common horn locations: the center of the thoracic pronotum and the dorsal surface of the head. We also suspect that this occurred early in the history of the scarabs, possibly in the common ancestor to all of the scarabs. This Jurassic beetle is thought to have lived in burrows and could have used these early horns in an ecological context not unlike what we observe in extant taxa. Today, there are five recognizable body regions with horns, raising the possibility that the patterning pathway was independently deployed additional times as well. It is not yet clear how readily the horn foci migrate across body segment boundaries. For example, could an evolutionary shift in horn location from the back of the head (the vertex) to the center (frons) or front (clypeus) of the head result from a gradual migration of the position of the focal cells activating horn growth? Or must these involve new cooptions of the patterning process, combined with suppression of expression of an earlier horn? Species exist with all possible combinations of these five horn locations, which may indicate that each of these horn types arose de novo; but convincing answers to these questions will have to wait until additional studies of horn patterning have been conducted. Regardless of how many times this pathway was coopted, it is likely that once expression was initiated, this patterning process provided a viable mechanism for subsequent evolutionary modifications to horn form.
If the patterning of beetle horns works in the same way that it does in other insect appendages such as Drosophila legs, then the patterning genes will have precise domains of expression that map to specific parts of the final structure. Critically, the process of patterning will be inextricably coupled with cell proliferation and disc growth. In Drosophila, many of the same signal interactions that specify the domains within a disc also stimulate and coordinate cell proliferation within those domains. This means that altered levels of expression of these patterning genes changes the final sizes of structures (Serrano and O’Farrell, 1997; Campbell, 2002; Johnston and Gallant, 2002). Furthermore, because these genes control proliferation within specific subsets of the disc that map precisely to corresponding parts of the final appendage, altered expression of these genes is predicted to change the shape of the developing structure (e.g., the size of the tibia relative to the femur in an insect leg, or the size of the dorsal surface of the tibia relative to the ventral surface).
These two critical features of patterning (the explicit spatial map generated by these molecular signals and the local nature of their effects on cell proliferation) link this developmental process with morphological