Although olfaction is a primary mode of communication, its importance in sexual selection remains understudied. Here, using the butterfly Bicyclus anynana, we address all the parameters of importance to sexual selection for a male olfactory signal. We show that variation in the male sex pheromone composition indicates male identity and male age. Courting males of different ages display small absolute (c. 200 ng) but large relative (100%) change of one specific pheromone component (hexadecanal) which, unlike the other components, showed no heritability. Females prefer to mate with mid-aged over younger males and the pheromone composition is sufficient to determine this preference. Surprisingly refined information is thus present in the male olfactory signal and is used for sexual selection. Our data also reveal that there may be no 'lek paradox' to resolve once the precise signal of importance to females is identified, as hexadecanal is, as expected, depleted in additive genetic variation.

Among the animals, the Lepidoptera (moths and butterflies) are second only to beetles in number of described species and are known for their striking intra- and interspecific diversity. Within species, sexual dimorphism is a source of variation in life history (e.g., sexual size dimorphism and protandry), morphology (e.g., wing shape and color pattern), and behavior (e.g., chemical and visual signaling). Sexual selection and mating systems have been considered the primary forces driving the evolution of sexual dimorphism in the Lepidoptera, and alternative hypotheses have been neglected. Here, we examine opportunities for sexual selection, natural selection, and the interplay between the two forces in the evolution of sexual differences in the moths and butterflies. Our primary goal is to identify mechanisms that either facilitate or constrain the evolution of sexual dimorphism, rather than to resolve any perceived controversy between hypotheses that may not be mutually exclusive.

For almost a century, biologists have used trait scaling relationships (bi-variate scatter-plots of trait size versus body size) to characterize phenotypic variation within populations, and to compare animal shape across populations or species. Scaling relationships are a popular metric because they have long been thought to reflect underlying patterns of trait growth and development. However, the physiological mechanisms generating animal scaling are not well understood, and it is not yet clear how scaling relationships evolve. Here we review recent advances in developmental biology, genetics, and physiology as they pertain to the control of growth of adult body parts in insects. We summarize four mechanisms known to influence either the rate or the duration of cell proliferation within developing structures, and suggest how mutations in these mechanisms could affect the relative sizes of adult body parts. By reviewing what is known about these four processes, and illustrating how they may contribute to patterns of trait scaling, we reveal genetic mechanisms likely to be involved in the evolution of insect form.