development, the fly's two wings grow from a structure in the larva known
as the wing imaginal disk (top images). (An imago is an insect in its final,
adult state.) The haltere grows from the larval haltere imaginal disk (bottom
images). Remember the UbxHox gene? Using staining again, we can detect the gene product
of Ubx. This reveals that the Ubx gene is naturally "off" in the wing
disk—note the absence of the bright green stain in the upper right
image—and is "on" in the haltere disk (lower right image).
Now you'll see what happens when the Ubx gene—just one of a large number of Hox genes—is turned off in the haltere disk.