AbstractDistal regions or “marginal ridge” areas of the dorsal mesothoracic disc of mature Drosophila larvae, not previously included in fate maps, were marked by microcauterizationin situ. Structures assumed to have primordia in the distal half of the disc (mesopleura, pteropleura, arch, ventral scutellum, ventral wing hinge and wing proper) are either missing, deficient, or damaged among adults surviving the operation. In contrast, neither the dorsal wing hinge nor mesonotum, which are known to have primordia in the proximal half of the disc, are similarly affected.The pattern of distribution of deformities in surviving adults indicates that (a) the anlage of the mesopleural sclerite is located in the distal edge portion or “marginal ridge” of the disc, (b) anlagen of pteropleura, arch and ventral scutellum may also be tentatively assigned to this location, (c) there are significantly high associations between deformities affecting the pteropleura and the ventral wing hinge, and (d) the anlage of the deep phragma of the postnotum may be more closely related to the first abdominal tergite than to the dorsal mesothoracic disc.The distal localization of pleural, arch, and ventral scutellar anlagen corresponds to the physical separation from mesonotal anlagen predicted from an earlier statistical analysis of genetic mosaics;i.e., these presumptive sclerites are separated from the mesonotal anlagen by the anlagen of the wing in the “pouch” area of the disc.The new information has been incorporated into a hypothetical fate map of the disc, which includes anlagen plans suggested by previ