Specialization into reproductive and non-reproductive castes is one of the defining traits of eusocial insects. Knowledge of the proximal causes of caste differentiation is therefore central to achieving an understanding of the evolution of eusociality. Castes are an example of a polyphenism, multiple, discrete phenotypes arising from a single genotype in response to differing environmental conditions. Here we focus on recent work in the social wasps to provide insight into how environmental conditions may trigger the development of caste across a range from independent- to swarm-founding social species. The amount of food larvae receive has long been recognized as a key input factor in the determination of caste, but that alone is insufficient to account for the range of combinations of size, development time, and caste among the female offspring of Polistes , an independent-founding wasp. Recent experimental work on P. fuscatus has shown that vibrations that are associated with the feeding of larvae are another essential environmental input in the determination of caste. We present a model of how vibrational signaling in the context of feeding larvae could interact with nutritional input to account for the developmental patterns seen in these wasps. Mapping the distribution of vibrational signaling onto a phylogeny of the social wasps suggests that this trait characterized the common ancestor of the subfamilies Vespinae + Polistinae, diversified in the independent-founding species, then was superseded by caste-determining mechanisms in the swarm-founding and vespine species that function more effectively in larger colonies.
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