Semiterrestrial and terrestrial crabs have evolved multiple strategies for aerial respiration. An uncommon strategy for aerial respiration is seen in porcelain crabs, genus Petrolisthes, where decalcified areas on the meral segments of the walking legs are used as respiratory structures. Here, the evolutionary history and adaptive significance of these structures in porcelain crabs is examined. Interspecific variation in leg membrane size is from 0% to 60% of the surface area of the meral segment. Leg membrane relative size is positively correlated with body size across species but not within one species, Petrolisthes cinctipes. Phylogenetic analyses suggest that leg membranes are ancestral to one of two eastern Pacific Petrolisthes clades. Comparative analyses using phylogenetic independent contrasts indicate a relationship between leg membrane relative size and body size that is phylogenetically independent. In large‐bodied intertidal species, whole‐animal lactate accumulation during aerial incubation is 200%–300% higher when the leg membranes are obscured, indicating that the leg membranes are functional respiratory structures in these species. Thus, it is possible that leg membranes have facilitated the evolution of larger body sizes by providing additional respiratory surfaces to accommodate the associated higher metabolic demands.