Meeting Abstract
Understanding the causes and consequences of evolution of larger or smaller body sizes in a lineage remains one of the important challenges of evolutionary biology. Insects have an unusual (for animals) respiratory system, transporting oxygen in the gas phase via air-filled tracheae. How is the morphology of the tracheal system adjusted as insects vary in 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, ranging in mass from 0.5 to 4 g. Animals were imaged at Argonne National Laboratories using x-ray synchrotron imaging, all with the same resolution (0.65 microns/pixel). Composite images were created by tiling, and we measured tracheal diameters and the fraction of body volume by point-counting using ImageJ. Tracheal diameters generally scaled isometrically, as did the percent of body volume occupied by the tracheal system within the head, thorax and abdomen. However, within the leg, tracheal volume scaled strongly hypermetrically, due to an increase in the number of tracheae with size. The length of meta- and meso-, but not prothoracic legs scaled hypometrically. These data indicate that evolution of larger species size in cockroaches requires specific modification of the tracheal system structure of the legs, perhaps to overcome the challenges of long-distance transport through these long, blind-ended structures. The increased tracheal content of legs in larger roaches will also reduce leg density, likely reducing cost of transport and risk of damage. Supported by NSF IOS 1122157 and IOS 1558052.
