The appearance of wings in insects, early in their evolution [1], has been one of the more critical innovations contributing to their extraordinary diversity. Despite the conspicuousness and importance of wings, the origin of these structures has been difficult to resolve and represented one of the "abominable mysteries" in evolutionary biology [2]. More than a century of debate has boiled the matter down to two competing alternatives-one of wings representing an extension of the thoracic notum, the other stating that they are appendicular derivations from the lateral body wall. Recently, a dual model has been supported by genomic and developmental data [3-6], representing an amalgamation of elements from both the notal and pleural hypotheses. Here, we reveal crucial information from the wing pad joints of Carboniferous palaeodictyopteran insect nymphs using classical and high-tech techniques. These nymphs had three pairs of wing pads that were medially articulated to the thorax but also broadly contiguous with the notum anteriorly and posteriorly (details unobservable in modern insects), supporting their overall origin from the thoracic notum as well as the expected medial, pleural series of axillary sclerites. Our study provides support for the formation of the insect wing from the thoracic notum as well as the already known pleural elements of the arthropodan leg. These results support the unique, dual model for insect wing origins and the convergent reduction of notal fusion in more derived clades, presumably due to wing rotation during development, and they help to bring resolution to this long-standing debate.

A new palaeodictyopterid nymph Bizarrea obscura gen.n. et sp.n. (Spilapteridae) and a new adult specimen of Homaloneura cf. dabasinskasi Carpenter are described from the Pennsylvanian (Moscovian) ironstone nodules of Mazon Creek (IL, U.S.A.). Both taxa share enlarged prothoracic lobes (interpreted by some as winglets), heteronomous meso-and metathoracic wing pads or wings, a slender abdomen with pointed laterotergites, and a unique division of the abdominal segments by two transverse sulci. An alternative hypothesis for the placement of Bizarrea within Homoiopteridae is considered on the basis of its large body size and relatively short wing pads. Based on the morphology of the new material, postembryonic development of wing pads in Palaeodictyoptera (Palaeodictyopterida) is reconsidered. Detailed investigation of the abdominal segments, including examination by scanning electron microscopy, reveals the presence of subcircular, sclerotized structures partially covered at the bases of the nymphal laterotergites I-VII, ?VIII. Based on their position and shape, these structures are interpreted as abdominal spiracles, and thus a terrestrial or semiaquatic habitat for these immatures is hypothesized. Moreover, our discovery of the same, supposedly homologous structures in the enigmatic Vogesonymphidae (Permoplectoptera), from the Middle Triassic of Grès à Voltzia in France, is evidence for the parallel coexistence of ancestrally terrestrial and derived aquatic lineages of Ephemerida (Ephemeropterida) in early Mesozoic ecosystems.This published work has been registered in ZooBank, http://zoobank.org/urn:lsid: zoobank.org:pub

The Late Palaeozoic insect superorder Palaeodictyopterida exhibits a remarkable disparity of larval ecomorphotypes, enabling these animals to occupy diverse ecological niches. The widely accepted hypothesis presumed that their immature stages only occupied terrestrial habitats, although authors more than a century ago hypothesized they had specializations for amphibious or even aquatic life histories. Here, we show that different species had a disparity of semiaquatic or aquatic specializations in larvae and even the supposed retention of abdominal tracheal gills by some adults. While a majority of mature larvae in Palaeodictyoptera lack unambiguous lateral tracheal gills, some recently discovered early instars had terminal appendages with prominent lateral lamellae like in living damselflies, allowing support in locomotion along with respiratory function. These results demonstrate that some species of Palaeodictyopterida had aquatic or semiaquatic larvae during at least a brief period of their post-embryonic development. The retention of functional gills or gill sockets by adults indicates their amphibious lifestyle and habitats tightly connected with a water environment as is analogously known for some modern Ephemeroptera or Plecoptera. Our study refutes an entirely terrestrial lifestyle for all representatives of the early diverging pterygote group of Palaeodictyopterida, a greatly varied and diverse lineage which probably encompassed many different biologies and life histories.

BackgroundThe systematic positions of the extinct insect orders Hypoperlida, Miomoptera and Permopsocida were enigmatic and unstable for nearly a century. The recent studies based on new material, especially from the Cenomanian Burmese amber, shed light on evolutionary history of Acercaria resolving Permopsocida as the stem group of Condylognatha. However, the knowledge of the remaining two orders differs significantly.ResultsIn this study, we describe new specimens and evaluate morphology of various structures with emphasis on the mouthparts and wing venation. Our results are primary based on revisions of the type specimens with a proper delimitation of taxa Hypoperlida and Miomoptera followed by their significance for the evolutionary history of Acercaria. Three new genera as Belmomantis gen. nov., Elmomantis gen. nov., and Mazonopsocus gen. nov. are designated as members of Palaeomanteidae. The Pennsylvanian Mazonopsocus provides a minimum age for calibration, in accordance to the presence of crown acercarians during the late Carboniferous.ConclusionsThis contribution demonstrates that Hypoperlida and Miomoptera are stem groups of Acercaria. The putative clade (Hypoperlida + Miomoptera) is appearing as potential sister group of (Psocodea + (Permopsocida + (Thripida + Hemiptera))).Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-017-1039-3) contains supplementary material, which is available to authorized users.

No abstract