tws⁵⁹ homozygotes develop into pharate adults, and many eclose, but immediately fall into the food. Those that do not show entirely uncoordinated movement in walking and overturn. They do not jump or fly. Extra sensory structures form. There are two sets of the shaft and socket instead of a single pair of each. The twin sockets are usually physically connected or fused to each other. Macrochaetae are duplicated or triplicated in mutants. The neuron and sheath cells are probably transformed into support cells. In other cases a single shaft protruded from a large socket with a bottle gourd shape, as for the fusion of two sockets. A minor aspect of the mutant phenotype is bristle loss. Duplication of the taste bristle is very rare. Wing margin chemosensory bristles show no altered morphology, though the long, non-innervated hairs on the posterior wing margin are duplicated.