2 SummaryThe spectrin cytoskeleton is required for development of the Drosophila neuromuscular junction (NMJ) but its role is unclear. Here we show that the muscle spectrin lattice functions to corral membrane-associated synaptic proteins and limit their lateral mobility. Drosophila adducin, Hts, is required for integrity of the spectrin cytoskeleton and disruption of Hts function results in failure of the corrals. The spectrin cytoskeleton is itself patterned at the muscle membrane by the engulfment receptor Draper (Drpr) through regulation of Hts. We find patches of membrane where the spectrin cytoskeleton is organized into bilaterally symmetric patterns, which coincide with a field of Drprdependent structures similar to phagocytic pseudopods. The bilaterally symmetric patterns are likely created by folds of the muscle membrane in the pseudopods. We present evidence that the folds trap nascent boutons of motor neurons, leading to boutons with a bilaterally symmetric organization of the postsynaptic membrane. Drpr thus acts as a sensor of synaptic damage that promotes synaptogenesis.