Fig. 2. Induction of leg structures in the Drosophila larval abdomen. (A) Lateral view of a wild-type first instar larval cuticle. Arrows point to Keilin's Organs (KOs), the rudimentary larval legs that appear in the thoracic segments. (B) Ventral view of a cuticle upon ectopic expression in the abdomen of Dll and btd. Arrows point to KOs that develop in otherwise normal abdominal segments. (C) Detail of A showing a wild-type thoracic KO (arrows). (D) Detail of B, showing the KO developing in abdominal segments (arrows; abdominal segments 4 and 5; more than 85% of the scored abdominal hemisegments show these KOs; n=20 embryos). (E) High magnification of a wild-type KO. (F) High magnification of a thoracic KO upon btd overexpression. (G) High magnification of an abdominal KO upon btd and Dll overexpression. KOs upon btd overexpression have more than the three hairs seen in the wild type. (H) Scheme of the proposed interactions giving rise to appendage primordia.