Phenocritical window of BrZ3 action. (A) Temperature step-up (tON) from 18°C to 29°C removed Gal80ts suppression to allow C161-GAL4 > UAS-BrZ3 expression within 2 to 3 h (2.3 to 3.5 h25°C). (B) Temperature step-down (tOFF) from 29°C to 18°C activated Gal80ts, removing BrZ3 expression by 12 h (approximately 6 h25°C). (C) Percent of pharate adult, dbd neurons with ectopic arbor after tON (squares) and tOFF (circles) steps, converted to equivalent hours APF at 25°C. Cumulative percentages included three levels of ectopic arbor severity: comb arbor only (black), any microtubule-containing arbor on the epidermis (gray), and any turn along the posterior muscle edge (white). Best fit variable slope sigmoidal curves. tON: 12 h, n = 10; 24 h, n = 92; 30 h, n = 39; 36 h, n = 116; 40 h, n = 73; 48 h, n = 62; 60 h, n = 26; 88 h, n = 60. tOFF: 29 h, n = 16; 34.5 h, n = 76; 41.4 h, n = 74; 55 h, n = 65; 69 h, n = 29; 83 h, n = 59. (D) Cartoons of the three grades of arbor phenotypes drawn from representative confocal images. The mildest phenotype category included dbds with dendritic arbor bending out of the muscle groove, and extending short, fine branches near the muscle. The intermediate category included longer branches that spread further onto the epidermis and were stabilized with microtubules. Full arbors had microtubule-containing large arbors on the epidermis with secondary branching.