Spatial requirements of the enlarged stemmata of mature larvae of Thermonectus basillaris (Harris) and Acilius mediatus (Say) are described and evaluated. Each species exhibits a medial shift in the origin of the abductor muscles with corresponding changes in the orientation of the abductor apodemes to compensate for greater stemmatal volumes. These changes disrupt an apparent plesiomorphic biomechanical system responsible for opening the mandibles, requiring alterations in the basal mandibular architecture evident on these aciliine species.