| Why is maximal insect size relatively small compared to that of vertebrates? Perhaps insect size is limited by the tracheal respiratory system's ability to deliver oxygen down longer tracheae. Developing insects provide a model system for studying effects of size. For example, grasshopper mass increases more than 100-fold by adulthood. Even more dramatic is the growth of Manduca sexta, in which mass increases over 7,000-fold during larval stages. Do these size increases correlate with increasing challenges for oxygen delivery? To answer this question, we used flow-through respirometry to investigate how safety margins for oxygen delivery change throughout ontogeny. We exposed insects to decreasing levels of atmospheric oxygen and recorded the point at which metabolic rates dropped. Older, larger grasshoppers had greater safety margins for oxygen delivery compared to younger, smaller grasshoppers and had a marked response to hypoxia that included increased breathing frequencies and tidal volumes. Older, larger caterpillars had safety margins that were identical to younger, smaller caterpillars, suggesting that oxygen delivery capacity matched metabolic need across instars. Thus, neither species showed patterns suggestive of increased problems with oxygen delivery in larger animals.; In addition to changes in size across instars, insects more than double body masses within an instar. For insects with sclerotized cuticles, like grasshoppers, external body dimensions are constrained during this time. Thus, increased tissue mass compresses air sacs, possibly impeding convective gas exchange. Despite increased ventilation frequencies; grasshoppers near the end of an instar had lower safety margins for oxygen delivery, partially due to decreased tidal volumes. Caterpillars nearing the end of an instar also had smaller safety margins, suggesting that insects generally may have a limited ability to increase oxygen delivery capacity within instars.; To determine whether these patterns were due to development or body size, we conducted similar tests for adult grasshoppers over a range of body masses. We found that regardless of size, all grasshoppers were able to breath at very low levels of oxygen (<5 kilopascals). Larger grasshoppers appeared to compensate for their larger size with higher ventilation frequencies relative to smaller species when phylogeny was controlled. |
