The embryos and first-instar larvae of the viviparous earwig Arixenia esau develop inside a maternal uterus and bear serial paired multilobed outgrowths on the dorso-lateral abdominal nota (terga); these outgrowths adhere to the uterine wall, collectively forming a dispersed placenta-like organ, and are shed at the first molt after birth. A 2019 anatomical and immunohistochemical study concluded that the outgrowths are probably de novo evolutionary novelties that act as muscle-driven haemolymph pumps supporting embryonic nourishment, but it left the central questions unresolved: the alternative interpretation — that the outgrowths are homologous to the dorsal (paranotal) component of ancestral wing primordia under the dual-origin hypothesis of insect wing evolution — could not be excluded because the molecular evidence was limited to Vestigial immunodetection, and the pumping function rested on static anatomy and volumetric estimates rather than direct measurement. The problem is therefore twofold: (1) a comparative developmental-genetic determination of whether the outgrowths arose de novo from the dorsal sclerites or are paranotal wing homologs, based on spatiotemporal expression of multiple wing/paranotum-associated regulatory genes across segments and species; and (2) a direct functional test of the haemolymph-pump hypothesis, with quantification of the outgrowths' contribution to embryonic nutrient and gas exchange.

Background

Adult winged insects (Pterygota) almost invariably lack non-sexual appendages on the abdomen; the few known exceptions, such as the abdominal appendages of male sepsid flies and the lateral abdominal sensory-secretory organs (LASSOs) of Bennini hemipterans, are celebrated precisely because they challenge this conserved body plan (Hoch et al. 2014). Immature stages are less constrained: mayfly nymphal gills, beetle larval tracheal gills, and pupal 'gin-trap' denticular outgrowths are all pregenital abdominal outgrowths, generally interpreted as respiratory or protective and — crucially for this problem — as either serial homologs of wings or de novo extensions of the body wall.

Arixenia esau is an earwig (Dermaptera) that lives epizoically — on the body surface of another animal — on bats roosting in Southeast Asian caves. It is one of only two earwig lineages with matrotrophic viviparity, a reproductive mode in which the mother directly provisions developing offspring rather than supplying only yolk (lecithotrophy); matrotrophy has evolved independently in many invertebrate groups and typically involves placenta-like maternal-embryonic contact organs (Ostrovsky et al. 2015). In Arixenia, embryos develop sequentially in two compartments: first in terminal ovarian follicles, then in a uterus-like dilation of the lateral oviduct, where advanced embryos and first-instar larvae float in fluid and, after hatching from the egg envelope, attach to the uterine wall (Tworzydlo et al. 2013). These attached larvae bear serial paired multilobed outgrowths on the dorso-lateral aspects of the abdominal nota (the dorsal exoskeletal plates, or terga) of the first eight abdominal segments; each outgrowth is subdivided into four lobes and the whole series remains until the end of intrauterine development, then degenerates and is shed with the larval cuticle at the first molt after birth (Bilinski and Tworzydlo 2019). Because the lobes adhere to the uterine epithelium at a series of contact sites where maternal and embryonic tissues are separated only by a thin permeable cuticle, they have been proposed to constitute a dispersed placenta-like organ involved in nourishment. A sibling study showed that these contact sites do mediate gas exchange, with a respiratory pigment (hemocyanin) synthesized in the larval fat body supporting a biphasic respiratory system (Jaglarz et al. 2019).

The source study (Tworzydlo et al. 2019) asked how these outgrowths arise and function. Using light and electron microscopy, 3D reconstruction, and immunodetection of Vestigial (Vg — a transcriptional co-activator that, with its partner Scalloped, is a key specifier of wing and haltere identity in Drosophila and other insects), it showed that the outgrowths bud from the lateral parts of the abdominal nota, that their lobes contain bundles of thin muscle fibers attached to the epithelial basal lamina with the lobe lumen integrated into the haemocoel (the body cavity containing haemolymph, the insect circulatory fluid), and that Vg protein, while present in whole-embryo homogenates and nerve-cord cells, is not expressed in the outgrowth tissues. From these static data the authors estimated that coordinated contraction of the lobe musculature could pump roughly 7-10% of the total larval haemolymph volume per contraction cycle, under three simplifying assumptions: simultaneous contraction of all 16 lobes, an average 10% shortening of muscle fibers, and one of two assumed haemocoel-occupancy scenarios. On this basis they concluded that the outgrowths are probably evolutionary novelties related to intrauterine (matrotrophic) development, functioning as haemolymph pumps.

The conclusion, however, faces a specific alternative. Under the dual-origin hypothesis of insect wing evolution — supported by fossil nymphal wing-pad morphology (Prokop et al. 2017) and by gene-expression studies of candidate wing homologs in crustaceans (Clark-Hachtel and Tomoyasu 2020) — insect wings arose from two sources: dorsal outgrowths of the thoracic terga (paranotal lobes) and proximal leg-exite (outer branch) elements. A serially arranged dorsal abdominal outgrowth in a winged insect is exactly the kind of structure that could represent the paranotal component alone. The absence of Vg from the outgrowths is weak counter-evidence because it is a single marker assayed by immunodetection, without staged comparative data or additional wing-patterning genes, and because co-option or loss of expression cannot be ruled out. Likewise, the pumping function has never been observed directly: no flow has been imaged, no maternal-to-embryo transfer rate has been physiologically quantified, and the contribution of outgrowth-driven flow to total nutrient and gas exchange is unknown.

Problem Statement

Determine the evolutionary origin and the physiological functioning of the pregenital abdominal outgrowths of the embryos and first-instar larvae of the viviparous earwig Arixenia esau, as posed by Tworzydlo et al. (2019). The outgrowths are serial paired multilobed structures (four lobes each) on the nota of the first eight abdominal segments; they adhere to the uterine wall and are shed at the first molt after birth. The problem comprises two linked components.

(1) Origin and homology. Establish whether the outgrowths are de novo novelties derived from the dorsal sclerites (the abdominal nota), as the source study tentatively concluded, or homologs solely of the dorsal (paranotal) component of ancestral wing primordia, as allowed by the dual-origin hypothesis of insect wing evolution. Discriminating evidence for this determination can come from comparative developmental and genetic analysis — for example, spatial and temporal expression of multiple wing- and paranotum-associated regulatory genes (beyond the single Vestigial marker used in 2019) in the outgrowth primordia, compared with stage-matched thoracic wing primordia and adjacent abdominal tergal tissue, across segments and species — i.e., a characterization of the developmental gene network operating in the outgrowth primordia in Arixenia and in appropriate comparative dermapteran material, together with an explicit argument for why the observed pattern does or does not support paranotal homology rather than co-option of the wing-patterning network.

(2) Function. Directly test the hypothesis that the outgrowths act as active haemolymph pumps, which currently rests only on anatomical linkage and volumetric estimates with simplifying assumptions. This requires functional evidence of the kind the source specifies — for example, live imaging of lumenal haemolymph flow during muscle contractions, physiological quantification of maternal-to-embryo transfer rates, or manipulation of outgrowth muscle activity with appropriate controls — and a quantification of the contribution of outgrowth-driven flow to embryonic nutrient and gas exchange relative to the other exchange surfaces of the placenta-like organ.

A complete answer resolves both components: the homology question by comparative developmental and genetic evidence with declared criteria, and the functional question by direct measurement and intervention. The two are scientifically linked — both bear on what this organ is and how matrotrophic viviparity in this lineage actually works — and neither component has yet been resolved.

References

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