Fig. 5.

Model depicting how changes in br expression follow the evolution of metamorphosis in insects. Each image is a cross section of the second thoracic segment. Gray arrowheads denote wing pads in nymphs, and black arrowheads indicate wing imaginal discs. The time of hatching is indicated by a vertical dashed line. In hemimetabolous insects, br (horizontal bar) is present throughout the nymphal stages. This expression correlates with a bout of differential growth that occurs late in embryogenesis, as well as the progressive differential growth of the wings that occurs during the nymphal stages. In holometabolous insects, the latter half of embryonic development is largely isometric, and this isometric growth phase persists until metamorphosis, when br permits differential growth of imaginal structures.