The Malpighian tubules are the insect's equivalent of a kidney. They use very high levels of V-ATPase on the apical membrane to pump protons, which are then exchanged for potassium. So the net effect is to excrete potassium.
If you look closely, you'll see that the nuclear staining is strongest in the central region. In particular, the upper tubule doesn't stain at all, and the lower tubule stains only very weakly. This is nice, because we happen to know that only the central region secretes urine! So this shows that V-ATPase pumping is associated with fluid production in this tissue.
(If you're interested, you can find out more about the Drosophila Malpighian tubule model here...)