Fig. 3. Distinct patterns of GAL4-EcR and GAL4-USP activation in the central nervous system at the onset of metamorphosis. hs-GAL4-EcR; UAS-nlacZ (GAL4-EcR) or hs-GAL4-USP; UAS-nlacZ (GAL4-USP) third instar larvae were heat treated, allowed to recover for several hours, staged at approx. 2 hours before puparium formation, and processed for histochemical staining. The CNS and attached leg imaginal discs are shown, with anterior to the right. (A) GAL4-EcR activation is prominent in the optic lobes of the brain. (B) GAL4-USP activation is weaker in this region, but strong in the ventral nerve cord. Neither fusion construct is activated in the leg imaginal discs at this stage (arrows).