Meeting Abstract
One hypothesis for why insects are smaller than vertebrates is that possessing a blind-ended tracheal respiratory system results in physiological challenges in oxygen delivery for larger insects. If this is the case, we might expect to see that larger insects have relatively larger gas transport structures. To test this possibility, we performed the first inter-specific study of the scaling of spiracle size, using scarab beetles, the insect clade with the most massive species. We took full body micro-CT scans of 17 individuals of 10 species. We measured cross sectional area of the spiracles’ opening, as well as the depth from the opening to the valve structure behind the spiracles’ atrium, and assessed the scaling of spiracular area, diffusive capacity (area/depth), and advective capacity (area2/depth). Data were log-transformed and slopes corrected for phylogenetic relationships among the species. Spiracles in the anterior portion of the animal, especially the large mesothoracic spiracle, showed hypermetric scaling, indicating that larger beetles have spiracles with a relatively higher diffusive and advective capacity (less resistance) compared to smaller ones, while the small posterior abdominal spiracles showed isometric scaling. For the metathoracic and first two abdominal spiracles, the pattern differed among subfamilies, with hypermetric scaling in Cetoniinae as compared to isometry in Dynastinae. These findings demonstrate that for the largest spiracles responsible for most gas exchange, some larger beetles have relatively larger spiracles, supporting a growing body of evidence that, unlike vertebrates, larger insects may selectively increase the relative size of specific respiratory structures to support their greater gas exchange needs.
