Fig. 4. Effects of Na⁺-free bathing saline on (A) fluid secretion rate and (B) transepithelial flux of salicylate across the main segment of isolated Malpighian tubules set up in the Ramsay assay. Malpighian tubules were bathed in control saline (solid bars) or Na⁺-free saline (hatched bars) containing 50 µmol l^–1 salicylate. 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. Secreted fluid droplets were collected after 40 min. Values are means ± s.e.m. (N=6–9 tubules). Asterisks indicate significant differences between control and experimental groups (^*P<0.05, ^**P<0.01; t-test, N=6–8).