Haeterius ferrugineus (Olivier, 1789)
This is a very local and rare species with a southern and mostly eastern UK distribution, it was first recorded at Hampstead in north London in 1848 and since then there have been a few scattered records from Surrey, Worcestershire and the Isle of Wight but while it seems to be established it has never been found in numbers although this is probably due to its lifestyle. For much of the 20th century it went unrecorded; it was found in Surrey in 1909 and then on the Isle of Wight in 1920 but was then absent until 2012 when it was found in Kidderminster (Brown, 2012). The species is associated with various ants, more particularly Formica sanguinea Latreille, 1798 and F. fusca Linnaeus, 1758 but possibly also Lasius niger (Linnaeus, 1758) and all the UK records have been from within nests, under nearby debris or among fungus close to the ant brood chamber. It is the only European member of the genus and is widespread and locally common in southern and central areas; it is considered to be strictly myrmecophilous and is found within nests of a wider range of species than in the UK. Adults are recorded living between two and four years when kept among ant nests under artificial conditions and have been observed mating and feeding on dead and injured ants and their early stages as well as mites within the nest, they are transported around the nest and have been observed being fed by the ants but nothing is known of their life-cycle.
Among the European fauna adults are distinct and should not be confused with any other species. 1.3-2.0mm. Very broadly-oval, almost continuous in outline and entirely brown to reddish-brown in colour, dorsal surface smooth and rather shiny, pronotum and elytra with sparse erect pale setae. Head widely transverse when viewed from above, weakly convex, smooth and with small transverse eyes, antennae 9-segmented, the basal segment greatly expanded internally, 2-8 gradually widened and the terminal segment forming a large and truncate club. Pronotum very transverse and widest at perpendicular posterior angles, anterior angles rounded and weakly produced, disc evenly convex between paired sub-lateral striae and there is a variable basal depression towards the lateral margin. Elytra transverse and evenly rounded to a continuous apical margin, each with a fine and usually abbreviated sub-humeral stria and three or four striae across the disc that extend almost to the apex, the erect setae arranged in longitudinal rows on or near these striae. Pygidium and propygidium smooth and shiny. Legs long and robust; femora narrow and simple, tibiae flattened and broad; straight internally and strongly angled externally. Tarsi 5-segmented and simple.