One major reason for the evolutionary success of insects is their unique tracheal respiratory system. However, during development when oxygen demands are high, the tracheal system may not be able to deliver adequate oxygen to the tissues. For example, in developing American locusts (Schistocerca americana), body mass can double during the intermolt period. Also during the intermolt period, jumping performance decreases while jumping muscle lactate production increases, suggesting increased oxygen delivery problems at the end of the instar. We hypothesize that decreased oxygen delivery during the instar is due to increasing body mass within the rigid exoskeleton compressing the air-filled tracheal system. To test this hypothesis, we identified early and late female sixth instar S. americana. Similar to previous work, body mass increased during the instar from 0.71g to 1.38g (94.3% increase) while femur length remained constant at 19.1mm. Using a Bruker SkyScan1272 micro-computed tomography (uCT) system, we scanned early and late sixth instars (n=8). We also scanned sixth instars within the intermolt period on day 4 and day 6 (n=3). uCT image (TIF) files were reconstructed using NRecon and the tracheal system was segmented using Amira 3D and Biomedisa. Total body and tracheal volumes were analyzed using Amira 3D. Although total body volume increased by 94% from the beginning to end of the instar, tracheal volume decreased by 80%. Early instar tracheal volume was 0.30mL, while late instar tracheal volume was 0.05mL. Therefore, the relative proportion of tracheal volume to body volume ratio decreases from 45% to 4.5% during the instar.
This relative proportion of tracheal volume to total body volume does not decrease linearly, however. Within the first four days of the instar, total body volume increased 24%, while tracheal volume decreased 34% from 0.30mL to 0.20mL. Between day four and day six of the intermolt period, tracheal volume decreases an additional 50% from 0.20mL to 0.09mL. Lastly, in the final stages of intermolt development, tracheal volumes decrease 35%. This suggests that total tracheal volumes of grasshoppers during intermolt development do not decrease incrementally. The relative proportion of tracheal volume to total body volume decreases 46% in the first four days, followed by a 63% decrease between day four and day six, and a final 51% decrease between day six and day eight of the instar. Our findings that tracheal volumes decrease during the intermolt period support previous work showing that late stage grasshoppers rely more on anaerobic metabolism during repeated jumping.