Evolution of cheaper workers in ants: A comparative study of exoskeleton thickness

The great evolutionary success of insects is partly linked to the innovation of the cuticle, which underlies a complex exoskeleton with varied functions. Cuticle development is costly because chitin and cuticular proteins require ingested nitrogen. Studying cuticle thickness on a comparative basis allows insight into the trade-off between offspring quality and quantity. This trade-off takes a particular meaning in ant workers because they are wingless, non-dispersers and generally sterile. We selected a comprehensive sample of 42 ant species (40 genera belonging to nine subfamilies) and measured cuticle thickness in workers using labour-intensive histological sectioning of the prothorax. Thickness varied substantially across genera, ranging from 1.3 to 109.8 µm. We correlated thickness with body size, measured as head width. Our results show a strong influence of body size, but also phylogeny, because workers in ‘formicoid’ species tend to have a thinner cuticle (two exceptions). The effect of body size is also obvious in species having size-polymorphic workers. We discuss the idea that a thinner cuticle is associated with miniaturization of workers, and its ecological consequences are buffered in a social environment. We reveal that the bulk of ant species have a very thin cuticle, and this represents cost savings that may partly underlie the massive increase in colony size during adaptive radiation.