various species have been transported with trade, this genus includes seven species, most of which are restricted to Europe; the greatest diversity is in south-eastern Europe and Italy and only two species occur north of the Alps (see below). Adults of most species have been recorded year-round but they have a rather restricted season of activity during spring and, occasionally early summer, the rest of the year being spent underground or secluded. Several have been recorded from subterranean mammal nests or among ant colonies but this is probably exceptional. Breeding and dispersal by flight occurs in the spring and adults visit blossom where they have been observed to feed on nectar. Teneral adults occur in spring early summer. Adults are diurnal and can be sampled by sieving litter and moss etc., they sometimes occur in numbers pitfall and flight-interception traps but otherwise are usually found as single specimens or pairs although very occasionally they may be swept from grassland in large numbers, probably coinciding with dispersal by flight. Our two UK species are widespread though local across England and Wales.
Among our UK fauna the species may be recognized by the elongate terminal antennomere and lateral margins of the head lacking a border, the penultimate maxillary palpomere is pyriform and the terminal segment diminutive, and the tarsal formula (though this should be irrelevant as the species are otherwise so distinctive) is 4-5-5 in both sexes. Other features shared by our species are as follows. Forebody with isodiametric microsculpture, pronotal pubescence obliquely directed, mesosternum without a median ridge, mesocoxal cavities carinate, and the first metatarsomere longer than the second but shorter, or barely as long as, 2+3 combined.
Our species were formerly included in separate genera (e.g. in Joy); rigidicornis in Semiris Heer, 1839, and obscurus in Callicerus. Semiris is now generally considered as a subgenus of Callicerus based on the morphology of the tenth antennomere. Our two species are readily separated as follows.
Callicerus obscurus 1
Callicerus rigidicornis 1
© Jan Klimaszewski
Callicerus obscurus 2
-Smaller, 2.5-3.0 mm. 10th antennomere long and cylindrical. Head evenly rounded behind small and rather protruding eyes. Pronotum quadrate or nearly so, broadest in front of the middle and angled laterally. Elytra longer and much wider than the pronotum.
C. obscurus Gravenhorst, 1802
-Larger, 3.5-5.0 mm. 10th antennomere quadrate in males, very slightly transverse in females. Head transverse and not so evenly rounded behind small and barely convex eyes. Pronotum broadest about the middle and curved laterally, Elytra broader than the pronotum and, measured along the suture, about as long.
C. rigidicollis (Erichson, 1839)
Callicerus obscurus Gravenhorst, 1802
This western Palaearctic species is locally common, though probably under-recorded, from Spain to Italy and northern parts of the Balkan Peninsula and Slovakia to the south and north to the UK, Denmark and into Central Fennoscandia to the north. The species was first recorded from North America in 2003, presumably from specimens introduced from Europe, and it now seems to be established in temperate parts of eastern Canada and the United States. In the UK it is locally common throughout Southern and Central England and less so in Wales, the West Country and further north into Southern Scotland. Adults occur year-round; they overwinter among moss or litter and are active over a long season from February until June or July, peaking in abundance during April and May. They are sometimes quoted as being largely subterranean, and are occasionally associated with mammal burrows and ant nests (on the continent with Lasius fuliginosus (Latreille, 1798) and L. brunneus (Latreille, 1798)) but beyond this they occur in a wide range of both damp and dry habitats. They often occur in open woodland, wooded parkland and arable land, often on heavy, poorly-drained soils in situations that remain humid for much of the spring and early summer. Active specimens have been recorded among litter on arable land and along hedgerows during mild winter spells and we have found them in local woodland among decaying terrestrial fungi in January, but they disperse by flight during the first warm days in February and at this time may be swept from emerging vegetation. Large numbers soon appear at blossom, at first on blackthorn (Prunus spinosa L.) and later on hawthorn (Crataegus monogyna Jacq.), at this time mating pairs may be seen on flowers or swarms of adults may be seen about higher branches laden with blossom. Numbers begin to fall in early June and by summer only single specimens are likely to be found, sometimes among woodland pitfall samples. Little is known of the biology but following reproduction in the spring it is likely that larvae develop through the summer and that the new generation of adults will go on the overwinter.
