Insects and myriapods bear pleurites — hardened lateral plates of the body wall that, in insects, form the pleuron anchoring the leg musculature and the wing articulation. A 2015 molecular developmental study of the red flour beetle Tribolium castaneum identified a cryptic proximal-most leg segment, the subcoxa, and showed that it gives rise to the larval pleurites, reviving the century-old subcoxal theory. That study left three linked questions open: whether myriapod pleurites also develop from a subcoxal segment (discriminating shared ancestry from convergent evolution of lateral sclerites), which proximal crustacean leg segment corresponds to the insect subcoxa, and whether subcoxal cells contribute to the wing (bearing on the gill-versus-paranotal debate on wing origin). Later work (2016-2021) substantially answered the wing-contribution thread, converging on a dual-origin view in which both tergal and pleural (subcoxal-region) tissues contribute to wings, with an exite (an outer branch of the leg) of an ancestral proximal leg segment incorporated into the body wall. However, the developmental origin of myriapod pleurites remains untested, and the precise segment-by-segment alignment between crustacean legs and the hexapod thorax is still contested, leaving a precise open core on the evolutionary fate of the subcoxa across mandibulate arthropods.

Background

The body wall of each thoracic segment of an insect consists of hardened plates called sclerites: a dorsal tergum, a ventral sternum, and lateral pleurites that together form the pleuron. The pleuron is not merely structural — it braces the base of the leg and, in winged insects (pterygotes), carries the wing articulation and serves as the attachment site for the direct wing muscles, so its evolutionary origin is integral to how the wing-bearing thorax was assembled. Each insect leg itself is a chain of segments (coxa, trochanter, femur, tibia, tarsus, pretarsus). The classical 'subcoxal theory', developed by R. E. Snodgrass and others in the early twentieth century, proposes that the pleurites derive from an additional, proximal-most leg segment — the subcoxa — that was evolutionarily flattened and incorporated into the body wall. Direct morphological evidence for an embryonic subcoxa was lacking until Coulcher, Edgecombe and Telford (2015) studied embryos and larvae of the red flour beetle Tribolium castaneum using in situ hybridization (a technique that reveals where specific genes are expressed in the embryo). Domains of the gene serrated, which marks the joints between leg segments, revealed six embryonic leg segments rather than the five of the larval leg, and the extra proximal-most segment — the subcoxa — was shown to move out onto the body wall and form the pleurites (the episternum and epimeron), matching the subcoxal theory's prediction. A subcoxal domain was also identified in the gnathal (mouthpart) appendages.

The phylogenetic context matters for what this finding implies. Myriapods (centipedes and millipedes) also have lateral sclerites, and they were once united with hexapods (insects plus springtails and allies) as 'Atelocerata' on the assumption of a shared terrestrial ancestor. It is now accepted that hexapods instead nest inside Pancrustacea (crustaceans plus hexapods), with myriapods placed outside that group, as the sister lineage within the larger clade Mandibulata. Most crustaceans lack pleurites, the exceptions being groups such as remipedes (a class of cave-dwelling crustaceans frequently recovered as the sister group of hexapods), in which a detached coxal fragment forms an anterior pleurite-like plate. This pattern leaves two alternatives for pleurite evolution: either pleurites have a deep mandibulate origin, or they evolved independently (convergently) in hexapods and myriapods — a question that hinges on whether myriapod pleurites, like those of Tribolium, develop from a subcoxal leg segment.

Because hexapods arose from within crustaceans, the insect body plan must be mapped onto the crustacean leg, which is built from proximal segments (a proximal coxa, then a basis, then more distal articles) whose number and morphology vary widely across crustacean groups. Since most crustaceans do not internalize proximal leg segments into the body wall, the insect subcoxa should correspond to some particular proximal crustacean leg segment, but which one is unresolved. Comparative morphology suggests the alignment insect subcoxa = crustacean 'coxa' and insect coxa = crustacean 'basis', an alignment consistent with the remipede and malacostracan coxal fragments, but it has not been tested with comparative developmental data.

The subcoxa also connects to the long-running debate on the origin of insect wings. Two classical hypotheses compete: the paranotal (tergal) hypothesis, in which wings evolved as lateral extensions of the dorsal body wall, and the gill or exite hypothesis, in which wings evolved from a branchable outgrowth (exite, e.g. an epipod gill) on the proximal leg — possibly on the subcoxa itself. Since 2016 a third, 'dual origin' synthesis has gained ground: studies in cockroaches, beetles, Drosophila, crickets and the crustacean Parhyale hawaiensis indicate that both tergal tissues and pleural (subcoxal-region) tissues, including a proximally positioned exite that was incorporated into the body wall, contributed to the evolution of the wing. A 2025 review by Ohde and Prokop summarizes the emerging view that an ancestral exite on a precoxa-like proximal leg segment was incorporated into the lateral body wall, with the relative contributions of the exite homolog, the pleuron and the medial tergum remaining the current focus of research. Within this framework, establishing the identity and fate of the subcoxa across myriapods and crustaceans is the unresolved developmental foundation beneath both the pleuron and wing-origin questions.

Problem Statement

Determine the evolutionary fate of the proximal-most (subcoxal) leg segment across mandibulate arthropods, in the two respects left open by the Tribolium evidence of Coulcher, Edgecombe and Telford (2015):

  1. Do the pleurites of myriapods (centipedes and millipedes) develop from a subcoxal leg segment, as the pleurites of the beetle Tribolium do, or do they arise as de novo sclerotizations of the lateral body wall? Relevant evidence may come from comparative developmental data (embryonic morphology and/or appendage-patterning gene expression) in myriapod trunk segments, tracing the primordia of the lateral sclerites relative to leg-segment markers, though other lines of evidence are equally admissible. This discriminates between a deep mandibulate origin of lateral thoracic sclerites and independent (convergent) evolution of pleurites in hexapods and myriapods.

  2. Which proximal leg segment of crustaceans, if any, is homologous to the insect subcoxa? Rival proposals include the morphology-based alignment under which the insect subcoxa corresponds to the crustacean coxa and the insect coxa to the crustacean basis, and the knockout-based alignment of Bruce and Patel (2020), in which two proximal crustacean leg segments were incorporated into the insect body wall; these are illustrative examples, and other candidate correspondences are equally admissible. Relevant evidence may come from comparative developmental data across crustacean lineages relevant to hexapod origins (e.g. remipedes, malacostracans (crabs, shrimps and their relatives), branchiopods (water fleas, brine shrimps and allies)) mapped onto current pancrustacean phylogeny, though the question is not restricted to that evidence.

Only the two questions above are in scope; the wing-contribution thread — whether cells of the subcoxal segment contribute to wing formation and whether wing-patterning genes such as vestigial are expressed in the subcoxal domain — has been substantially answered affirmatively by the 2016-2021 dual-origin studies and is settled background, not part of this task.

References

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