fly's body has three main divisions: head, thorax, and abdomen.
We'll focus on the thorax, which itself has three main segments. In a
normal adult fly, the second thoracic segment features a pair of wings, while
the third thoracic segment has a pair of small, balloon-shaped structures
called halteres (see arrow in inset). A modified second wing, the haltere
serves as a flight stabilizer. In order for the pair of wings and the pair of
halteres (as well as all other parts of the fly) to develop properly, the
fly's suite of Hox genes must be
expressed in a precise way and at precise times.