Lomechusa Gravenhorst, 1806
This small Palaearctic-wide genus includes about thirteen species of highly specialized myrmecophilus rove beetles; of which only a few are at all well-known and widespread e.g. L. emarginata (Paykull, 1789), L. paradoxa Gravenhorst, 1806 and L. pubicollis Brisout de Barneville, 1860, occur across Europe and reach higher latitudes. The other European species are more restricted; L. bordonii Hlaváč, 2005 occurs on Sicily and L. bifoveolata Brisout de Barneville, 1860 is known from Spain and France around the Pyrenees, while L. atlantica (Koch, 1937) occurs further south, in Morocco. The Asian species also have rather restricted distributions; L. kishimotoi Hlaváč, 2005 and L. yunnanensis Hlaváč, 2005 occur in China (Sichuan and Yunnan provinces respectively), L. barbarae (Schilow, 1977) from eastern Russia, L. sinuata (Sharp, 1888) from Japan, and L. malaisei (Scheerpeltz, 1965) from Myanmar. The remaining species are from Tajikistan (L. dvoraki Hlaváč, 2005) and Uzbekistan (L. stangei (Reitter, 1910)). Two species, L. emarginata and L. paradoxa occur in the UK although a further species, L. strumosus (Fabricius, 1792), which is now included in Lomechusoides Tottenham, 1939, will be found in older literature.
All species are rare and infrequently recorded, probably as a result of their cryptic lifestyle. In general the adults inhabit nests of ants of the genus Myrmica Latreille, 1804 from late summer until the spring, at which time they leave and disperse to find other nests in which they will oviposit and in which the larvae will develop, these are usually of various species of Formic Linnaeus, 1758. Specific host associations are known for many of the beetles but there are likely to be others. Larvae develop through the winter and pupate within the host nests and the resulting adults are often morphologically distinct, depending upon which species of ant they developed alongside, thus many Lomechusa are known from a range of colour and structural varieties. This complex relationship probably began with a more general association with Formica species and gradually evolved to include Myrmica because the latter genus continues brooding through the winter, unlike Formica, and so the beetle larvae are assured of a plentiful food source during their development. These relationships depend on complex chemical and behavioural signals that allow ants to accept the beetles
Lomechusa emarginata 1
Lomechusa emarginata 2
and to tolerate their larvae; once accepted into the nests adults are carried by the ants into their brood chambers where they are fed by the ants but they also prey on developing ant larvae. Eggs are laid among detritus in brood chambers and the resulting larvae are fed by worker ants, they will also predate ant larvae but receive most of their food from the workers. Adult beetles possess trichomes which release various substances that allow the ants to accept them into the nests, among these are so called appeasement chemicals but also some which are vital in allowing worker ants to ‘adopt’ their larvae at the appropriate time. Upon first contact, usually outside the host nests, ants will often try to attack the adults but the beetles will raise the abdomen and exude tiny droplets of ‘appeasement’ fluids which are readily taken by the ants and which quickly change their behaviour, stimulating them to take the beetles back to the nest. Adults produce a similar abdomen-raising response when approached by a non-host species of ant but here they will produce defensive chemicals that will deter the ants. Adults of most species spend most of their time within nests but when they disperse in the spring and autumn they are attracted to lights and they sometimes appear in flight-interception or light-traps, and some species have appeared in pheromone traps operated to attract clearwing moths. A good way to sample adults is to search under rocks and debris in the vicinity of nests during spring and early summer, at this time the adults may be found active among the ants or near to their runs.
Species of Lomechusa are very distinctive and might only be confused with members of the closely-related genus Lomechusoides (see below). They are small to medium sized rove beetles, 3.2-6.1 mm, robust with a small head, parallel-sided body and long appendages, very finely pubescent and pale to dark brown in colour, often with the pronotal margins and elytra paler than the rest of the body, in most the appendages are entirely brown. Head usually dull and densely microsculptured, smoothly convex but with a depression between large convex eyes that are about the same length as the temples, labrum transverse and usually bearing rather dense golden setae, antennal tubercles well-developed, mandibles sharp, weakly curved and smooth internally. Maxillary palps four-segmented but appearing three-segmented due to the diminutive basal segment, segment two wider than and about as long as segment two, apical segment short and rounded apically. Labial palpi three-segmented; basal segment much wider and longer than the second, and the terminal segment about as long as the second segment but much narrower. Antennae long and slender (but variable), the basal segment hardly longer than but much broader than the others, segment two much shorter than segment three, 4-10 quadrate to slightly elongate, and long (longer than 9&10 combined). Pronotum variable, even within a species, but strongly transverse, broadest across acute and produced posterior angles and narrowed to rounded anterior angles and a more-or-less straight apical margin. Basal margin produced medially and strongly sinuate before the angles, surface dull with fine microsculpture and usually with various depressions, often parallel to the lateral margins or inside the posterior angles. Scutellum (often hidden by the produced pronotal margin) large, triangular and sculptured as the pronotum. Elytra transverse, as long as or shorter than the pronotum, straight laterally and slightly broadened from rounded shoulders to weakly produced outer angles, basal margin gently sinuate, surface flat and uneven, finely and densely punctured, without striae or series of larger punctures. Hind wings always fully developed. Abdomen broad and at least as long as the forebody and elytra combined, finely punctured and shiny black or with apical tergites dull due to microsculpture, paratergites 3-7 (usually the first three visible tergites) with tufts of prominent hairs (trichomes), tergites 2-5 sometimes with outstanding lateral setae (characteristic for each species), tergites 7 & 8 shorter than preceding tergites. Tergite 8 produced laterally and strongly excavate medially. Front coxae elongate (about 2.5X longer than wide), middle coxae small and slightly transverse, hind coxae about 2X wider than long. Legs long, especially the middle and hind femora and tibiae, and covered in fine pale pubescence, tibiae generally as long as the corresponding femur. The sexes are very similar but differ in the structure of the eighth tergite or the abdominal sternites. Our UK species may be distinguished as follows (mostly from Joy, 1932):
-Hind margin of elytra almost straight, legs stouter, femora with tufts of pubescence underneath near apex. 5.5-6.5 mm. [Hind angles of thorax almost right angles. Pronotal disc smooth and shiny, lateral margins dull and finely microsculptured. Lateral pronotal margins and base of elytra with fine long black setae.]
