Ants use their mandibles for catching prey, cracking seeds, cutting leaves, or for the construction of nests and the tender care of brood. The functional morphology of the mandibles reflect the species' adaptations to particular foraging habits and social life. The versatility and specialization of the mandibles depend directly on the design and physiology of the mandible closer muscles and their component fibers. A comparative video analysis of the closing movements of ant mandibles revealed that the maximal velocity varies considerably among species. The speed is correlated with the morphology of the mandible closer muscle, the largest muscle in ants. It is composed of two morphologically very distinct fiber types: long fibers with short sarcomeres (sarcomere length approximately 2 microM) showing all the structural attributes of fast muscle fibers, and shorter fibers with longer sarcomeres (sarcomere length approximately 5 microM) exhibiting the characteristics of slow and powerful fibers. Ants with fast-moving mandibles have a very high proportion of fast closer fibers, whereas the muscles of ants that cannot perform fast mandible movements have only a few or no fast fibers at all. Fast fibers always attach directly to the solid apodeme, while slow fibers often attach to thin apodeme threads. We suppose that the latter kind of fiber attachment is disadvantageous for fast contracting fibers but helps the ants to make better use of the space in the head capsule.

No abstract

No abstract

To be able to perform swift and powerful movements, ant mandible closer muscles are composed of two subpopulations of muscle fibres: fast fibres for rapid actions and slow fibres for forceful biting. All these fibres attach to a sturdy and complex apodeme which conveys force into the mandible base. Fast muscle fibres attach directly to the apodeme. Slow fibres may attach directly or insert at individual thin filament processes of the apodeme which vary in length. Comparisons of different ant species suggest two basic principles underlying the design of mandible muscles. (1) Ants specialized for fast mandible movements generally feature long heads which contain long fast muscle fibres that attach to the apodeme at small angles. Their muscles comprise only a few filament-attached fibres and they maximize speed of action at the expense of force output. (2) Ants performing particularly forceful mandible movements, such as seed cracking, rely on many short parallel muscle fibres contained within a broad head capsule. Their slower muscles incorporate a large proportion of filament-attached fibres. Two simple models explain how the attachment angles are optimized with respect to force and velocity output and how filament-attached fibres help to generate the largest power output from the available head capsule volume.

No abstract