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.

Department of Zoology Faculty of Science Charles University Viničná 7 128 44 Praha 2 Czech Republic

Division of Entomology Natural History Museum and Department of Ecology and Evolutionary Biology 1501 Crestline Drive Suite 140 University of Kansas Lawrence KS 66045 USA; Division of Invertebrate Zoology American Museum of Natural History Central Park West at 79th Street New York NY 10024 5192 USA

Institut de Systématique Évolution Biodiversité ISYEB UMR 7205 CNRS MNHN UPMC EPHE Muséum national d'Histoire naturelle Sorbonne Universités 57 Rue Cuvier CP 50 Entomologie 75005 Paris France

Institute of Biology Pedagogical University of Kraków Podbrzezie 3 31 054 Kraków Poland

Institute of Systematics and Evolution of Animals Polish Academy of Sciences ul Sławkowska 17 31 016 Kraków Poland

Senckenberg Gesellschaft für Naturforschung Senckenberganlage 25 60325 Frankfurt Germany

References provided by Crossref.org

Thoracic and abdominal outgrowths in early pterygotes: a clue to the common ancestor of winged insects?

Ecomorphological diversification of the Late Palaeozoic Palaeodictyopterida reveals different larval strategies and amphibious lifestyle in adults

Palaeozoic giant dragonflies were hawker predators

New insights on basivenal sclerites using 3D tools and homology of wing veins in Odonatoptera (Insecta)