We examined specific leg structures in eastern cicada killers (Sphecius speciosus) to determine whether they were sexually dimorphic as a result of adaptation or allometry. Females were on average larger than males by every measure, and female hind tibial spurs were longer than those of males, even independent of body mass. The femora of females were not significantly wider relative to femur length than those of males, but the hind tibiae of females were significantly wider than the respective tibiae of males relative to tibia length. Spurs have concave surfaces that contact the loose soil and brush-like structures where spurs meet the substrate; however, these details are essentially the same in males and females. Enlarged hind tibial spurs in females appear to be an exaptation evolved primarily by increase in spur size relative to body size, without changes in shape or external microstructure.