Euplectus nanus (Reichenbach, 1816)
This western Palaearctic species is widespread across central and northern Europe although this distribution is rather patchy and, because there have been very few people working the genus, our understanding is likely to be very incomplete, it is known from southern France and northern Italy north to the UK and beyond the Arctic Circle in Scandinavia. Through most of central Europe it is locally common in suitable situations but further north is very local, generally scarce and may be declining due to habitat abuse, it occurs very locally across England and Wales as far north as the Scottish border and may be locally common but for much of this range there are very few records. Adults are associated with a range of deciduous trees in various stages of decay; they often occur among moist and fragmentary heartwood in stumps and trunks and less so among fallen timber, they have also been found under bark and among fallen leaves around trunks, and they occasionally occur near ant nests although the species is not thought to have any particular association with ants. According to Pearce (1957) they occur chiefly under bark and in rotten beech, oak and elm etc. In northern Europe it is also known from decaying conifer wood but this is rare. Little is known of the biology but, typical of the family, adults probably predate mites while larvae feed on decaying vegetation and all stages require high levels of moisture. Adults occur year-round, they usually appear in small numbers (unlike some other members of the genus), especially during the warmer months when they may vanish altogether. They may be found by sieving or extracting suitable material at any time but samples should examined under a lens due to the tiny size of the beetles.
Identification can be very difficult with these tiny beetles, more especially as the only viable key to our UK species is Claude Besuchet’s 1973 in Die Kafer Mitteleuropas Vol. 5 (1973), this includes good morphological characters but in general only males can be identified and here usually by dissection (see below for a link.) Historically the species has often been confused with E. kirbyi Denny, 1825, and this has rendered many European records in need of verification. The species is tiny, 1.4-1.6 mm, and very typical of the genus; elongate, flattened and rather parallel-sided, dark reddish-brown to black with pale brown appendages and finely pubescent above. Head broadest across small and protruding eyes and slightly narrower than the pronotum, with broadly-curvet temples and long, weakly-converging cheeks, surface with two
© Lech Borowiec http://www.cassidae.uni.wroc.pl/Colpolon/index.htm
deep furrows which converge from the base and a well-defined triangular pit from the middle of the base to the posterior margin of the eyes (this character is otherwise present only in E. kirbyi). Antennae inserted above the external margin of the mandibles, two basal segments large and elongate, 3-8 quadrate or slightly transverse, 9 and 10 strongly transverse and the terminal segment long and broad. Pronotum slightly transverse, broadest in front of the middle and narrowed to rounded anterior angles and obtuse posterior angles, lateral margins variably granulate but without distinct teeth, very finely punctured but strongly sculptured; usually with strong median longitudinal furrow, an uneven transverse furrow behind the middle and a strong depression towards each lateral margin. Elytra slightly elongate, gently curved from rounded shoulders to almost truncate apical margins, lateral margins with distinct keels that are visible from above, surface finely punctured throughout; without regular striae but with a complete and well-impressed sutural striae and several short longitudinal impressions from the base. Abdomen finely punctured throughout, the first three visible tergites strongly bordered and the first two with short oblique keels from the base, those on the first tergite divided by a shallow transverse impression. In males the fifth sternite has a deep transverse impression across the base bordered apically by two rounded lamellae and a weak impression, and the sixth sternite has two glabrous depressions linked by a transverse furrow. In females the last tergite is produced into a small blunt tubercle.
E. kirbyi, which is the only UK species with a similar basal depression on the head, is on average smaller, 1.2-1.4 mm, and paler, most specimens are reddish-brown. A good character is the transverse impression between the keels on the basal abdominal tergite which is much deeper in kirbyi than in nanus. Typical specimens may be assigned by comparison but otherwise males will need to be dissected, there is a good key (in German but which translates well with google) with line drawings of the genitalia here.