Biomechanics of the insect bite apparatus

Abstract

In this thesis, I investigate the morphology and biomechanics of the insect bite apparatus using polymorphous leaf-cutter ants as study organism. I first quantify the key morphological determinants of bite force and discuss the spatial constraints of muscle geometry imposed by the insect exoskeleton. Across the substantial size-range of leaf-cutter ant workers, the bite force capacity increases with a strong positive allometry, driven by a disproportional increase in the volume of the mandible closer muscle combined with a muscle fibre arrangement that enables high volume-specific muscle forces. Second, I derive and validate a biomechanical model that links the geometry of the musculoskeletal bite apparatus to changes of the mandibular opening angle. I then deploy this model to extract the force-length properties of the mandible closer muscle via in-vivo bite force measurements. Third, based on these results and the previously extracted morphological parameters, I predict the scaling relationship between maximum bite force and body mass, and compare this prediction to direct force measurements. Leaf-cutter ants generate among the highest weight-specific bite forces ever reported for any animal, displaying an exceptional level of morphological and physiological adaptation to the high mechanical demands of herbivory. Fourth, I turn my attention to ontogenetic changes of the bite apparatus, from freshly-eclosed adults to fully-matured foragers. In the week following eclosion, the bite apparatus undergoes a considerable biomechanical development, driven by substantial muscle growth and increasing head capsule rigidity. As a result, young leaf-cutter ants are likely incapable of engaging in any colony tasks involving leaf-cutting. Fifth, although fully-matured foragers can generate much larger bite forces than callows, their mandibles require significantly higher forces to cut the same material. These mandibular cutting forces are largely size-independent, and for callows, they approach a theoretical minimum for the tested material, suggesting geometric adaptations for high mandible ‘sharpness’. The results of this work add important aspects to the discussion on age- and size-related foraging behaviour in social insects, and more generally, underline the importance of biomechanics in insect herbivory.