Decreasing tracheal system volume during intermolt development in grasshoppers

Abstract

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 hypothesized 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 0.73g to 1.3g (78% increase) while femur length remained constant at 19.3mm. Using a Bruker SkyScan1272 micro‐computed tomography (uCT) system, we scanned early and late sixth instars (n=5). 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 53% during the instar, tracheal volume decreased by 145%. Early instar tracheal volume was 0.31 mL, while late instar tracheal volume was 0.05 mL. Therefore, the relative proportion of tracheal volume to body volume ratio decreases from 44% to 5.2% during the instar. Our finding that tracheal volume decreases during the intermolt period supports previous work showing that late stage grasshoppers rely more on anaerobic metabolism during repeated jumping.