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FIGURE 14.8 Development of a branched beetle horn. Horn disc from a late-stage prepupa of a male rhinoceros beetle (Trypoxylus [Allomyrina] dichotoma, Dynastinae) showing the folded tubes of epidermis (a) that, once unfolded, will comprise the branched adult horn (b). Four distinct axes of proximal–distal outgrowth are visible in this disc (arrows, inset) corresponding to each of the distal branch tips of the final horn. All branches are already formed by the time the animal pupates (a and c), suggesting that the evolution of a branched horn shape in this lineage resulted primarily from genetic modifications to the patterning processes that control cell proliferation (horn growth). However, the grooves between horn branches are more pronounced in the adult than in the pupa (b and c show the same individual), suggesting that some remodeling of horn shape also occurs during the pupal period.



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