Figure 6
Partial phylogeny for the families and subfamilies of scarab beetles (Coleoptera: Scarabaeoidea) illustrating lineages with head or pronotal (thorax) horns (approximate no. species in parentheses). The majority of species with exaggerated horns are concentrated within three distantly related clades (Geotrupidae, Scarabaeinae, Dynastinae) that collectively represent only 20% of extant scarab species. For this reason, horns are thought to have arisen multiple times independently within the superfamily, leading Darwin and others to speculate on the ‘special tendency’ of scarabs towards evolution of enlarged horns. However, all but three of the included clades contain either rudimentary horns (1), at least a few genera or species with enlarged horns (2) or pupal horns (3), and one completely hornless subfamily contains mutant individuals with horns (4). Even the lucanids, which primarily produce enlarged mandibles as weapons, contain species with head and pronotal horns (eg Sinodendron). At least nine clades also contain threshold mechanisms for dimorphic horn expression (5). We interpret these as evidence for an inherited developmental capacity for the production of horns, and suggest that the ancestor of the scarabs may have had both horns and horn dimorphism. Representative horned species from primarily ‘hornless’ clades: (a) Pleocoma marquai (Pleocomidae); (b) Odontotaenius disjunctus (Passalidae) (c) Sinodendron cylindricum (Lucanidae); (d) Theodosia viridaurata (Cetoniinae); (e) Fruhstorferia sexmaculata (Rutelinae); (f) mutant ‘horned’ individual of Pterorthochaetes armatus (Ceratocanthidae). Tree topology partially from Smith et al (in review); Browne and Scholtz, 1998; Browne and Scholtz, 1999. Node ages based on fossil records (Krell 2000). Species numbers and taxon descriptions from Ratcliffe and Jameson (eds) Generic guide to new world scarabs (http://www-museum.unl.edu/research/entomology/Guide/index4.htm).
