Meeting Abstract

54.6  Tuesday, Jan. 6  The Scaling of Critical P_O2 in Coleoptera LEASE, HM*; KLOK, CJ; KAISER, A; HARRISON, JF; University of New Mexico, Willamette University; Arizona State University; Midwestern University; Arizona State University hlease@unm.edu

Because diffusion rates are highly dependent on distance, and because tracheal length increases with size, gas exchange has traditionally been thought to be more difficult for larger insects. However, intraspecific studies in grasshoppers and caterpillars, and interspecific studies in grasshoppers suggest that critical P_O2 values do not increase with body size in insects. The lack of a positive correlation between body size and critical P_O2 may occur because larger insects have enhanced respiratory capacity (Greenlee and Harrison 2004), partially attributable to increased relative investment in the tracheal system as body size increases. For example, intraspecific and interspecific tracheal volume have been shown to scale with mass^1.3 (Lease et al. 2006, Kaiser et al. 2007) for some insect groups. However, as yet the effect of body size on critical P_O2 has not been measured for the single group of insects (beetles) for which there is interspecific data on tracheal scaling. In this study, we address this deficiency by measuring the critical P_O2 for CO₂ emission rates across 4 orders of magnitude of body size (1 mg to 4 g) of two families of Coleoptera (Tenebrionidae and Scarabaeidae). We exposed adult beetles to progressively lower oxygen levels, and measured their ability to maintain CO₂ emission rates. As predicted, absolute metabolic rates increased with beetle body size at both normoxic and hypoxic conditions; and as oxygen levels decreased, mean CO₂ output for all species decreased. Critical P_O2, however, was independent of size. These data suggest that tracheal conductance increases as insects get larger, enabling similar oxygen delivery safety margins for large and small beetles. These data support the hypothesis that larger insects achieve these similar oxygen delivery safety margins by increasing relative investment in the tracheal system.
This research was partially supported by NSF IBN 0419704 to JFH.