Linking shape conspicuous asymmetry with shape covariation patterns and performance in the insect head and mandibles

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.