The following essay explores this question. The deeper question of the relationship between the function of spineless in insect morphogenesis and its relation to the function of the dioxin receptor in vertebrates, is not be dealt with here. When spineless was ectopically expressed in Drosophila, a few flies survived to the pharate adult stage. In this way, the effects of ectopic spineless expression on adult structures have been studied. The results indicate that spineless is a primary determinant of distal antennal identity.
Ectopic expression causes transformation of the maxillary palp and distal leg to distal antenna.
Palps are also often deleted. Surprisingly, ectopic spineless induces ectopic antennal structures in the rostral membrane between the maxillary palp and the normal antenna. These range from small patches of third antennal segment and arista to entire ectopic antennae, and are always arranged in mirror symmetry to the normal antenna. Ectopic spineless expression also causes transformation of the distal leg to arista. In some cases, aristal-claw intermediates are present, indicating that aristae can arise by transformation of claws.