Ninomiya and Yoshizawa (2009) established a homology scheme for the wing-base (pteralia) sclerites of Odonata — dragonflies and damselflies — with those of mayflies (Ephemeroptera) and of all other winged insects (Neoptera), using the basal wing hinge and the subcostal veins as conserved landmarks. Their analysis showed that the polarity of the resulting wing-base character states — the undivided versus separately divided condition of the second axillary sclerite, the basiradiale and the median plate; the separated versus fused median notal wing process (and possibly also the posterior notal wing process); the dorsal expansion of the basalare; and the presence or absence of axillary folding lines — cannot be determined, because the order in which the three major lineages of winged insects diverged remains unresolved (the Palaeoptera problem). The open problem is to determine the ancestral wing-base condition of the winged insects (Pterygota), which requires resolving the basal pterygote branching order (Palaeoptera versus Metapterygota versus Chiastomyaria) or providing topology-independent evidence such as exceptionally preserved fossils, together with settling the exact boundaries within the odonate axillary plate. Morphological, phylogenomic and mitogenomic studies up to 2026 support each of the three competing hypotheses.

Background

Pterygota are the winged insects, comprising three major lineages: Ephemeroptera (mayflies), Odonata (dragonflies and damselflies), and Neoptera (all remaining winged orders, characterized by the ability to fold the wings flat over the abdomen). The order in which these three lineages split from their common ancestor is a century-old controversy known as the Palaeoptera problem, with three competing hypotheses: Palaeoptera (Ephemeroptera + Odonata as sister groups), Metapterygota (Odonata + Neoptera), and Chiastomyaria (Ephemeroptera + Neoptera). The wing base or pteralia is the set of small sclerites (hardened cuticular plates) that articulates the wing with the thorax: among them the first, second and third axillary sclerites (1Ax, 2Ax, 3Ax), which transmit wing movement and, in Neoptera, mediate wing folding; the basalare and subalare, epipleural sclerites serving as attachment sites for the direct flight muscles; the median plate, a sclerite associated with the base of the anal and mediocubital vein system; and the basiradiale, the basal sclerite associated with the radius vein. The median notal wing process (MNP) and the posterior notal wing process (PNP) are projecting processes of the thoracic notum (dorsal plate), the PNP articulating with the third axillary sclerite; separation of the MNP from the scutum is clearly observed in Ephemeroptera and Odonata, whereas the PNP is separated from the scutellum (the posterior division of the notum) in Ephemeroptera and some Neoptera but fused with it in Odonata, where a clear line at its base may nonetheless mark a structural border. Axillary folding lines are the creases along which neopteran wings fold. The odonate wing base is highly modified: several of these elements are merged into a composite axillary plate, whose internal boundaries are difficult to recognize. Because of this extreme divergence, homology of the odonate wing base with that of other pterygotes had been considered intractable — in extreme interpretations even motivating hypothetical dual origins of insect wings. Ninomiya and Yoshizawa (2009) cut through this by using highly conserved, unambiguously identifiable structures — the basal wing hinge and the subcostal veins — as landmarks, and established a homology scheme for the odonate wing base with those of Ephemeroptera (previously worked out by Willkommen and Hörnschemeyer, 2007) and Neoptera. However, establishing homology is only half of the task: deciding which character state is ancestral (the polarity) requires a resolved phylogeny of the three lineages. Under the Palaeoptera hypothesis the undivided palaeopteran condition of the 2Ax–basiradiale–median plate complex is equally parsimoniously ancestral or derived, whereas under Metapterygota or Chiastomyaria the neopteran condition of separated sclerites is more likely apomorphic (derived) and the undivided condition ancestral.

Problem Statement

Determine the ancestral condition of the wing base in the winged insects (Pterygota) under the Ninomiya–Yoshizawa homology framework. Two parts of this question are open. First, the polarity of the wing-base character states, which cannot be determined without a resolved branching order of the three major pterygote lineages — Ephemeroptera, Odonata and Neoptera (the Palaeoptera versus Metapterygota versus Chiastomyaria question) — or equivalent topology-independent evidence (for example, exceptionally preserved stem-pterygote fossils showing the wing-base sclerites): is the undivided condition of the second axillary sclerite (2Ax), basiradiale and median plate, seen in the living palaeopterans, the ancestral pterygote state or a derived fusion; is a separated median notal wing process (MNP) ancestral, as Kukalová-Peck (1983, 1991) predicted — and likewise a separated posterior notal wing process (PNP), with its odonate fusion derived — or is the separated condition derived in those Neoptera and Ephemeroptera that show it; and what is the ancestral state of the dorsal expansion of the basalare and of the axillary folding lines? Second, the exact boundaries within the odonate axillary plate — in particular whether the narrow membranous region observed in Zygoptera represents the border between the first and second axillary sclerites and reflects an ancestral odonate condition. A complete answer addresses the two parts separately. For the character polarity it must fix the ancestral state of each listed wing-base character: for every character it must state how the ancestral state depends on the unresolved branching order, giving the state under each of the three candidate topologies; a bare assertion that the polarity is unknown is not a determination. For the odonate axillary plate it must either settle the internal boundary or state, with reasons, that the boundary remains undetermined and name the evidence that would decide it. Settling one part does not require settling the other.

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