Intermolt development reduces oxygen delivery and jump performance in grasshoppers

Insects are the most evolutionarily successful animals in part due to their efficient air-filled tracheal respiratory system. However, during development when oxygen demands are high, the tracheal system may not be able to deliver adequate oxygen. During the intermolt period in Schistocerca americana grasshoppers, body mass doubles. This increased body mass may compress tracheae and reduce jump performance. To test this hypothesis, we examined tracheal volumes, jump performance, and lactic acid production at the beginning and end of 6^th instar. First, using a Bruker SkyScan1272 micro-computed tomography system, we found tracheal volume decreased by 82%,  while total body volume increased 97%. Therefore, the relative proportion of tracheal to body volume decreased from 44.5% to 4.27% during the instar (p< 0.0001). Second, we forced early-stage and late-stage grasshoppers to jump in oxygen atmospheres (5%, 10%, 21%, 40% O₂: balance N₂). After jumping, grasshoppers were immediately frozen in liquid nitrogen for a fluorometric lactate assay.  After two minutes of jumping, late-stage grasshoppers had a 1.6-fold reduced jump rate in extreme hypoxia (5% O₂; p< 0.03), but showed a 1.5-fold increase in jump rate in hyperoxia (40% O₂; p< 0.02). No differences were seen in moderate hypoxia (10% O₂) or normoxia (21% O₂). Finally, late-stage instar grasshoppers produced significantly greater jumping muscle lactate regardless of oxygen level (p< 0.0001).  The reduced tracheal volumes of late-stage grasshoppers impair jump performance in hypoxic conditions and also result in significantly greater lactate production during jumping regardless of oxygen treatment. Funded by NSF award 1531850 to SDK.