Fig. 1. (A-C) Drosophila labial discs, their proximodistal orientation, the resulting adult appendage, and wild-type expression patterns of Pb and Scr. (A) The labial discs are attached to the mouthparts of larvae. (B) Their point of attachment is at the proximal end of the disc, the peripodial stalk. (C) The labial discs fuse to form the proboscis, the appendage used for feeding. (D-F) Pb (green) and Scr (red) are co-expressed throughout wild-type labial discs except for the proximalmost cells (A. M. Pattatucci, PhD thesis, Indiana University, 1991; Cribbs et al., 1992; Johnston and Schubiger, 1996). L, labial disc; T, thoracic leg disc. White dots indicate stalks of labial discs only. The distal ends of most of the labial discs point towards the right.