A juvenoid compound known as methoprene has no effect on growth and respiratory metabolism in penultimate instar larvae that contain endogenous juvenile hormone. In the last larval instar the juvenoid induces enormously large somatic growth and postpones pupal ecdysis although it does not increase the overall metabolic intensity beyond certain limits. The metabolic changes caused by the juvenoid were more pronounced when connected with formation of the supernumerary larval instars. After this developmental change the larvae further continued to grow on and turned finally into the conspicuous giants. The overall metabolic capacity of the supernumerary larval tissues surpassed the above limitations determined by the body of a normal last instar larva. However, unlike in some other species, there was no hypermetabolic response to juvenoid treatments in this material.