Body size as a magic trait in two plant-feeding insect species.

This article investigates body size as a "magic trait"—a trait under divergent selection that also contributes to assortative mating—in two sympatric pine-feeding sawfly species, *Neodiprion lecontei* and *Neodiprion pinetum*. The study finds that body size differences correspond with host pine needle thickness, with *N. lecontei* (using thicker-needled pines) being larger than *N. pinetum* (using thin-needled white pine), and that egg size is under divergent selection related to host needle width. Mating assays demonstrate strong size-assortative mating within and between species, contributing to variable prezygotic reproductive isolation that correlates with body size differences. These findings support body size as a magic trait facilitating speciation-with-gene-flow in these sawflies and suggest that similar mechanisms may be common in plant-feeding insects, though further genetic and ecological studies are needed to confirm pleiotropy and assess the relative importance of body size among reproductive barriers.