Winged insects (Pterygota) comprise three extant lineages — mayflies (Ephemeroptera), dragonflies and damselflies (Odonata), and the remaining winged insects (Neoptera) — and which of the three diverged first remains one of the longest-standing unresolved questions in insect phylogenetics. Each of the three possible branching orders has received strong apparent support from some dataset: the Palaeoptera hypothesis groups Ephemeroptera and Odonata as sister lineages; the Metapterygota hypothesis places Ephemeroptera as the earliest-diverging branch with Odonata sister to Neoptera; and the Chiastomyaria hypothesis places Odonata as the earliest-diverging branch with Ephemeroptera sister to Neoptera. Morphological, nuclear-genomic, and mitochondrial-genomic data still support different answers, and dedicated reanalyses show the phylogenomic signal itself is internally conflicted and highly sensitive to outgroup choice, taxon sampling, and substitution model. The open problem is to determine the true evolutionary relationship among the three lineages — equivalently, whether mayflies are the earliest-diverging extant winged insects and whether Palaeoptera is monophyletic — with evidence robust to these known sources of systematic error.

Background

Insects are the most species-rich group of animals, and all winged insects belong to a single clade, the Pterygota. Extant winged insects split into three primary lineages: Ephemeroptera (mayflies, aquatic nymphs with a short-lived adult stage), Odonata (dragonflies and damselflies, aerial predators with aquatic nymphs), and Neoptera (all remaining winged orders, from cockroaches to beetles, flies, and moths). The name of the problem comes from 'Palaeoptera' ('ancient wings'): mayflies and odonates cannot fold their wings flat over the abdomen at rest, whereas Neoptera can, which traditionally suggested that Ephemeroptera and Odonata share a more recent common ancestor with each other than with Neoptera. Three competing topologies have been debated for decades. (1) The Palaeoptera hypothesis: Ephemeroptera + Odonata are sister groups, together sister to Neoptera; support includes wing non-folding and, in modernized form, detailed studies of insect head anatomy that control for concerted convergence (independent evolution of similar character states across lineages, which can mimic shared ancestry). (2) The Metapterygota hypothesis: Ephemeroptera diverged first, and Odonata + Neoptera form a clade (Metapterygota, named for shared 'true metamorphosis' traits such as the absence of adult moulting and indirect sperm transfer); historically supported by wing venation, mandible morphology, and respiratory anatomy, and by some combined molecular–morphological analyses. (3) The Chiastomyaria hypothesis: Odonata diverged first, and Ephemeroptera + Neoptera are sisters; originally proposed on characters such as direct sperm transfer, and supported by several single molecular markers and by some mitochondrial-genome analyses. Resolving which topology is true matters because the order of these deepest splits determines how the common ancestor of winged insects (the protopterygote) is reconstructed — including the sequence in which flight-related characters evolved, and how Palaeozoic fossil nymphs with abdominal and thoracic outgrowths are interpreted in scenarios for the origin of insect wings. The controversy persisted through the molecular era: an 18S/28S ribosomal-DNA study explicitly titled 'The Palaeoptera Problem' (Hovmöller et al., 2002) and a major review of insect phylogeny (Trautwein et al., 2012) recorded strong apparent support for all three topologies from different datasets. In the phylogenomic era, morphological head-character analyses (Blanke et al., 2012, 2013), denser taxon sampling of nuclear data (Thomas et al., 2013, 113 taxa), and the 1478-gene 1KITE transcriptomic analysis (Misof et al., 2014) variously favoured monophyletic Palaeoptera, and a dedicated reanalysis (Simon et al., 2018) found overall phylogenomic support for Palaeoptera but also substantial hidden signal for Chiastomyaria that standard tree-inference methods do not detect: as gene partitions accumulate, statistically strong support can flip between the two hypotheses. Mitochondrial genomes (mitogenomes, the closed circular genome of the mitochondrion, widely used for deep insect phylogeny) are likewise split: analyses under the site-heterogeneous CAT-GTR substitution model (a model allowing amino-acid preferences to vary across sequence sites, which reduces artefacts from heterogeneous sequence composition) strongly supported Palaeoptera (Song et al., 2019), whereas Bayesian analyses of an expanded mitogenome dataset with different outgroup choices — the lineages used to root the tree, here bristletails, silverfish, or diplurans — strongly supported Chiastomyaria (Yu et al., 2021). The deepest pterygote internodes are short (a rapid ancient radiation), so little signal separates the hypotheses while long branches and heterogeneous composition of outgroups and markers can dominate the inferred topology. As of 2025–2026, specialist literature continues to treat higher-level palaeopteran relationships as unresolved: the largest odonate mitogenome study concludes mitogenomes fall short for exactly these deep relationships (Huang et al., 2025), and new three-dimensional morphological work on Carboniferous fossils finds the thoracic anatomy of Odonata and Ephemeroptera to be separate specializations, with the fossil pattern resembling Neoptera (Prokop et al., 2026), leaving the reconstruction of the protopterygote open.

Problem Statement

Determine the true evolutionary relationship among the three extant lineages of winged insects — Ephemeroptera (mayflies), Odonata (dragonflies and damselflies), and Neoptera (all other winged insects): which of the three possible branching orders holds, i.e. (i) Ephemeroptera + Odonata as sister groups (Palaeoptera hypothesis, making mayflies and dragonflies jointly the earliest-diverging extant winged insects), (ii) Ephemeroptera as the earliest-diverging lineage with Odonata + Neoptera as sisters (Metapterygota hypothesis), or (iii) Odonata as the earliest-diverging lineage with Ephemeroptera + Neoptera as sisters (Chiastomyaria hypothesis)? Equivalently: is Ephemeroptera the earliest-diverging extant winged insect lineage, and is Palaeoptera monophyletic? An acceptable answer must establish one of the three topologies for the extant fauna with evidence whose conclusion is robust to the documented sources of systematic error in this problem — outgroup choice, taxon sampling, substitution-model class (including site-heterogeneous models), and data class (morphology, nuclear genome, mitochondrial genome, fossils) — or must demonstrate quantitatively that available evidence cannot distinguish the three hypotheses. No restriction to a particular data type, marker, model, or taxon subset is implied; the problem is posed in its natural generality.

References

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