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Brachyura and Anomala (or Anomura), also referred to as true and false crabs, form the species-rich and globally abundant group of Meiura, an ingroup of Decapoda. The evolutionary success of both groups is sometimes attributed to the process of carcinization (evolving a crab-like body), but might also be connected to the megalopa, a specific transitional larval phase. We investigate these questions, using outline analysis of the shields (carapaces) of more than 1500 meiuran crabs. We compare the morphological diversity of different developmental phases of major ingroups of true and false crabs. We find that morphological diversity of adults is larger in false crabs than in true crabs, indicating that taxonomic diversity and morphological diversity are not necessarily linked. The increasing morphological disparity of adults of true and false crabs with increasing phylogenetic distance furthermore indicates diverging evolution of the shield morphology of adult representatives of Meiura. Larvae of true crabs also show larger diversity than their adult counterparts, highlighting the importance of larvae for biodiversity studies. The megalopa phase of Meiura appears to be plesiomorphic, as it overlaps between true and false crabs and shows little diversity. Causes may be common evolutionary constraints on a developmental phase specialized for transitioning., (© 2024. The Author(s).)

Loss of biodiversity and especially insect decline are widely recognised in modern ecosystems. This decline has an enormous impact due to the crucial ecological roles of insects as well as their economic relevance. For comparison, the fossil record can provide important insights on past biodiversity losses. One group of insects, for which a significant decline over the last 100 million years has often been postulated, but not demonstrated quantitatively, is Neuroptera (lacewings). Many adult lacewings are pollinators, while the larvae are mostly predators, which becomes very obvious from their prominent stylet-like mouthparts. We investigated the fossil record of larvae of all neuropteran lineages as well as a large share of extant neuropteran larvae. Based on these, we performed an outline analysis of the head with stylets. This analysis provides a quantitative frame for recognising the decline of lacewings since the Cretaceous, indicating also a severe loss of ecological roles., (© 2023. The Author(s).)