-Hind margin of elytra distinctly sinuate, and legs more slender. 3.8-4.5 mm. [Pronotum microsculptured and dull throughout, pronotum and base of elytra without fine long black setae.]
-Third antennomere about 1.25X longer than the second. Lateral pronotal margins narrowed and strongly sinuate from obliquely produced and strongly acute posterior angles. Posterior margin of tergites 1-3 raised across the middle. 3.5-4.5 mm. [Several named varieties based mostly on overall colour or the shape, surface structure or pubescence of the pronotum) Typically dark brown with the elytra and pronotal margins pale brown, antennae dark brown with the terminal segment pale apically, legs brown with paler tarsi.
-Third antennomere about 1.5X longer than the second. Lateral pronotal margins gently sinuate and almost parallel-sided from widely-rounded anterior angles to slightly acute posterior angles that hardly project. Posterior margin of abdominal tergites 1-3 not raised across the middle. 3.7-4.3 mm. Colour variable, often similar to emarginata.
Lomechusa emarginata (Paykull, 1789)
L. emarginata is widespread across Europe from the Pyrenees to Italy, Ukraine and parts of European Russia in the south and extending north into the UK, Denmark and the Baltic countries where it reaches into southern Norway and Finland and central provinces of Sweden. The species occurs from lowland to middle mountain altitudes; it is very sporadic and scarce throughout southern and central regions but more frequent in the north; being locally common in Fennoscandia. In the UK it is widespread though very local across Southern and Central England and Wales and there are scattered records further north to Cumbria. in general the species is infrequently recorded, no doubt because both ant hosts need to be present in order for it survive, but adults come to light traps through the spring and summer and the recent popularity of trapping for all types of insects, rather than just Lepidoptera, is producing more records. Typical habitats are open grassland, especially on calcareous soils, and sparsely vegetated upland and mountain areas, and there are many records from disturbed areas such as wasteland and old quarry workings etc. Adults are present year-round; they are active from March until October and peak in abundance during April and May when they often disperse by flight although they may occur in light traps at any time. Summer hosts include Formica fusca L, F. cinerea Mayr, and F. sanguinea Latreille. Winter hosts include a range of species of Myrmica Latrielle, 1804, and among those recorded (throughout the European range) are M. scabrinodis Nylander, M. laevinodis Nylander, M. ruginodis Nylander, M. rubra L. (which is perhaps the most frequently recorded) and M. schencki Viereck.
Lomechusa emarginata (Paykull, 1789)
L. paradoxa is a very local and rare insect of Central and Northern Europe from the Pyrenees to Austria and Slovakia and extending north into the UK, Denmark and the Baltic countries where it is recorded from Southern Sweden but not Norway or Finland, further east there are a few scattered records from European Russia and Turkey. In the UK it is generally rare and, although formerly more widespread, is now more-or-less confined to coastal or near-coastal sites in the West Country, particularly in Cornwall although there are also recent records from Dartmoor and Exmoor. Typical habitats are dry sandy grassland and moorland, sometimes in grazed areas, but also on sparsely-vegetated rocky or stony soils above cliffs etc. Adults have been recorded from January until October; they peak in abundance from March until April, and again in September, no doubt reflecting their periods of dispersal from winter to summer hosts etc. Summer hosts include Formica rufibarbis and F. fusca, and winter hosts include Myrmica ruginodis, M. laevinodis, M. scabrinodis, M. rugulosa, and M. rubra. Adults may be found in nests or under stones and debris nearby, and at certain times in the spring they may be present in nests of both their winter and summer hosts before migration is complete.