Phenoloxidase converts L-DOPA and dopamine into melanin, an easily detectable, dark-brown pigment. UAS-prophenoloxidase insertions can be used to detect GAL4 expression in much the same way that UAS-lacZ insertions are used to convert X-gal into blue pigment, but the melanin typically appears faster, the tissue does not need to be fixed before staining, and L-DOPA and dopamine are much cheaper than X-gal. Also, UAS-proPO insertions can be handy for situations where fluorescence microscopy with UAS-GFP insertions is not convenient, such as classroom exercises.
Melanin expression in GAL4 patterns is not seen in live flies, because prophenoloxidase requires proteolysis for activation and is catalytically inactive under normal circumstances. One can bypass the need for proteolysis when staining by adding isopropanol to the incubation buffer.