Fig. 5. Effects of 1 mmol l^–1 cAMP on the rate of fluid secretion by the main segment of isolated Malpighian tubules set up in the Ramsay assay. The first secreted droplet was collected at 30 min (solid bars) and the second secreted droplet (hatched bars) was collected 30 min after adding cAMP. Experimental larvae were raised for 10 days on a 10 mmol l^–1 salicylate-enriched diet. Control larvae were raised on a salicylate-free diet. Values are means ± s.e.m. (N=8–10 tubules). Asterisks indicate significant differences in fluid secretion rate before and after adding cAMP (^**P<0.01, ^***P<0.001; paired t-test, N=8–10).