Grasshoppers eat plant leaves by biting off pieces small enough to ingest. Chewing leaves involves complex physical properties of both the grasshopper mouthparts and the leaf.
When a grasshopper or a person chews off a piece of leaf, the leaf must fracture. Leaves can resist fracture by elastic “bend but don’t break” properties in response to force. Efficient fracture requires the insect to apply force while limiting the ability of the leaf to deform.
The mandibles of leaf feeding grasshoppers typically have a row of sharp pointed “teeth”. The teeth apply force to a narrow area of the leaf. A leaf portion between two adjacent teeth is a short distance, limiting the distance it can stretch. This facilitates fracture of the leaf. The mandibles close on both sides of a leaf simultaneously holding the leaf so it cannot bend to dissipate the force. The labrum of the grasshopper contains a notch where a leaf can be held, further stiffening the leaf and limiting its deformation.
To successfully detach a leaf piece from a leaf, the initial fracture must propagate. The resistance to propagation is known as leaf toughness. Leaf toughness is analogous to a “tough” piece of meat that resists chewing into small pieces. Toughness varies greatly among leaves and is an important component of food acceptance in insects.
Grasshoppers typically feed perpendicular to the leaf edge. With one mandible on each side of the leaf. The teeth of grasshopper mandibles are wedge structures. During the bite, the fracture is propagated along a straight line. The mandibles bite such that they separate a narrow strip of leaf along the edge from the bulk of the leaf. At the bottom, the mandibles extend to the edge. Each bite of the mandibles cuts two sides of a rectangle, the leaf edge being the other two sides. The width and length of the detached leaf piece is the maximum size suitable for ingestion. Like a kitchen utensil specialized for a single purpose, grasshopper mandibles are specialized for fracturing leaves and cutting ingestible pieces.