KIRKTON, S.D.*; HARRISON, J.F.: Effects of Allometric Growth on Oxygen Sensitivity and Jumping Performance in the American Locust
Body size changes within instars and across the lives of insects could have significant respiratory and locomotory consequences because of possible problems with oxygen delivery in larger insects. We investigated the effect of across and within instar growth on performance during repeated jumping in the American locust, Schistocerca americana. Larger (older) grasshoppers fatigued more rapidly than smaller (younger) instars, both when comparisons were made across and within instars. We hypothesized that the poorer endurance of larger grasshoppers was due to reduced oxygen delivery relative to tissue metabolic needs. Grasshoppers ranging from 2nd instars (0.08 g) to adults (2 g) were forced to jump up to 20 minutes in oxygen atmospheres ranging from 5 kPa O2 to 60 kPa O2. Hyperoxia had no significant effect on the endurance time of any age grasshopper. As juveniles increased in size/age endurance time became increasingly sensitive to hypoxia; specifically endurance time in 12 kPa O2 atmospheres relative to that in 21 kPa O2 decreased with size. This result suggests that larger juveniles have less tracheal oxygen delivery capacity (or safety margin) relative to leg muscle oxygen needs. Adult endurance time was unaffected by hypoxia, suggesting that leg muscle activity metabolism in adults is nearly completely anaerobic. Body size changes within an instar also affected repeated jumping performance. Within the 6th instar, grasshoppers that were four days older have 40% lower endurance times. In contrast to the general trend toward reduced endurance in larger (older) animals, adults that were 28 days old had similar endurance times when compared to 50% lighter 2 day old adults, suggesting that adult legs may undergo a compensatory mechanism to improve jump performance.
