Amphotis marginata (Fabricius, 178)
Three species of Amphotis occur in Europe (or four, as there may be a distinct Canary Island endemic) but only the present one is widespread, the distribution is mostly central, from France to Italy and the Balkans in the south, north to the UK and southern Fennoscandia, and extending east into western Russia. It is sporadic and may be locally common in southern and central areas but is generally rare in the north, there are only a few widely scattered records from the UK and it was first recorded from Sweden in 2014. Adults are associated with the ant Lasius fuliginosus (Latreille, 1798), which is generally common across Europe including much of England and Wales, although they disperse by flight and often occur on flowers in the spring and have been recorded from saproxylic fungi in the autumn, more generally they are active from April to November, peaking in abundance in June and July, and overwintering under stones or among wood debris close to or within the host nest. The ants generally nest among decaying wood in old deciduous trees, often oaks but also a range of other species, and forage over a wide area by leaving scent trails and it is along these trails that the adult beetles wait for ants returning to the nest laden with food, they recognize host-specific chemicals and pheromones and are able to follow trails until they find one in regular use by the ants. Ants do not usually attack the beetles but are curious and often wait while the beetle approaches head-on and begins to mimic the food-begging behaviour of other ants in the nest, this is not always successful but often the ant will regurgitate a drop of food onto the beetle’s head which is consumed immediately. Under experimental conditions the beetles are successful after every four or five attempts with the ants, in many cases the ants simple ignore them and continue towards the nest although sometimes they will spot and receive stimulation but then ignore it and move on after a few seconds. The beetles are rarely attacked but when this happens they are able to clamp down onto the soil with the appendages withdrawn under the widely explanate pronotal and elytral margins and in this way the ants are usually unable to bite or turn the beetles over, moreover the beetles are well-sclerotized and so the ant mandibles are unable to puncture or otherwise gain purchase. Except in the spring when they may disperse or visit flowers in hot weather, the beetles are strictly nocturnal, spending the day under stones etc.
Amphotis marginata 1
© Lech Borowiec http://www.cassidae.uni.wroc.pl/Colpolon/index.htm
Amphotis marginata 2
© U.Schmidt https://www.kaefer-der-welt.de/index.htm
and sometimes in numbers, near to ant trails or sometimes within the nest and becoming active as the ants begin to forage. Although it has been demonstrated that Amphotis responds specifically, both in chemical and behavioural terms, to L. fuliginosus they have also been able to solicit food by regurgitation from other ant species, although at much lower success rates, including Camponotus ligniperda (Latreille, 1802), a wood-nesting species absent from the UK, Formica pratensis Retzuis, 1783, and F. sanguinea Latreille, 1798, which have very restricted UK distributions, F. fusca Linnaeus, 1758, which is widespread and locally common in the UK, and F. rubra (Linnaeus, 1758) which is absent. Several other species of ant e.g. Lasius niger (Linnaeus, 1758), Tapinoma erraticum (Latreille, 1798) and Tetramorium caespitum (Linnaeus, 1758), either ignored the beetles completely or attacked them. Little is known of the species development but mating has been observed in the spring and larvae, presumed to be this species, have been recorded among nest material in early spring and summer. The species is probably relies on a high density of ant nests in established trees in old and undisturbed woodland for its successful dispersal and occurrence and in some areas of northern Europe there are signs of a recent decline.
Adult beetles are about the same size as their host, 3.8-5.8mm. A very distinctive broadly-oval and flattened species with wide and flat explanate margins to the pronotum and elytra; various other UK species have this general form but Amphotis may be recognized by the overall appearance and the form of the basal antennomere which is greatly expanded internally and rather oblong in form. Overall colour dark brown to reddish brown with paler explanate margins and several pale markings to the elytra; two at the base of each and a variable but often complete and continuous irregular transverse mark just behind the middle. Head transverse, smooth and finely punctured, labrum truncate and weakly emarginate anteriorly, eyes small and very convex, mandibles large, curved and expanded externally, strongly bifid, from above often hidden beneath the expanded basal antennomere, antennal club circular and compact. Pronotal disc finely and rugosely punctured and finely and sparsely pubescent, explanate margins smooth and shiny, anterior angles strongly produced forward. Elytral disc strongly and densely punctured, explanate margins smooth and shiny, the striae represented by raised longitudinal carinae. Legs short and robust; femora hidden from above, tibiae broad and sinuate externally, apical margin truncate and produced, each with a robust and sharp spur on the inner angle. Tarsi 5-segmented, the terminal segment about as long as the others combined, claws smooth and without a basal tooth.