Gnathisotoma Cassagnau 1957

Gnathisotoma Cassagnau, 1957

Cassagnau (1957) established the genus Gnathisotoma for the new species bicolor which was found in the central Pyrenees. Later Najt (1981) redescribed bicolor and described a new species G. deharvengi from snowfields in the Pyrenees and reported another possibly new species (known only from ecomorphic specimens) from the Cantabrian mountains in Spain. Yoshii (1990) described two more species from glaciers in Chile, G. patagonica and G. pallens, and also included the Antarctic species Isotoma klovstadi Carpenter, 1902 in Gnathisotoma. Stevens et al. (2006) transferred klovstadi to the genus Desoria. Further species of Gnathisotoma are not known.

In the original diagnosis of the genus Cassagnau (1957) stressed the modified maxillae of bicolor. Najt (1981) defined Gnathisotoma on the reduced number of sensilla on the tergites, the modified maxilla and the absence of frontal setae on the ventral tube. Yoshii (1990) used the subsegmentation in proximal part of dens as a single diagnostic character for Gnathisotoma. Potapov (2001) used the presence of four long maxillary lamellae and the absence of anterior setae on ventral tube as diagnostic characters separating Gnathisotoma from Desoria. However, none of the above characters are clearly inclusive of all species and unique to the genus. There is only a gradual difference between the maxilla of the Pyreneean Gnathisotoma and some Desoria species (tigrina -group). Also some Desoria have the sensillary equipment of tergites as much reduced as in Gnathisotoma. The significance of the ventral incision (subsegmentation) at base of dens is unclear on a generic level. The presence of the Gnathisotoma described below from Alaska with maxilla almost identical to bicolor but with 1–2 frontal setae on ventral tube and an increased number of tergal sensilla make the situation more confused and questions the status of Gnathisotoma as separate genus from Desoria.