When a key innovation becomes redundant: Patterns, drivers and consequences of elytral reduction in Coleoptera

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.