Archisotoma megalops is a scarce littoral species with an apparent northerly and eastern bias to its distribution. However, it may have been overlooked elsewhere. The body is blue/black to greyish in colour, a PAO is present and the eyes each have eight ocelli. The largest specimen I have seen was 2.5 mm in length (note the much larger size compared to Archisotoma besselsi).
     Thibaud & Palacios-Vargas (2001) sunk Archisotoma subbrucei Delamare Deboutteville 1954 as a junior synonym of Archisotoma megalops. The definitive feature for identification is the structure of the maxilla (Fig. 1: compare this photo with Fig. 347 in Gisin (1960) of 'Archisotoma subbrucei' = Archisotoma megalops). The presence of an inner tooth on the claw is not a defining character as it is not reliably present (e.g. Fig. 2).
     The mucro of Archisotoma is very distinctive with three large teeth (Figs. 3, 4).

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Fig. 2 (right): Foot of leg 3 of Archisotoma megalops from Boston, Lincolnshire collected in August 1952 (identity of collector unknown).

Fig. 1 (above): Maxilla (max) of Archisotoma megalops collected from Seaford in June 1946 by Bagnall.

Figs. 3 (right above) and 4 (right below): Two levels of focus of mucros of the same specimen of Archisotoma megalops shown in Fig. 2). The mucro has three large teeth.