Differences in larval and adult morphology due to Hox gene differences. (A, B) Larva and adult of Drosophila, a dipteran. An arrow points to one of the halteres of the adult. The larva lacks prolegs; its anterior end is at the left. (C, D) Larva and adult of Danaus plexippus, the monarch butterfly, a lepidopteran. The anterior of the caterpillar is to the left, and a proleg is indicated by an arrow. The adult has hindwings rather than halteres. The regulation of Hox genes determines the presence of prolegs, and the targets of Hox genes determine whether the third thoracic segment is to generate halteres or hindwings. (A courtesy of M. Tyler; B courtesy of E. B. Lewis; C courtesy of G. Savage; D by Bill Beatty/Visuals Unlimited.)