Changes in Grasshopper Jumping Muscle During Intermolt Development

Abstract

During the sixth instar, the American locust (Schistocerca americana) doubles in mass, but its body size remains relatively constant throughout the instar due to its rigid exoskeleton. Previous work has shown that tracheal air sac size is reduced at the end of the intermolt period. For late stage grasshoppers, this change in the tracheal function correlates with reduced jump performance and their inability to maintain resting metabolic rates in hypoxia. We hypothesized that if late stage grasshoppers had a potential reduction in oxygen delivery, their jumping muscle would rely more on anaerobic metabolism. Early sixth instar (day 2) and late sixth instar (day 8) female grasshoppers were randomly assigned into five jumping groups (0, 30, 60, 120, or 300 seconds; n=10). During repeated jumping, the number of jumps was recorded for each animal. Immediately after jumping, grasshoppers were frozen in liquid nitrogen. A fluorometric assay was developed to measure lactate in the jumping muscle. After 30 seconds of jumping, femoral muscle lactate levels increased 12‐fold in the day 2 animals and 23‐fold in the day 8 animals. Regardless of time spent jumping, day 8 grasshopper jumping muscle had 20‐25% more lactate than day 2 jumping muscles. We also measured lactate dehydrogenase (LDH) and citrate synthase (CS) activity levels in separate day 2 and day 8 sixth instar female grasshoppers (n=12). While jumping muscle CS activity did not vary during the instar, day 8 grasshoppers had a 28% increase in jumping muscle LDH activity. Therefore, we conclude that decreases in the tracheal volume during the intermolt period correlate with an increased reliance on anaerobic metabolism in the jumping muscle, which leads to greater lactate production and higher rates of fatigue during repeated jumping.