Hyper‐development of the tracheal system in larger insects

Abstract

Why are insects small? One hypothesis is that possession of a tracheal respiratory system precludes large sizes due to distance effects on diffusion. In this study, we used phase-contrast synchrotron-x rays to image the tracheal system of Schistocerca americana grasshoppers varying in mass by 100x. Images of grasshoppers of various ages clearly showed that most first instars completely lacked air sacs; a few contained small air sacs in the head or within the thorax near the legs. Adults had more tracheae of much larger diameters, and an extensive system of air sacs. We used a point-counting method to quantify the proportion of the image devoted to tracheae or air sacs. The proportion of tracheae and air sacs in the image each increased strongly and linearly with size, indicating a progressive increase in investment in the tracheal system. Measurements of the proportional change in air sac diameters indicated that 2nd instar grasshoppers compressed their air sacs diameters by only 20%, while 4th instar and older grasshoppers compressed their air sac diameters by an average of 80%, showing that the increased investment in air sacs leads to a increase in convective gas exchange in these grasshoppers. Increasing investment in tracheal structures must either displace other tissues or require increased cuticular growth, potentially providing an ultimate limit on insect size. Supported by NSF IOB 0419704 to JFH.