Quantitative studies of plasma levels of protein, lipid, and soluble carbohydrate in larvae of O. 8tygia reveal discontinuities at days 6 and 10 of development. From day 3 to 6 plasma levels of protein and lipid increase sharply. After day 6, there is a decline in the concentration of plasma protein which is accelerated. at the onset of pupation. The plasma lipid concentration rises abruptly at day 10, but then declines with the onset of puparium formation. The concentration of soluble carbohydrates in the haemolymph falls slightly between days 6 and 7 of development and commences to rise at about day 10; still higher levels are reached in the quiescent larva and early prepupa.