|Index to this page| |See Embryonic Development: Getting
Started| The insect body plan consists of head, thorax, and
abdomen. The thorax is built from three segments, T1, T2, and
T3. Each carries a pair of legs; hence insects are six-legged
creatures. In most of the insect orders, T2 and T3 each carry a
pair of wings (the honeybee is an example). However, flies
belong to the insect order diptera; they have only a single pair
of wings (on T2). The third thoracic segment, T3, carries
instead a pair of balancing organs called halteres. In
Drosophila, a gene called Ultrabithorax (Ubx) acts within the
cells of T3 to suppress the formation of wings. By creating a
double mutation in the Ultrabithorax gene (in its introns, as it
turned out), Professor E. B. Lewis of Caltech was able to
produce flies in which the halteres had been replaced by a
second pair of wings. Ultrabithorax (Ubx) is an example of a
"selector gene". Selector genes are genes that regulate (turning
on or off) the expression of other genes. Thus sel