The haltere, a club-shaped balancing structure supported by the thorax muscles, is one of the fruit fly's many limbs. Fruit flies typically have a single pair of these appendages. The digital image featured below was recorded on a confocal laser scanning microscope using a fluorescently labeled specimen.

Fruit Fly Imaginal Disc: Haltere Specimen: Drosophila Imaginal Disc Technique: Fluorescence (Triple Label)

The fruit fly has a single pair of wings, but crawling, hopping, and climbing also serve as primary forms of mobility. Abnormal limb formation in fruit flies, due to induced and natural mutations, has been the subject of a substantial body of research in genetics and biology. In particular, the formation of legs that protrude from the head in place of antennae is a perplexing phenomenon that scientists found to be a result of a defect in the Antennapedia gene. Investigations of this mutation is led to the discovery of the hox genes, a group of genes responsible for regulating body formation in vertebrates.