TRIMBLE, A.C.: Musculoskeletal Control of Flight in the Hawkmoth Manduca sexta: a new model coupling muscle forces to wing motion.

Insect flight arises from complex interactions of the nervous system, muscles, exoskeleton and aerodynamics. Physical coupling of the power muscles to the wings through the thoracic exoskeleton forms the core of the flight apparatus. The original model dates back to Snodgrass (1927), and forms the basis of our current understanding of the transduction of muscle forces to wing motions in indirect flight mechanisms. While the fundamental roles of the muscles remain undisputed, the physical mechanism of tergal displacement that couples muscle forces to wing motion has been debated for over 50 years. I used finite element analysis on a three-dimensional mathematical model of the thorax from the hawkmoth Manduca sexta to explore the mechanisms that physically couple contraction of the dorso-longitudinal muscles to depression of the wings. The model is based on cryosections of the thorax of an adult male Manduca. Analyses of this complex model and simplified versions show that: (i) the muscle forces are transmitted along the margin of the tergum, not through vertical displacement of the dorsal surface, and (ii) muscle contraction produces dorso-lateral displacement of the tergal rim at the wing hinge. These results contradict previous flight mechanism models that suggest vertical tergal displacement as the coupling mechanism between the dorso-longitudinal muscles and the wing hinge.