Captain America without the shield: elytra loss and the evolution of alternative defence strategies in beetles

The paper uses a thought experiment based on Captain America’s loss of his shield to frame the biological question of how beetles cope with the loss or reduction of their protective elytra. Beetles (Coleoptera), with over 380,000 described species encompassing nearly a quarter of all known animal species, owe much of their evolutionary success to the transformation of forewings into hardened elytra. Elytra provide broad benefits including mechanical protection of abdomen and hind wings, thermoregulation, water conservation, aid in flight and swimming, water harvesting, mimicry/camouflage, and acoustic communication. Despite these advantages, elytra reduction (brachelytry) has independently occurred multiple times across beetle lineages, likely driven by different factors (e.g., increased manoeuvrability or resource savings), though these drivers remain untested. The central question posed is how brachelytrous beetles can withstand selection pressures without their key protective structures and what alternative defensive strategies may have evolved to compensate.

- Elytra reduction (brachelytry) has evolved independently multiple times across beetle lineages, with likely diverse drivers (e.g., manoeuvrability, resource savings), none empirically tested to date. - Multiple alternative defence strategies are associated with reduced elytra and may compensate for the loss of primary protective covers: • Hardened, thickened abdominal tergites (“plate armour”) in rove beetles (Staphylinidae). • A unique 2-way asymmetric hindwing folding mechanism in rove beetles that enables full protection of flight wings under very short elytra. • Warning (aposematic) coloration in groups such as Chauliognathus (Cantharidae) and Balanophorus (Melyridae). • Batesian mimicry in longhorn beetles (e.g., Necydalinae, Cerambycinae tribes Molorchini, Hesthesini) that resemble wasps; exposed hind wings may enhance the mimicry. • Defensive abdominal glands in many rove beetles that release repellent secretions directed at attackers. • Explosive emission of hot, noxious gases in bombardier beetles (Carabidae: Paussinae, Crepidogastrini). • Male bioluminescent lanterns in some glowworm beetles (Phengodidae), potentially acting as aposematic signals. - The review suggests these innovations likely compensate for elytra loss, and in some cases, shortened elytra may even enhance the effectiveness of alternative defences (e.g., mimicry performance, improved aiming of chemical emissions). - Contextual background: Coleoptera diversity exceeds 380,000 described species, nearly 25% of known animal species; elytra likely originated by the Carboniferous via progressive sclerotization and gene network co-option.

The review addresses the question of how beetles manage the selective pressures associated with reduced elytra by compiling evidence for a suite of alternative defence strategies that accompany brachelytry. These include morphological fortification of exposed abdominal segments, specialized wing-folding mechanisms preserving flight capability, visual deterrents (aposematism and mimicry), chemical defences (repellents and explosive reactions), and bioluminescent signaling. Collectively, these strategies demonstrate plausible compensatory pathways enabling survival without full elytral protection. The paper emphasizes that causal direction is unresolved: alternative defences may have evolved in response to elytra loss, or conversely, the emergence of effective alternative defences may have relaxed selection for long elytra. In some systems, reduced elytra could actively improve defence performance (e.g., increased mimicry credibility or better targeting of chemical sprays). Beyond biological inference, the author argues that pop culture analogies provide a valuable heuristic and communication tool, supporting hypothesis generation and public engagement with complex evolutionary concepts.

The analysis indicates that beetles with reduced elytra exhibit diverse alternative defence strategies, including armored abdominal tergites, specialized wing-folding protecting flight wings under short elytra, aposematic coloration, Batesian mimicry, chemical repellents, explosive defensive emissions, and bioluminescence. Whether these traits arose directly in response to elytra reduction remains unclear; alternatively, the emergence of effective defences could have made long elytra redundant. In some cases, shortened elytra may enhance defence effectiveness (e.g., mimicry, chemical targeting). Thought experiments and pop culture analogies are proposed as useful tools for developing theoretical templates, identifying gaps, guiding empirical tests, and improving science communication and education by making complex natural phenomena accessible to wider audiences.

- The review is conceptual and illustrative; it does not present new empirical tests of drivers or consequences of elytra loss. - Causality between elytra reduction and alternative defences remains unresolved. - Hypothesized drivers of brachelytry (e.g., manoeuvrability, resource savings) have not been empirically tested and may differ among lineages. - Examples emphasize select taxa; generalizability across Coleoptera requires broader comparative and phylogenetic analyses.