In social insects, caste morphologies are different depending on their tasks allocated in their colonies. Although many ant species generally possess two female castes: winged queen and wingless worker, in some Myrmecina species, some colonies produce a wingless reproductive caste instead of alate queens. This wingless reproductive caste is termed 'intermorphic' queen, and its morphological features are intermediate between winged queen and worker. In the present study, we analyzed the morphological features of the three female castes to explore the adaptive meaning of the intermorphics in terms of reproductive strategy, in addition to the developmental and evolutionary origin in Myrmecina nipponica. We first performed the principal component analysis based on the morphometric data of the three female castes of adults (alate queen, intermorphic queen and worker). The results of analysis showed that allometry of the intermorphic queen differ from those of alate queen and worker. In intermorphics, compound eyes and gasters were more developed in comparison with heads and thoraxes. We also examined several body parts in detail by scanning electron microscopy in the three female castes. The morphological features of intermorphics varied widely compared with the other two castes. Intriguingly, some parts of intermorphics were queen-like, while others were worker-like. Our findings suggest that the morphological features of intermorphic queens have specialized reproductive strategy involving budding.
