Transformation of posterior portion of haltere towards wing, loss of A1 tergite and appearance of T3 legs on A1. Ventral setal belt of A1 is transformed toward that of T3 and ventral pits appear on abdominal segments A1 to A7, inclusive.

Hemizygotes display transformation of posterior haltere towards wing, loss of first abdominal tergite and appearance of one or two legs from the first abdominal segment. In late embryos and larvae ventral setal belt of the first abdominal segment is transformed toward that of the third thoracic segment. Ventral pits and Keilin organs appear on abdominal segments A1 to A7, inclusive.

Ubx^bxd-100/Ubx^bxd-125 flies have a number of abnormal transversal fascicles in the CNS; the posterior internal T3, posterior ventral T3, anterior dorsal A1, anterior internal A1, anterior ventral A1, posterior dorsal A1 and occasionally the posterior dorsal T3 fascicle are affected.