Understanding the causes and consequences of evolution of larger or smaller body sizes in a lineage remains an important challenge in evolutionary biology. We studied how the morphology of the tracheal system varies with insect size. Based on comparisons with vertebrates, we expect either isometric scaling (as found for lungs) or hypometric scaling (as found for vertebrate capillaries), with the latter matching the hypometric scaling of metabolic rate. We tested these expectations using one-to-three individuals of ten species of cockroach, and ten species of scarab beetles; species varied by more than an order of magnitude in body mass and included some of the largest extant species. Cockroaches were imaged at Argonne National Laboratories using x-ray synchrotron imaging, while scarab beetles were imaged at Virginia Tech by microCT. We measured the fraction of body volume occupied by the tracheal system for the major body compartments. In general, the tracheal system scaled isometrically in the head, thorax and abdomen. However, within the legs, tracheal volume scaled strongly hypermetrically, due to an increase in the number (roaches) and size (scarabs) of tracheae and/or air sacs with size. These data strongly suggest that evolution of larger species of insects requires increased relative investment in the tracheal system of the legs to overcome the challenges of long-distance transport through these long, blind-ended structures, limiting the maximal size of insects. Supported by NSF IOS 1122157 and IOS 1558052.