Temperature gradients in the flight muscles of Manduca sexta imply a spatial gradient in muscle force and energy output.

There is a significant dorso-ventral temperature gradient in the dominant flight muscles [dorsolongitudinal muscles (DLM(1))] of the hawkmoth Manduca sexta during tethered flight. The mean temperature difference was 5.6°C (range=3.8-6.9°C) between the warmer, ventral-most subunits and the cooler, dorsal-most subunits. As force generation in muscle depends on temperature, the mechanical energy output of more dorsal subunits will differ from that of deeper and warmer muscle subunits. To test this hypothesis, we isolated the dorsal subunits and the ventral subunits and recorded both single and 25 Hz (wingbeat frequency) isometric contractions at a range of temperatures. Our data show that the contractile dynamics of the various regions of the DLM(1) are similarly affected by temperature, with higher temperatures leading to reduced contraction times. Furthermore, using standard electromyography, we showed that the different regions are activated nearly simultaneously (mean time difference=0.22 ms). These observations suggest that the existence of a temperature gradient will necessarily produce a mechanical energy gradient in the DLM(1) in M. sexta.