Intercastes in the ant Aphaenogaster rugulosa (Formicidae: Myrmicinae): morphological variation and adaptive potential

Among eusocial insects, ants show the most advanced morphological division of labor with large winged queens, smaller wingless workers, and phenotypically intermediate castes. These intermediate castes may have evolved from the selection of developmental errors called intercastes. Intercastes are unusual phenotypes that show uncoordinated expression of worker and queen traits. These rare specimens provide remarkable insights into the advanced morphological evolution of the division of labor unique to ants. While they have been reported in several genera of the subfamily Myrmicinae, a single recent study described intercastes in the genus Aphaenogaster (210 spp). Here, we provide the first record of intercastes in an Asian species, Aphaenogaster rugulosa, endemic to the Japanese island of Yonaguni-jima. Using allometric measurements, scanning electron microscopy and dissections, we describe the trait combination of seven intercastes and compare it to worker and queen phenotypes. We discuss the modular origin of these rare phenotypes and elaborate on the potential role of intercastes in ant evolution.