We examine the functional morphology of the mandibular apparatus, including its driving muscles, of the generalist insect Periplaneta americana using a combination of μ-computed tomography and geometrical modelling. Geometrical modelling was used to determine the changes of the mean fibre angle and length in the mandibular adductor muscle over the physiological range of mandible opening. The roughly scissor-like mandibles are aligned along the dorso-ventral axis of the head and are characterised by sharp interdigitating distal teeth, as well as a small proximal molar region. The mechanical advantage of the mandibles, i.e. the ratio between inner and outer levers, ranges between 0.37 to 0.47 depending on the considered incisivus. The mandibular abductor muscle is comprised of eight muscle fibre bundles, which are defined by distinct attachment positions on the sail-like apodeme protruding from the medio-lateral basis of the mandibles into the head lumen. Compared to carnivorous, herbivorous, or xylophagous insects, the relative volumes of the mandibular abductor and adductor muscle are small. Dependent on the mandible opening angle, the mean fibre angle of the adductor muscle ranges from 34° to 21°, while mean fibre length changes from 1.24 mm (closed mandible) to 1.93 mm at maximum mandible opening. Many of the specific morphological features found in the chewing apparatus of P. americana, such as the presence of a mola in combination with distal incisivi, small relative muscle size and the intermediate fibre angle can be understood as adaptations to its omnivorous life style.

Comparative morphology, insect, head, mouth parts, skeleton, muscles, mandibles, biting, chewing