KIRKTON, S.D.*; HARRISON, J.F.; Univ. of California, San Diego; Arizona St. Univ.: Within instar development reduces locomotory performance but not oxygen delivery to the jumping muscle in the American locust

During development within an instar, juvenile grasshoppers nearly double their body mass; however, major tracheae and spiracles only increase in size at the molt, and the increased body mass compresses air sacs. As a consequence, critical PO₂�s for metabolic rate are higher for resting grasshoppers measured later in an instar. O₂ delivery problems for older grasshoppers may be especially important during periods of increased aerobic demand, such as locomotion. In this study, we investigated how age within an instar affected jumping performance and O₂ delivery in Schistocerca americana grasshoppers. Within an instar, the growth of the jumping muscle does not match the increases in body mass. Grasshoppers measured late in an instar jumped less frequently and had reduced muscle-specific power outputs as compared to those measured early in an instar. Late instar grasshoppers also produced more lactic acid during repeated jumping than younger ones. Animals measured early in an instar had significantly higher whole body mass-specific O₂ consumption at rest and during jumping than older grasshoppers; however, the jumping muscle-specific O₂ consumption rates were similar. Jumping muscle-specific O₂ delivery was maintained by increasing femoral width during the instar, allowing femoral air volume to remain constant during an instar, even though the whole body air volume decreases strongly. Our data suggests that despite the stable muscle-specific O₂ consumption, aerobic ATP production by the jumping muscle does not match the power needs of late instar grasshoppers. Therefore, older grasshoppers rely more on anaerobic ATP production, which may contribute to fatigue and reduced locomotory performance.