Stalk-eyed flies have their eyes and antennae projected on lateral extensions of the head known as eyestalks. It has previously been shown that this trait is under sexual selection, as females prefer to mate with males with the largest eyespan in some stalk-eyed fly species. However, the developmental biology underlying eyestalks is unknown. In this thesis I investigated how this exaggerated trait is constructed. In Diptera, the eyes, antennae and dorsal head are derived from the fusion of two eye-antennal imaginal discs. The disc morphology of two species of stalk-eyed fly, Cyrtodiopsis dalmanni and Sphyracephala beccarri is very similar to Drosophila melanogaster. However, an extra disc region exists in the stalk-eyed fly which is absent in D. melanogaster. Using key regulators of D. melanogaster head development, I showed that these genes have conserved expression in the eye-antennal imaginal disc of stalk-eyed flies. The expression of genes that specify the extremes of the dorsal head in D. melanogaster was preserved in the stalk-eyed fly disc despite their disparate head morphology. Also, although the adult antenna and eye are adjacent in stalk-eyed flies, the disc regions that will form these structures are widely separated. A fate map of the disc regions was deduced from in vivo culture of disc fragments. This work confirmed my expression data. Surprisingly, the regions that will form the palpus and antenna are adjacent in the eye-antennal disc, yet the adult structures they will form are widely separated. Conversely, the regions that will form the eyestalk and antenna are separated in the disc even though in the adult they are adjoining. To further explore these issues I successfully performed a D. melanogaster P-element enhancer trap screen in which I found several genes involved in head capsule morphogenesis and also produced and characterised a cDNA library from the larval stage of C. dalmanni (Diopsidae).