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FIGURE 14.5 Evolution of horn dimorphism. Species differ in the presence and nature of dimorphism in horn expression (males = closed circles; females = open circles). In Onthophagus taurus (a) and O. nigriventris (b), large adult males produce either a head (O. taurus) or a thoracic (O. nigriventris) horn that is not present in small males (male dimorphism) or females (sexual dimorphism). In O. sloanei (c) the patterns of horn dimorphism are reversed: small male and female adults produce a head horn that is not present in large males. In O. pentacanthus (d) all individuals of both sexes produce the horn, but the relative sizes of these horns differ between males and females. Insets are SEM photographs of the head and thorax of male (a,b,d) or female (c) individuals, with horns colorized to indicate their physical location. Reprinted with permission from Emlen et al. (2005a).

The major adult structures in metamorphic insects form from isolated “pockets” of cells called imaginal discs, analogous in many ways to the limb buds of vertebrates (Truman and Riddiford, 2002). In Drosophila, these discs have been especially thoroughly studied (Kojima, 2004; Weihe et al., 2005), but discs occur in all metamorphic insects, and arguably, disc-like patterns of growth occur in nonmetamorphic insects and other

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