Bilateral symmetry is widespread across animals, yet, among bilaterians, many cases of conspicuous asymmetries evolved. This means that bilaterally homologous structures on the left and right sides display divergent phenotypes. The evolution of such divergent phenotypes between otherwise similarly shaped structures can be thought to be favoured by modularity, but this has rarely been studied in the context of left-right differences. Here, we provide an empirical example, using geometric morphometrics, to assess patterns of asymmetry and covariation between landmark partitions in a grasshopper with conspicuously asymmetric mandibles. Our morphometric data confirms the presence of strictly directional conspicuous asymmetry in the mandibles and surrounding structures. Covariance patterns and tests hint at a strong integration between mandibles despite their divergent morphologies, and variational modularity with the head capsule. While mandibles have been selected to achieve a key-and-lock morphology by having interlocking shapes, the developmental modularity required to achieve this seems to be overwritten by developmental and/or functional integration, allowing the precise matching required for feeding. The consequent conflicting covariation patterns are reminiscent of the palimpsest model. Finally, the degree of directional asymmetry appears to be under selection, although we find no relationship between bite force and mandible shape or asymmetry.

The dataset consists of raw image stacks of CT scans of several Schistocerca gregaria (grasshoppers) heads, and the landmarks digitized on these heads. The animals were purchased from a commercial pet shop (Fressnapf) in Bonn. CT scans were acquired on Bouin solution fixed heads with mandibles fully closed, using a Bruker SkyScan 1272 (Voltage = 50 kV, current = 200 µA, image pixel size = 6.0 µm or 7.5 µm) and reconstructed using NRecon (Bruker). Landmarks were digitized in the software Morphodig v. 1.6.7 (Lebrun, 2018), and replicated once for all specimens.