Insects acquired powered flight roughly 400 million years ago, but the evolutionary origin of the wing — the structure that enabled their dominance — is still contested. Bruce and Patel (2020) knocked out five leg-patterning genes in the amphipod crustacean Parhyale hawaiensis and argued that insect wings are not new structures: two proximal leg segments of an ancestral pancrustacean were incorporated into the insect body wall, carrying a precoxal leg outgrowth (exite) up onto the back, where it later became the wing. The problem this leaves open — phylogenetically tracing the exite-to-wing transformation across relevant crustacean and hexapod lineages and identifying the regulatory and morphogenetic changes that turned a leg exite into an articulated, muscularized, flight-capable wing, including the relative contributions of the exite homolog, the pleuron, and the medial tergum — remains open; per a 2025 review, recent evo-devo work supports and refines the exite-incorporation hypothesis, and the tissue-contribution question is the field's current focus.

Background

Winged insects (pterygotes) account for the majority of described animal species, and the wing is one of the most consequential morphological innovations in animal evolution. How the first wing arose has been debated for over a century. Three classes of hypothesis dominate. The tergal (paranotal) hypothesis derives the wing from a fixed lobe of the dorsal body wall plate (the tergum or notum). The exite hypothesis derives it from a movable lateral branch of the leg base: an exite is an outgrowth on the outer (lateral) side of a leg segment, and in many crustaceans such proximal exites serve as gills (epipods); Averof and Cohen (1997) argued from gene expression that insect wings descend from such ancestral gill-like leg branches. The dual (composite) hypothesis holds that the wing combines a tergal contribution with a leg-derived (pleural or exite) one, as supported paleontologically by Prokop and colleagues (2017) from wing pads of Paleozoic insect nymphs, developmentally by Niwa and colleagues (2010), who found in two basal insects — the mayfly and the bristletail — that the wing-patterning genes wingless and vestigial are expressed in candidate wing-precursor outgrowths and proposed that the wing arose by integration of two developmental modules, and by Linz and Tomoyasu (2018), who showed in the beetle Tribolium that abdominal wing serial homologs — structures in other body segments that share the wing's developmental construction — retain both tergal and pleural components.

The modern toolkit that made the present question possible pairs two model arthropods: the amphipod crustacean Parhyale hawaiensis, a marine crustacean amenable to CRISPR–Cas9 targeted mutagenesis (a technology for precisely disabling a chosen gene), and the beetle Tribolium castaneum, a standard insect genetics model. A set of conserved patterning genes builds both legs and wings: Distal-less and dachshund (dac; in Parhyale present as two paralogues, dac1dac1 and dac2dac2) mark distal and medial leg regions, while wing specification relies on genes such as nubbin, vestigial, pannier, and apterous. Bruce and Patel (2020) knocked out five leg-patterning genes in Parhyale and aligned the mutant phenotypes with the corresponding insect phenotypes. The alignment implied that two proximal leg segments present in the common ancestor of insects and crustaceans — including a precoxa (the leg segment proximal to the coxa, bearing a lateral exite) — were not lost in insects but incorporated into the body wall (the insect pleuron, the lateral body wall sclerite between tergum and leg), moving the precoxal exite dorsally onto the back, where it later formed the wing. On this account the Parhyale tergal plate (a dorsal wall extension of the leg base) and the insect wing are homologous, and wings are modified ancestral structures rather than novel outgrowths of the body wall.

