Apical bulkheads are under tension. (a) Left: Still images from a BC labeled with SiRactin the frame before (−1f), the frame directly after (+1f), and 40 frames after (+40f) ablation of the apical membrane (see also Video 3). Ablation line is indicated in red, kymograph line is indicated in yellow. Scale bar: 5 µm. Right: Kymograph revealing BC deflation following ablation of the apical membrane. Red arrowhead indicates moment of laser ablation. (b) Schematic representation of the laser ablation of individual apical bulkheads. Expected ablation volume in red. (c) Left: Still images from a BC labeled with SiRactin the frame before (−1f), the frame directly after (+1f), and 40 frames after (+40f) ablation of an individual apical bulkhead (see also Video 4). Ablation line is indicated in red, kymograph line is indicated in yellow. Scale bar: 5 µm. Right: Kymograph revealing an immediate local expansion of the BC following ablation of the apical bulkhead. Red arrowhead indicates moment of laser ablation. (d) Left: Still image from a BC labeled with SiRactin before control ablation next to an apical bulkhead and laser ablation of an apical bulkhead. Scale bar: 5 µm. Right: Kymographs revealing that only ablations of apical bulkheads, but not control ablations, causes an immediate local BC expansion. Red and blue arrowheads indicate the moment of laser ablation. (e) Quantification of local BC expansion upon laser ablation of apical bulkheads (n = 38) and control laser ablations (n = 21), obtained in three independent experiments. Data are color-coded by biological replicate, where dots represent individual ablations, triangles represent the mean of each biological replicate, and the horizontal bars represent the average of the means ± SEM. **, P < 0.01 using an unpaired Student’s two-tailed two-sample t test of the means of each biological replicate.