evolution. Even subtle genetic changes to the levels of expression of these patterning genes can have significant and predictable consequences for the shapes and sizes of adult insect appendages. For these reasons, the limb-patterning pathway has been a major focus for studies of the developmental basis for morphological evolution in arthropods. In the case of beetle horns, we think that subtle changes in the levels of expression of these patterning genes may underlie at least two types of changes in horn form, the evolution of horn location and horn shape (Emlen et al., 2006).

Because cells exposed to high levels of hh, wg, and dpp signals become the distal tip of an appendage, their domains of expression determine the precise physical location of a horn. Genetic changes to any of these domains (e.g., an increase in the expression of wg) would shift the relative location of the domain boundaries, changing the respective point of intersection. Thus, a different cluster of cells would be induced to become the distal tip, and there would be a shift in the precise physical location of the outgrowth. Consequently, genetic modifications to the domains of expression of these patterning genes comprise a plausible mechanism for this trajectory of horn evolution, for example, the migration of a horn from the center to the sides of the head (Fig. 14.2 Lower).

In addition, because changes in the domains of expression of these same genes can duplicate or bifurcate appendages, this same process could give rise to a multitude of evolutionary changes in horn shape. One horn could be split into 2 or even 3, as in Onthophagus fuliginosus (Fig. 14.3). This mechanism could even account for the addition of forks or branches to horns (Fig. 14.8). Comparative studies of horn patterning are still in their infancy, but the behavior of this pathway in the appendages of other insects, combined with existing evidence for species differences in horn patterning (e.g., Moczek and Nagy, 2005; Moczek et al., 2006b) suggest that this mechanism underlies at least some of the evolutionary diversification of horn form.

Insulin signaling couples trait growth with nutrition, and for this reason, this pathway comprises another likely mechanism for horn evolution. The InR pathway is activated within each of the imaginal discs, and the sensitivity of each disc to these insulin signals will determine to a large extent how that particular structure will grow. Consequently, genetic changes in the expression or activity of elements in the InR pathway could cause specific traits to become more or less sensitive to insulin signals,with profound consequences for subsequent patterns of growth of that trait. Increased sensitivity of horn disc cells to insulin is predicted to increase