A new species of the endemic Australian genus Mesostoca (M. austini) is described and illustrated. The Praonopterinae is synonymized with the Mesostoinae through transfer of its type species, Pranopterus laevis. Keys to the genera and species of Mesostoinae are provided. Male internal and external reproductive structures, and final larval instar cephalic structures of Mesostoinae are described for the firt time. M. austini was reared from an unidentified gall on Banksia. The relationships of the Mesostoinae are reassessed in the light of this new information.