Insect muscles – biological ‘actuators’ in a confined space

Frederik Püffel, PhD
Postdoctoral Research Associate
Imperial College London,UK
Research Interests
Biomechanics, functional morphology, cutting mechanics, biomimetics, social insects, polyethism

Many insects have a quasi-rigid exoskeleton which limits the total space available for internal organs, body fluids and muscles. For musculoskeletal systems with large size-specific demands, this space limit drives the need to arrange muscle ‘optimally’ in order to generate sufficient muscle work, output force and tendon displacement. One of the key challenges arises from the discrepancy between external and internal muscle attachment area: Muscle fibres typically originate from the inner surface of the exoskeleton and insert to an internal cuticular tendon of considerably smaller size. As a consequence, muscle fibres are often arranged in radial fashion, and at a pennation angle to the central tendon. Fibre pennation is a crucial parameter for muscle performance: it determines muscle attachment area, and thus the total number of fibres that can attach to the tendon; it affects the magnitude of muscle force that is transmitted via the tendon; and it enables ‘displacement gearing’, so altering the fibre strain required for a given tendon displacement. A second design parameter, which has received considerably less attention than fibre pennation, is the filament attachment of muscle. In some insect, muscle fibres may attach to the tendon not directly, but via thin cuticular filaments. Filaments reduce fibre rotation per unit strain, and so modulate tendon displacement; they also increase the effective internal attachment area of the tendon, so increasing muscle volume and cross-sectional area without the need of an enlarged tendon. The design space spanned by fibre pennation, filament-attachment, and the external and internal dimensions of the skeletal system, presents an exciting area for comparative work on insects, and may also be informative for comparable robotic systems based on artificial muscle.

Frederik Püffel is a Postdoctoral Research Associate in the Department of Bioengineering at Imperial College London. He passed his PhD viva only two month ago, defending his thesis on the “Biomechanics of the insect bite apparatus”, and now continue working in the Evolutionary Biomechanics Group with Dr. David Labonte. He joined the group in 2018 after completing a degree in biomimetics in Bremen, Germany. Before that, he studied acting in drama school for a year. The longer he does research, the more he recognises the similarities between drama and science. Both require careful observation and an exceptional attention to detail; and they both offer room for creativity and playful exploration. The organism that he explored in his PhD, and continue working on today, forms the arguably most complex non-human societies: leaf-cutter ants. He investigate the physical boundaries of their intricate foraging behaviour by quantifying their biting and cutting abilities.