A. Representative images and quantification of Drosophila midguts treated with the pH indicator Bromophenol Blue. Moreover, quantification of the three categories of Drosophila of indicated conditions. The three categories can be divided into: homeostasis, a well-defined acidic (yellow colored) copper cell region (CCR) is flanked by basic (blue colored) anterior midgut (AM) and posterior midgut (PM). "Perturbed A", the acidic region is lost and the whole gut is basic; "Perturbed B", the strongly acidic region is lost, and the remainder of the gut also becomes less basic. The numbers of quantified guts from left to right are 90, 90, and 90. Error bars show the SD of three independent experiments. 