A bivoltine species: Males which I have studied (all apparently freshly emerged) were collected in late May and June; two females, presumed to belong to this species, were taken in July and August. R.C.L. Perkins (1919g) reported only a single brood for A. pilipes (almost certainly misidentified nigrospina) in Oxfordshire, although in Bedfordshire, Chambers (1949) states that there it is bivoltine.
On the Continent the species is also bivoltine, flying from late May to late June, and again from the end of July to the beginning of September (Kocourek, 1966).
F.K. Stöckhert (1954), Kocourek (1966) and a recent revision of the central European bee fauna by Dylewska (1987) recognise this species as distinct from A. pilipes. However, Warncke (1986) and Westrich (1989) consider the present taxon to be a no more than a form of the latter, as transitional variants exist between it and typical A. riparia. As circumstantial evidence, Warncke reports that both forms are declining in southern Germany. To him it is logical that if they represent a single species then it would be expected that both forms would decrease in abundance together. Nevertheless, to me, the converse is also true: two closely related species could similarly share the same fate.
From the morphological characters presented in the above key and from the ecological evidence outlined below (and contrasted with that for A. pilipes), I accept that these two taxa are very closely related sister species. Koucourek (1966) also states that A. nigrospina elsewhere in Europe exists independently of the various geographical races of A. pilipes.
Care is required to distinguish between the males of the two species: to the naked eye that of the present species typically has its thoracic hairs pale grey, whereas in its relative these hairs appear a dingy brown. Unfortunately females are so similar and variable that it is often impossible to identify them with certainty. Several of the very subtle key characters presented by Dylewska (1987) to both sexes are less than satisfactory, especially for British specimens. Obviously this species requires further research to ascertain its true taxonomic status.