2.5-3.0 mm. Elongate and rather parallel-sided with the forebody narrower than the elytra and abdomen. Dorsal surface finely pubescent, black with brown elytra, antennae and legs variable but usually substantially brown. Forebody densely shagreened and finely punctured, elytra dull and finely punctured throughout, abdomen shiny, sparsely and very finely punctured. Head slightly narrower than the pronotum and without lateral borders, eyes small and slightly protruding and temples evenly curved so that the basal margin is almost semi-circular, surface without impressions or structure. Penultimate maxillary palpomere pyriform and much wider than the second segment, terminal segment diminutive. Antennae long and slender, three basal segments elongate, 4 and 5 quadrate, 6-9 transverse, 10 elongate and longer than 8+9 in males and quadrate in females, terminal segment very long; in males about 3X longer than wide and longer than 8-10 combined, in females about as long as 9+10 combined. Pronotum quadrate or nearly so; widest behind a curved anterior margin and narrowed to obtuse posterior angles and a sinuate basal margin, surface weakly convex and obscurely depressed about the base, the pubescence directed obliquely back from the centreline. Elytra transverse, much longer and wider than the pronotum, slightly widened from rounded shoulders to a sinuate basal margin, pubescence obliquely directed backwards across the disc. Abdomen weakly curved and almost parallel-sided, the tergites strongly bordered laterally and depressed across the base. Legs long and slender, femora only weakly thickened, tibiae without outstanding setae and with tiny terminal spurs. Tarsi 4-5-5, without bilobed or modified segments.
Callicerus rigidicornis (Erichson, 1839)
This western Palaearctic species occurs very locally from lowland to middle mountain altitudes (1600 m in the Pyrenees) throughout Europe from Portugal to Greece and Ukraine in the south and extending north to the UK and Denmark; it is generally more frequent in warmer southern and central regions and scarcer with increasing latitude. It is absent from the Mediterranean islands and North Africa, and the eastern limit if the distribution is the Asia Minor and the eastern shores of the Black Sea. It was first recorded from Canada (Ontario) in 2001 and now seems to be established but so far (although the Nearctic species are being revised) it appears to be absent from the United States. In the UK it is very local throughout Southern and Central England and Wales, sporadic and rare in the West Country and there are records from Scotland and Ireland. Adults are present year round, they overwinter in the ground or below moss or litter and are active from March until July, peaking in abundance during May and being rarely recorded beyond July. The species occurs in a variety of fairly damp habitats on most soil types, chalk, clay and sand; it has been recorded from the banks of streams and rivers, damp deciduous and mixed woodland and wooded parkland, heaths and arable land as well as disturbed areas including domestic gardens. Dispersal occurs by flight in the spring and at this time both sexes may be found in numbers on a variety of blossom and shrubby flowers such as hawthorn (Crataegus monogyna Jacq.) and gorse (Ulex L.) where they are thought to feed on nectar. Mating occurs in the spring and gravid females have been recorded in May but otherwise little is known of the biology; that adults are only rarely recorded during autumn and winter might suggest a subterranean lifestyle with adults emerging in the spring only to mate and disperse. Adults are easily obtained by beating flowering shrubs, and here they usually occur in numbers, otherwise they may be found by sieving litter or by pitfall trapping. They occasionally occur in flight-interception traps but do not seem to be attracted to light, suggesting a predominantly diurnal lifestyle, and specimens occasionally occur in winter flood refuse.
3.5-5.0 mm. Long and parallel-sided with the forebody and abdomen narrower than the elytra, dorsal surface with fine pale pubescence throughout. Forebody rather dull black to dark brown, elytra variable but usually substantially brown, abdomen dark brown with the apical margins of the tergites and sternites paler, appendages pale brown, the antennae often to some extent darkened. Head transverse, broadest across small and weakly convex eyes and rather unevenly narrowed to a broad neck, surface smoothly convex and with distinct isodiametric microsculpture. Penultimate maxillary palpomere pyriform and at least 2X wider than the second segment, terminal segment diminutive. Antennae long and thin; the third segment about as long as the first and longer and wider than the second, 4-8 quadrate or nearly so and gradually, though only very slightly, increasing in width, terminal segment as long as or very slightly longer than 9+10 combined in both sexes. Pronotum transverse, broadest at or slightly in front of the middle and evenly curved to rounded angles, surface rather flat with a variously-developed longitudinal median impression, with isodiametric microsculpture (which is usually weaker than that on the head) and fine and rather dense punctures, pubescence, for the most part, directed obliquely back from the midline. Elytra slightly dilated from rounded shoulders to a sinuate basal margin, wider than the pronotum and abdomen and about as long, measured along the suture, as the pronotum. Elytral surface with dense granulose punctures and fine and shallow transverse microsculpture, pubescence for the most part directed obliquely back from the suture. Abdomen parallel-sided or gently curved laterally, with strong lateral borders and moderately strong and dense punctures which become sparser towards the apex. Third tergite (usually the first visible) in males with a fine but obvious raised median ridge towards the apical margin. Legs long and slender, femora simple, tibiae without outstanding setae and with tiny apical spurs. Tarsi 4-5-5 with all segments simple; the basal metatarsomere almost as long as the fifth and obviously longer than the second.