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FIGURE 14.4 Evolution of horn allometry. Horn length–body size scaling relationships shown for the head horns of nine Australian species of Onthophagus, representing a well-supported monophyletic clade within the phylogeny of Emlen et al. (2005b).

beetle horn, and we illustrate how each of these mechanisms could contribute to the above trajectories of horn evolution.

THREE STEPS TO BUILDING A BEETLE HORN

Step 1:
Making an Axis of Outgrowth

Beetle horns form as long tubes, localized regions of epidermal tissue that undergo a burst of proliferation at the end of the larval period, just before pupation (Fig. 14.6) (Emlen and Nijhout, 1999). As these horn cells proliferate, they fold in on themselves to produce a compact disc of epidermal tissue that unfurls to its full length during the pupal molt. In these respects, beetle horns develop in a way very similar to the traditional appendages in beetles and other insects (e.g., wings or legs), and it now appears that similar mechanisms may be involved.



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