Jakub Goczał

Recent progress in beetle palaeontology has incited us to re‐address the evolutionary history of the group. The Permian †Tshekardocoleidae had elytra that covered the posterior body in a loose tent‐like manner. The formation of elytral epipleura and a tight fit of elytra and abdomen were important evolutionary transformations in the Middle Permian, resulting in a tightly enclosed subelytral space. Permian families were likely associated with dead wood of gymnospermous trees. The end‐Permian extinction event resulted in a turnover in the composition of beetle faunas, especially a decline of large‐bodied wood‐associated forms. Adephaga and Myxophaga underwent a first wave of diversification in the Triassic. Polyphaga are very rare in this period. The first wave of diversification of this suborder occurs in the Jurassic, with fossils of Elateriformia, Staphyliniformia and Cucujiformia. The Cretaceous fossil record has been tremendously enriched by the discovery of amber inclusions. Numerous fossils represent all major polyphagan lineages and also the remaining suborders. Improved analytical methods for documenting and placing extinct taxa are discussed. Different factors have played a role in the diversification of beetles. The enormous number of species associated with flowering plants, and timing and patterns of diversification in phytophagous lineages indicate that the angiosperm radiation played a major role in beetle macroevolution. Moreover, the evolution of intimate partnerships with symbionts and the acquisition of novel genes—obtained from fungi and bacteria via horizontal gene transfers—facilitated the use of plant material as a food source and were key innovations in the diversification of plant‐feeding beetles.

The transformation of the fore wings into strongly sclerotized protective covers (elytra) is considered a fundamental evolutionary innovation of the megadiverse order Coleoptera. Surprisingly, these multifunctional structures have been reduced in many distantly related groups of beetles. Patterns, drivers and the evolutionary implications of this modification have never been comprehensively discussed. In the present study, we surveyed the entire order Coleoptera to analyse the patterns of elytral shortening and loss, with a special focus on prevalence, forms, degree of reduction and the functional background of this significant deviation from the coleopteran ground plan. Our analysis revealed that about 20% of all extant species (roughly 88,000 out of 442,275 spp.), distributed across all four suborders, have shortened or even absent elytra. The elytral loss was more frequent within the polyphagan series Elateriformia and Staphyliniformia. Moreover, we found that elytral reduction has independently occurred multiple times in the evolutionary history of Coleoptera and that it has been driven by a wide array of selective drivers. One of the main drivers is the improved flexibility of the uncovered abdomen and the correlated increased manoeuvrability in narrow spaces, as well as the option of using the flexible abdomen as a steering organ or to facilitate mating. Another common driver is mimicry, where exposed metathoracic wings potentially improve the overall similarity to hymenopteran models. Exposure of the abdomen can facilitate the targeted release of defensive abdominal gland secretions and was most likely a crucial step towards establishing relations with social insects enhanced by chemical communication. In the Elateriformia, and rarely in other lineages, elytral loss is a consequence of paedomorphosis, related to a specific resource‐allocation strategy. In many groups of beetles with reduced elytra, alternative defensive strategies can be found. This includes, for instance, aposematic coloration, chemical defence, mimicry or bioluminescence. Direct drivers of elytral loss in many groups remain unclear, and more studies are needed to understand the evolutionary background and implications of this significant morphological modification in Coleoptera.