Mysteries of oxygen and insect size

What sets the maximal size of insects? Results suggest that larger insects invest a greater fraction of their body as respiratory system (tracheal hypermetry). Simple models indicate that tracheal hypermetry prevents increases in PO2 gradients down the tracheae of larger insects. Tracheal hypermetry may lead to spatial constraints within the exoskeleton that provide a maximal limit on insect size. However, this theory must be tested by examination of the world’s largest insects, and examination of the effect of size on tracheal morphology and use of convection. Most insects exhibit developmental plasticity in response to rearing in different oxygen levels, but these plastic effects are likely due to other mechanisms such as direct effects on feeding behavior or downstream effects of oxygen-induced proteins on cell size.