*nigricans Bagnall, 1939
*spitzbergenensis (Lubbock, 1898)
Archisotoma besselsi is a moderately common littoral species that is probably present around the entire coast of Britain and Ireland. The body is blue/black to greyish in colour, a PAO is present (Fig. 1) and the eyes each have eight ocelli (two of which are small and difficult to resolve). The species is said to reach 1.5mm, but UK specimens seem not to exceed 1.2 mm. The definitive feature for identification is the structure of the maxilla (Fig. 2 - compare this photo with Fig. 344 in Gisin (1960)). Another useful character is the thick spur-like seta above the claw on the tibiotarsus of leg 3 (shared only with the extremely tiny Archisotoma theae).
Bagnall (1939) described Archisotoma nigricans from Whitburn, Durham which had 'PAO exceptionally elongated, parallel-sided, and in its transverse length about six times the diameter of the nearest ommatidia and usually greater than the width of Ant.I.' Steve Hopkin examined all of Bagnall's specimens in the NHML slide collection including the holotype and paratypes, and reported that in none of these specimens is the PAO more than three times the length of the largest ocellus (e.g. Fig. 1). In addition, the maxilla of these specimens is identical to that of Archisotoma besselsi (Fig. 2). Thus Archisotoma nigricans is definitely a junior synonym of Archisotoma besselsi, leaving the insoluble puzzle of why Bagnall thought its PAO was so long.