Involved in determining which thoracic imaginal disk cells will form wings and halteres, perhaps by interacting with other nuclear regulatory proteins. When in combination with scalloped (sd), it acts as a transcriptional activation complex that regulates gene expression in the wing. Binding to sd switches the DNA target selectivity of sd. Required and sufficient for cell proliferation at the dorsal/ventral (D/V) boundary of the wing imaginal disk. Also required for cell proliferation in the wing imaginal disk, mediated via activation of E2f. By interacting with Dhfr, may control genes involved in DNA replication. The Ser-rich protein domain within the C-terminal region interacts with the C-terminus of sd to form a complex which acts as a selector for wing development. Interacts with Dhfr. Expressed in the developing wing primordia initially along the D/V wing boundary, and by the late third larval instar, maximal expression is seen in cells at the D/V wing disk boundary. Less expression is seen in cells located farther from this boundary. Loss of vestigial function selectively eliminates wing and haltere formation.