1. ¹Division of Biological Sciences, The University of Montana, Missoula, MT 59812, USA
2. ²Department of Entomology, Washington State University, Pullman, WA 99164-6382, USA
3. ³Department of Genetics, North Carolina State University, Raleigh, NC 27695-7614, USA

Correspondence: DJ Emlen, Division of Biological Sciences, 104 Health Science Building, The University of Montana, Missoula, MT 59812-1002, USA. E-mail: doug.emlen@mso.umt.edu

Received 1 November 2005; Accepted 5 June 2006; Published online 19 July 2006.

Abstract

Beetle 'horns' are rigid outgrowths of the insect cuticle used as weapons in contests for access to mates. Relative to their body size, beetle horns can be enormous. They protrude from any of five different regions of the head or thorax; they are curved, straight, branched or bladed; and their development is often coupled with the nutrient environment (male dimorphism) or with sex (sexual dimorphism). Here, we show that this extraordinary diversity of horns can be distilled down to four trajectories of morphological change – horn location, shape, allometry and dimorphism – and we illustrate how the developmental mechanisms regulating horn growth could generate each of these types of horn evolution. Specifically, we review two developmental pathways known to regulate growth of morphological structures in Drosophila and other insects: a limb-patterning pathway that specifies the location and shape of a structure, and the insulin pathway, which modulates trait growth in response to larval nutrition. We summarize preliminary evidence indicating that these pathways are associated with the development of beetle horns, and we show how subtle changes in the relative activities of these two pathways would be sufficient to generate most of the extant diversity of horn forms. Our objective is to intuitively connect genotype with phenotype, and to advocate an informed 'candidate gene' approach to studies of the developmental basis of evolution. We end by using this insight from development to offer a solution to the long-standing mystery of the scarabs: the observation by Darwin, Lameere, Arrow and others that this one family of beetles appeared to have a 'special tendency' towards the evolution of horns.