Figs. 2–4. 2 in Changes in Mandibular Musculature and Morphology in Response to Stemmatal Enlargement in Larvae ofThermonectus basillaris(Harris) andAcilius mediatus(Say) (Coleoptera: Dytiscidae: Dytiscinae)

Figs. 2–4. 2) Lateral view of Agabus punctatus showing distribution and relative size of corneal lenses of the stemmata. Each stemma is identified with an Arabic number (Schöne 1951, 1962). 3) Lateral view of Thermonectus basillaris, showing distribution and relative size of corneal lenses of the stemmata along with the cellular sacs (cs) of stemmata one and three. 4) The mandibular abductor muscle (Ab) of Thermonectus basillaris is overlain on the mandibular architecture (Md) of Agabus punctatus to illustrate the biomechanical problem resulting from stemmatal expansion. The medial displacement of the abductor apodeme places its line of action virtually inline with the mandibular joint. For a lever system to function the force vector must be at least some distance from the fulcrum.