neur, bib and fng reporter expression was observed during the formation of ball-and-socket joints in the leg. These drivers are expressed in different but overlapping cell populations in the developing joint. bib and neur are expressed in the deep region of the cavity, flanked by fng expression in cells projecting into the ridge and cells on the ventral side of the cavity. bib was expressed distal to neur, with an overlap of about two cells. Shortly after the onset of ball formation, bib-expressing cells are in contact with the ball cuticle. The expression of neur appears to overlap largely with bib-expressing cells and to extend dorsally to them. bib and neur expression domains are four to six cells wide along the anteroposterior axis. When the ball has fully grown, the apical surfaces of bib-expressing cells comes into contact with the ventral side of the ball, and then the apical surfaces further shifted distally, eventually contacting the newly formed lip. Consistently, the apices of the ventral portion of neur-expressing cells (co-expressing bib) relocate to the ventral side of the ball, and the apical surfaces of their dorsal subsets move with the edge of the extending socket, until covering the ventral part of the socket. Meanwhile, the dorsal fng-expressing cells expand their apical surfaces, and when the socket cuticle forms at the bottom of the cavity, they

cover a large dorsal portion of the socket cuticle. Live imaging was used to follow the bib- and neur-expressing cells.