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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 specimens I have seen reached a maximum length of 1.2 mm (1.5 mm is reported in the literature). 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.' I have examined all of Bagnall's specimens in the NHML slide collection including the holotype and paratypes. 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. I believe that the first Archisotoma species Bagnall found was Archisotoma pulchella which he assumed was Archisotoma besselsi. Thus when he found the true Archisotoma besselsi he described it as the new species Archisotoma nigricans. However, I am at a loss to understand how he thought the PAO was so long.
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