Fig. 9. Stimulation of chitin synthesis in midgut extracts by serine proteases. Relative chitin synthase activity was measured by the incorporation of N-acetyl-D-[U-¹⁴C]glucosamine into alkali-insoluble material obtained from crude extracts of the anterior midgut of fifth instar larvae. (1) Activities of untreated extracts (0.4 nmol mg^–1 h^–1) were set to 100%; (2–4) relative activities in the presence of trypsin at a concentration of 10 µg µl^–1 (2), chymotrypsin at a concentration of 10 µg µl^–1 (3), and 5 mmol l^–1 Pefabloc SC serine protease inhibitor (4). Mean values (±s.e.m.) from three independent experiments are given as a percentage of the control.