also sensitive to larval nutrition (Tu and Tatar, 2003), coordinate the timing of many developmental events, including molting and metamorphosis (Nijhout, 1994; Gilbert et al., 1996), as well as both the onset and cessation of cell proliferation in the different imaginal discs (Truman and Riddiford, 2002; Emlen and Allen, 2004). All these signals influence the insulin-producing cells in the brain and coordinate circulating levels of insect insulins (Nijhout, 2003; Tu and Tatar, 2003; Colombani et al., 2005).
By the time that the cells in a horn disc (or any of the traditional imaginal discs) initiate the outgrowth portion of the patterning cascade and begin their burst of proliferative growth, they are bathed in a milieu of circulating whole-animal physiological signals whose levels depend critically on the nutritional state of the animal. Several of these signals have been shown to modulate the rate of cell proliferation in these growing tissues in a way that couples their growth with nutrition. The result of this process is a beetle horn of the appropriate length relative to the final body size attained by that individual.
Both of the above developmental processes (steps 1 and 2) combine to stimulate cell proliferation and growth within developing horn discs. Together, they specify the total amount of growth that occurs. By the time the animal sheds its larval cuticle and molts into a pupa, these growth processes appear to be largely completed and the densely folded discs of horn tissue unfurl to form the fully extended fluid-filled tubes visible in pupae (Figs. 14.6 and 14.8).
One of the most exciting recent discoveries regarding beetle horn development was the observation by Moczek and colleagues (Moczek, 2006b; Moczek et al., 2006a) that these pupal horns often undergo extensive remodeling during the pupal period. The regions of pupal horns that subsequently are removed by local apoptosis map to domains of expression of some of the same patterning genes that presumably were involved in the initial formation of the horn outgrowth (Moczek, 2006b; Moczek et al., 2006b). Pupal remodeling of horns is only just beginning to be explored, but already it is clear that during this process, the final shape of the adult structure can be modified (e.g., by producing a narrower and/or more curved final horn; see Fig. 14.6), and it can also lead to the complete loss of the horn from the adult phenotype in some species. Many scarab species produce a thoracic pupal horn that is not retained in the adult, and in these species this horn is completely reabsorbed by all individuals during the pupal period (Moczek, 2006b). However, in other species with thoracic pupal horns, the horns are initially grown by all individuals but then selectively reabsorbed by only a subset (e.g., females, small males),