Homozygotes show partial transformation of haltere to wing and the number of bristles in the first abdominal segment is reduced compared to wild type. 83% of mutant metathoracic legs have a small apical bristle in the anterior compartment, similar to that found in the wild-type mesothoracic leg. The number of big bristles present n the posterior compartment of the metathoracic basitarsus is reduced, indicating a partial transformation into the basitarsus of the wild-type mesothoracic leg (which has no bristles).

Abdominal segments A1 and A2 show transformation towards thoracic identity in mutant embryos; the dorsal transverse 1 muscle is lost and the lateral transverse muscles have a thinner, more thoracic-like morphology than normal. The ventral acute 2 muscle is still present in the mutant A1 and A2 segments.

Dorsal chordotonal organs of T2 and T3 are transformed into T1-like lateral chordotonal organs whose axons project into the more posterior segment. This phenotype is not 100% penetrant. Extra monoscolopoid chordotonal organs appear in the ventral regions of T3 and A1, when all components of dorsal chordotonal organ 3 are present. Ectopic chordotonal organs are rare in other Ubx mutants. In general there is a loss of specific cells, sensory organs or entire cell clusters from PS6 without replacement by elements from PS5 or PS4. PNS elements that disappear from A1 with high frequency in Ubx^MX17 appear with low frequency in the thoracic segments of Ubx^IVa.hs, and vice versa.

Homozygotes and Ubx^MX17/Ubx^MX19 transheterozygotes show a slight but clear transformation of haltere to wing. The transformation appears stronger in trans to Df(3R)P9. Only the distal part of the appendage is affected. There is a partial transformation of the first abdominal segment to thorax as indicated by the reduced number of bristles. There is no detectable embryonic phenotype.

Weak allele.