Effects of Temperature on Dragonfly Nymph Prey-Strike Performance
During the aquatic nymphal stage, dragonflies possess a modified labium that functions as a high-speed raptorial appendage used in prey capture. During labial protraction, a preparatory phase of slow protraction is followed by the release of a latched knob-like structure in the elbow region of the labium, which results in a fast strike phase when the appendage rapidly accelerates. The strike phase appears to be elastically powered, where relatively slow muscle contraction before and during the preparatory phase stores energy in associated elastic elements, and this energy is recovered via rapid recoil of the elastic structures once the latch disengages to produce a ballistic high-speed movement. I hypothesized that the performance (e.g., velocity, acceleration, power) of the muscle powered preparatory phase should be thermally dependent, whereas the performance of the prospectively elastic strike phase should be thermally independent. To test this hypothesis, individual nymphs were recorded (two cameras at 1000 Hz) capturing prey (California blackworm) across a temperature range of 15–35°C. I found significant temperature effects on the duration, distance, and average velocity of labial protraction during the preparatory phase. This result was expected, given that the shortening velocity of ectotherm muscle is known to be temperature dependent. In contrast, all kinematic variables of the strike phase maintained peak performance across all temperatures. The previously documented energy storage and latch characteristics involved in this prey-capture mechanism, coupled with the high observed performance and thermal robustness presented in this study, provide strong evidence that dragonfly nymph labial protraction is an elastically powered movement like those observed in some other vertebrate and invertebrate systems.