The claim remains genuinely contested. Ohde and colleagues (2022) showed that in the cricket Gryllus bimaculatus the lateral margin of the tergum itself acts as a growth organizer — a region that directs disproportionate growth of surrounding tissue — via Wnt, Fat–Dachsous, and Hippo signaling (a conserved circuit linking cell-cell communication to organ-size control), and argued that wings evolved by expansion of lateral tergal tissue. Clark-Hachtel and colleagues (2021) documented wing-related tissues on both the tergum and the pleuron of the German cockroach. Fisher and colleagues (2021) proposed that the insect wing gene regulatory network (GRN; the linked set of transcription factors and signaling pathways that pattern a structure) is better understood as a versatile, evolutionarily older module for building bilayered cuticular outgrowths of the body wall generally. Bruce and Patel (2022) extended the incorporation model to the water flea Daphnia, arguing its carapace is a cryptically persisting exite homologous to the Parhyale tergal plate and the insect wing. Synthesizing evo-devo and fossil evidence, Ohde and Prokop (2025) describe an emerging but incomplete convergence: an ancestral precoxa bearing an exite came to lie within the hexapod lateral tergum, and the outstanding question is the relative contribution of the incorporated exite homolog, the pleuron, and the medial 'true' tergum to the wing — that is, the actual mechanics and sequence of the transition.

Problem Statement

Determine whether, and if so how, an ancestral crustacean precoxal leg exite was transformed into the insect wing — that is, test the evolutionary transition proposed by Bruce and Patel (2020) and, where it is supported by expression, functional, and where possible fossil evidence, reconstruct it and establish the developmental-genetic mechanisms that effected it. This comprises three scientifically linked aspects of one open question:

  1. Phylogenetic tracing. Establish where wing-homologous exite-derived structures occur across the pancrustacean tree by extending gene-expression and comparative analysis of wing- and leg-patterning genes beyond Parhyale, Tribolium, and Daphnia to relevant intermediate and outgroup crustacean and hexapod lineages — including apterygote hexapods (wingless insects such as bristletails and silverfish) and early-diverging pterygotes — with functional (loss-of-function) analysis only where technically feasible, so that the proposed sequence of morphological changes (dorsal migration of the exite, incorporation of proximal leg segments into the body wall) is traced lineage by lineage rather than inferred from the two endpoints.

  2. Developmental-genetic mechanism. Identify the regulatory and morphogenetic changes — cis-regulatory and trans-acting modifications to the gene regulatory network, and the changes in musculature, articulation, and epithelial outgrowth that redeploy exite development to a dorsal, laterally hinged position and elaborate it into a muscularized, articulated, flight-capable wing — including the mechanisms by which two proximal leg segments became morphologically incorporated into the insect body wall.

  3. Tissue contributions and generality. Resolve the relative contributions of the incorporated exite homolog, the pleuron, and the medial tergum to the finished wing, and determine how general the resulting scenario is across Pancrustacea, identifying exceptions or alternative evolutionary trajectories where they exist.

An answer is complete when it supplies an explicit, evidence-supported scenario for the origin of the wing: either the exite-to-wing transition, integrating the comparative lineage evidence, the developmental-genetic mechanisms of segment incorporation and exite redeployment, and resolution of the relative contributions of the incorporated exite homolog, the pleuron, and the medial 'true' tergum — the tissue-contribution question identified by Ohde and Prokop (2025) as the field's current focus — or, where that transition is not supported, a demonstrated alternative origin resolved on that scenario's own terms (its comparative lineage evidence, its mechanism of wing-precursor formation, and its resolution of the contributing tissues); partial evidence from a single taxon or a single gene does not by itself constitute a complete scenario.

References

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  2. Niwa, N., Akimoto-Kato, A., Niimi, T., Tojo, K., Machida, R. & Hayashi, S. (2010). Evolutionary origin of the insect wing via integration of two developmental modules. Evolution & Development 12(2):168–176. DOI: 10.1111/j.1525-142x.2010.00402.x. https://doi.org/10.1111/j.1525-142x.2010.00402.x
  3. Prokop, J., Pecharová, A., Nel, A., Hörnschemeyer, T., Krzemińska, E., Krzemiński, W. & Engel, M. S. (2017). Paleozoic nymphal wing pads support dual model of insect wing origins. Current Biology 27(2):263–269. DOI: 10.1016/j.cub.2016.11.021. https://doi.org/10.1016/j.cub.2016.11.021
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