Convergent evolution and convergent loss in the grasping structures of immature earwigs and aphidlion-like larvae as demonstrated by about 100-million-year-old fossils

Abstract

Convergent evolution is a common phenomenon, independently leading to similar morphologies in different evolutionary lineages. Often similar functional demands drive convergent evolution. One example is the independent evolution of grasping structures in different lineages of Euarthropoda, though the exact morphology of these grasping structures varies significantly. In this study, we investigated grasping apparatuses with two movable counteracting structures as well as some related structures, exemplified by the stylets (compound structures of mouthparts) of aphidlion-like larvae (part of Neuroptera or lacewings) and the cerci of immature earwigs (Dermaptera). For the stylets of aphidlion-like larvae, studies have pointed to a significant loss in morphological diversity in the last 100 million years. We used quantitative morphology to evaluate if a similar process has also occurred in the cerci of earwigs. The cerci of extant immature earwigs exhibit two distinct types of morphologies in the modern fauna: elongated cerci divided into several ringlets with a feeler-type function, and pincer-like stout cerci. In some fossil immature earwigs, however, the cerci are generally elongated but undivided and roughly occupy the morphospace between those of the two modern cerci types; hence this fossil cerci morphology appears to have been lost. To some extent, a comparable loss is also found in certain lacewing larvae. Outgroup comparisons suggest that the morphologies no longer present today are in fact not ancestral, but instead specialised, hence their loss is possibly resulting from disruptive evolution in earwigs as well as lacewings. We discuss the possible functions of these specialised grasping structures.