Fig. 2. Segmental transformations of two Uab alleles. For each genotype, a dorsal view of the abdominal tergites is shown on the left, and a close-up of the ventral sternites is on the right. The cuticle of the second abdominal segment (A2) is marked with a bracket in each panel. All panels show female cuticles, except that the dorsal picture of Uab^HH1 shows a male. Wild type: the tergite of the first abdominal segment is narrower than in the second abdominal segment, and it lacks the band of pigment and large bristles at the posterior margin. On the ventral side, the first abdominal segment lacks a sternite. The sternite of the second abdominal segment has fewer bristles than that of the third abdominal segment, and these bristles all point directly posterior. The wild-type second abdominal sternite also includes a clear anterior patch, called the Wheeler's organ (marked W.O.). Uab^R1 homozygote: the A1 tergite is enlarged, and has pigmentation and bristles like that of the A2 segment. There is a clear sternite in the A1 segment, although it is not completely transformed to a more posterior type. Uab^HH1 homozygote. These animals rarely eclose as adults. The A1 tergite appears fully transformed to the character of the A2 or A3 segment. This individual also shows an extra half tergite anterior to A1. This tergite also has pigment and large bristles typical of the A2 tergite. The A1 sternite resembles that of A3. Note that the A2 sternite also resembles that of A3, as judged by bristle number and orientation, and by the lack of a Wheeler's organ.