Sitaris muralis (Forster, 1771)
This species is locally common in southern and western Europe but rare and sporadic elsewhere, there are a few widely scattered records from Germany, Austria, Moravia and Slovakia, only a single record from Poland and it is absent from the Baltic and Fennoscandia but it seems to be increasing in range and abundance, at least in central Europe, possibly due to climate change as it is thermophilic, and so might be expected from more northerly regions in the future. More generally it is thought to be native to the eastern Palaearctic region with Europe representing the western limits of its distribution. Here it is a very local and rare species of south and southeast England; it was known from the Oxford district from 1906 until the mid-1940s but there have been very few modern records, mostly from the New Forest but also Kent, Oxford and Dorset. The species is relatively large, conspicuous and unlikely to be mistaken for any other but it may be under recorded due to its lifestyle and habitats, it is a kleptoparasite with larvae developing within nests of various solitary bees; it is associated primarily with the hairy-footed flower bee, Anthophora plumipes (Pallas, 1772) - a large bumblebee-like species which often nests in old walls-but also the rare potter flower bee, A. retusa (Linnaeus, 1758)-a formerly widespread but species which has drastically declined in recent decades-and species of Osmia Panzer, 1806 and perhaps also Bombus terrestris (Linnaeus, 1758). The hosts are cavity nesters and while they may utilize natural habitats in dead wood or embankments etc. they often nest in old walls where the mortar has become soft or in pipes or crevices around doors and windows, and many Sitaris records have been from such artificial situations which are not generally habitats inspected by coleopterists. Adults appear in the summer and remain close to their emergence sites, they mate soon after emergence but do not feed, they remain active through the summer and may disperse by flight looking for suitable host nesting sites where they will lay a batch of up to two thousand eggs around the base of nearby herbaceous plants, these remain dormant until the autumn when small triungulin larvae emerge and remain concealed to overwinter. Triungulins become active the following spring, they climb plant stems and wait in flowers
for visiting bees, they are inconspicuous but sometimes large masses of larvae aggregate and become obvious, they attach themselves to any visiting bee and when present in numbers they resemble masses of small lice, hence the common name of bee-lice. Attached to a female bee they will then be carried back to the nest but when attached to a male they will wait until the bee mates and transfer to the female. Inside the nest they remain on the bee while it constructs a larval cell and, when the bee has deposited an egg, detaches at an opportune moment and drops into the cell, when the bee has sealed it off the triungulin then consumes the egg, this stage lasts about a week and enables it to metamorphose into a quite different apodous eruciform larvae which are able to complete their development by consuming honey, the triungulin is unable to develop further until it has consumed the egg and shed its skin and so, along with large numbers of triungulins that do not make it to the nest, the vast majority will not survive. These eruciform larvae are fully-developed by the autumn when they transform into a prepupal form in which they will overwinter. Pupation occurs in the spring and adults emerge from early summer. They are very poor fliers and walk slowly and clumsily, when disturbed they drop to the ground, roll into a ball and play dead while displaying the bright warning colour on the elytra, they are best looked for on bright sunny days when they rest on walls etc exposed to the sun.
Adults are absolutely distinctive among our UK fauna and should not be mistaken for any other species. 7-15mm. Head, pronotum and scutellum black, elytra dark brown or grey with the base extensively yellow. Head transverse with widely rounded temples and narrow, transverse and emarginate eyes. Vertex and frons almost flat, with dense, strong punctures and short dark pubescence. Antennae 11-segmented and filiform, second segment short and quadrate, the rest elongate. Pronotum transverse, widely rounded anteriorly and strongly sinuate before sharp posterior angles, basal margin straight, surface rugose and unevenly punctured. Scutellum flat and very large; strongly narrowed from the base and curved laterally, apical margin broad and sinuate. Elytra with broad rounded shoulders and strongly narrowed to curved apical margins, without striae and finely and quite densely punctured throughout, the sutural margins strongly sinuate and diverging from the base. Wings fully developed and exposed above the abdomen. Male abdomen extending just beyond the elytra, in gravid females greatly extended with broad pale membranes between the sclerites, otherwise with a crumpled appearance. Legs long and slender with femora widely visible from above, tibiae narrow and only weakly expanded toward a truncate apex, spurs fine and short. Tarsi 5-5-4 with all segments elongate and slender, the basal and apical segments about equal in length, claws smooth, long and only weakly curved.