Fig. 5. The pH within the AP changes in response to changes in ambient pH in vivo. Top, larva exposed to bromothymol blue in vivo in pH 4 media (RS 4). Under these conditions bromothymol blue enters the AP. Bromothymol blue is yellow at pH <6.8 and blue at pH >6.8. In acid media, the interior of the AP is thus acidic (pH <6.8). Bottom, AP that have been `loaded' by exposure of larvae to bromothymol blue in acidic media followed by transfer to pH 11 media (RS 11) for at least 24 h. Note that the rectal lumen is acidic under these conditions.