Phylogenetic and developmental studies into the evolution of an insect novelty.

The insects possess one of the most instantly recognisable bodyplans. This thesis addresses the evolution of one characteristic feature of the insects: the intercalary segment of the head. This small, appendageless segment is the homologue of the ancestral crustacean second antennal segment and its evolution underlies the loss of the second pair of antennae in the insect head. There is little consensus between different methods of phylogenetic reconstruction as to which crustacean group the insects are most closely related to. This question is addressed by compiling a multigene dataset and running a number of Bayesian phylogenetic analyses to investigate the effects of analysing the data under different models of evolution. In addition, Bayes factor hypothesis tests addressing the position of the insects within the Pancrustacea are described. The rest of the thesis addresses the developmental changes underlying the evolution of the intercalary segment. Almost everything that is known about the development of this segment in the insects comes from Drosophila. However, it is not clear exactly what constitutes the segment in the fly embryo. Specifically, it is unclear whether a pair of lobes behind the Drosophila stomodeum---the hypopharyngeal lobes---belong to the intercalary or mandibular segment. Using a detailed comparison of expression patterns between Drosophila and the red flour beetle Tribolium, the segmental affinity of these lobes is resolved. Finally, a screen to identify potential candidate genes for patterning the intercalary segment is described. The screen makes use of the Berkley Drosophila Genome Project expression pattern database to identify genes expressed in the segment of the fly. Having identified orthologues of the genes in Tribolium using the genome sequence on BeetleBase, their expression patterns are examined in the beetle. Genes with conserved expression are deemed good candidates for a more widespread role in patterning the segment