A normal adult fruit
fly, enlarged 40 times. To the right the fly's
ill-fated cousin, a mutant with 4 wings but no
balance organs. This now famous little "monster"
was a starting point for Lewis
in his research on homeotic
found that the extra pair of wings was due to a
duplication of an entire body segment, the 2nd
thoracic segment. Inactivity of the first gene of
the bithorax-complex in the appropriate
larval segment caused other homeotic genes to
respecify the 3rd thoracic segment into one that
forms wings instead of halteres.