Several body dimensions were measured in 29 spp. The relationship between pairs of dimensions was determined by allometric analysis. In general, the wings of dragonflies become relatively longer and narrower as body mass increases; fliers have longer but thinner wings than perchers of comparable size. These trends are probably the result of differences in energetic and structural requirements in flight. Thoracic diameter and volume increase more rapidly than expected as thoracic mass increases and are relatively greater in fliers than in perchers. Thoracic dimensions are partly determined by increased selection for insulating, sub-cuticular air sacs in large fliers.