Figure 10 in The evolution of Bolbites onitoides (Coleoptera: Scarabaeidae: Phanaeini): its phylogenetic significance, geographical polychromatism and the subspecies problem

Figure 10. Phylogenetic placement of Bolbites onitoides among the Phanaeini and the evolution of iridescent colouration in this clade. A growing body of evidence has accumulated over the past two decades confirming that the horneless Bolbites is a phanaeine lineage that is more closely associated to horned Phanaeina than to the primitively hornless Gromphadina (see references in the text). If this is correct, the evolution of iridescence from the plesiomorphic opaque condition preceded that of the typically male cephalic and pronotal ornamentation found widely among the Phanaeina. As a consequence, the selective pressure that led to the evolution of the two features cannot have been the same. A more likely scenario is that iridescence first evolved serving another function in the common ancestors of the Phanaeini – e.g. thermoregulation or aposematism – and was inherited serving this function by the the Gromphadina and Phanaeina alike. Then, iridescence was putatively co-opted independently during the parallel evolution of cephalic horns by the Phanaeina less Bolbites and the gromphadine clade Gromphas aeruginosa + G. lemoinei to serve a new role in the evaluation of male horns during sexual interactions. Sexual selection thus co-opted for a new use a feature that had originally evolved under natural selection serving another function. This characterizes iridescence as an exaptation for horn evaluation, not an adaptation for this function. Species illustrated (from top to bottom): Eucranium cyclosoma Burmeister, 1861, Oruscatus davus (Erichson, 1847), Gromphas lemoinei Waterhouse, 1891, G. dichroa Blanchard, 1846, Bolbites onitoides Harold, 1868, a red Coprophanaeus saphirinus (Sturm, 1826), a blue C. saphirinus, and Sulcophanaeus menelas (Castelnau, 1840).