LETTER doi:10.1038/nature11898 Amphibious flies and paedomorphism in the Jurassic period Diying Huang 1 , Andre ´ Nel 2 , Chenyang Cai 1 , Qibin Lin 1 & Michael S. Engel 3,4 The species of the Strashilidae (strashilids) have been the most per- plexing of fossil insects from the Jurassic period of Russia and China 1,2 . They have been widely considered to be ectoparasites of pterosaurs or feathered dinosaurs, based on the putative presence of piercing and sucking mouthparts and hind tibio-basitarsal pincers purportedly used to fix onto the host’s hairs or feathers 1–6 . Both the supposed host and parasite occur in the Daohugou beds from the Middle Jurassic epoch of China (approximately 165 million years ago) 7,8 . Here we analyse the morphology of strashilids from the Daohugou beds, and reach markedly different conclusions; namely that strashilids are highly specialized flies (Diptera) bearing large membranous wings, with substantial sexual dimorphism of the hind legs and abdominal extensions. The idea that they belong to an extinct order 2 is unsupported, and the lineage can be placed within the true flies. In terms of major morphological and inferred behavioural features, strashilids resemble the recent (extant) and relict members of the aquatic fly family Nymphomyiidae. Their ontogeny are distinguished by the persistence in adult males of larval abdominal respiratory gills, representing a unique case of paedomorphism among endopterygote insects. Adult strashilids were probably aquatic or amphibious, shedding their wings after emergence and mating in the water. Order Diptera Strashila daohugouensis sp. nov. Etymology. The name refers to the locality of the type specimen. Holotype. Nanjing Institute of Geology and Palaeontology (NIGP)155020, a complete male with details of head, associated with female allotype, without counterpart. Referred material. Allotype NIGP156171; paratypes NIGP155017, NIGP155018, NIGP155019, NIGP155022, NIGP155023, NIGP155024, NIGP155025, NIGP155027 and NIGP156170. Locality. The Xiayingzi outcrop of the Daohugou beds, Daohugou Village, Ningcheng County, Inner Mongolia, China. The exception is NIGP115019 (see Supplementary Information). Horizon. Jiulongshan Formation, Middle Jurassic epoch. Diagnosis. The new species greatly resembles the type species, Strashila incredibilis, except that it has shorter and more swollen male metafemora (see Supplementary Information for further details of the new species). The enigmatic Jurassic-period insect Strashila incredibilis displays a hypognathous head with compact antenna and a short ‘suctorial beak’, an apterous thorax, prominent chelate hind legs, and fleshy lateral abdominal appendages 1 . Only a few strashilid males have been reported from the Late Jurassic epoch of Russia 1 and Middle Jurassic epoch of China 2,9 , and therefore knowledge of these enigmatic insects has been very limited. Recent descriptions add little critical informa- tion with regard to their functional morphology and systematic affi- nities, and attribute strashilids to an extinct, putatively ectoparasitic order 2 . The present study is based on 13 new specimens of 2 different forms, Strashila daohugouensis sp. nov. (9 males, 2 females) and Vosila sinensis (2 males) (we demonstrate that Vosila sinensis and Parazila saurica are the same species; for details see Supplementary Informa- tion) from Daohugou. These specimens are rare among the 100,000 fossil insects recovered from Daohugou and among the collections of the Nanjing Institute of Geology and Palaeontology. Although definitive Middle Jurassic ectoparasites (fleas) on verte- brates were reported recently from the Daohugou outcrops 5,10 , earlier speculation regarding strashilids as terrestrial ectoparasites can be rejected owing to an alternative hypothesis stemming from the dis- covery of hitherto unknown large membranous wings (Fig. 1i) and females of Strashila, in two cases preserved in copula (Fig. 1a, e). Males and females have identical head morphologies but differ strongly in the absence of the hind leg pincers in females, which excludes ectoparasit- ism on terrestrial vertebrates as these are reinterpreted as sex-related structures for grasping the female rather than a host’s integument. In addition, unlike ectoparasitic insect lineages, the body is cylindrical in strashilids (Fig. 1e), rather than dorsoventrally or laterally flattened. Male strashilids have genitalia of antliophoran type 1,2,4 that are thought to have terminalia characteristic of Holometabola and are close to those of Mecopteroidea, but with primitive volsellae (small accessory clasping sclerites) 2 . This has led to the suggestion that they are an extinct order and a sister group to the Mecoptera 2 . In fact, the male genitalia are virtually identical with those of the recent nemato- ceran flies 11 (for example, Chironomidae, Sciaridae or some Nym- phomyiidae) rather than Mecoptera. Examination of the genitalia in finer detail reveals that they are clearly dipteran. The male tergite VIII shows a rounded anterolateral process (Figs 1c, f and 2c, and Supplementary Fig. 3i), and it has a volsella of typical dipteran mor- phology, gonocoxites articulated on the posterolateral sides of the transverse tergite, and gonostyli with terminal grasping hooks. The presence of these features in combination reinforces the placement of strashilids among basal lineages of the true flies 12,13 (Figs 1c, f and 2c, and Supplementary Figs 3i and 6a). The relatively small, oval, prognathous head bears a pair of large compound eyes with numerous facets and a pair of lateral ocelli, which are not completely or virtually lost or reduced as in ectoparasitic insects (Fig. 1b, h and Supplementary Figs 1a–d, 2b, c, e, i and 3b). Together with the presence of short antennae, this supports further the affinity of the Strashilidae with true flies. The mouthparts are vestigial, remarkably reduced in both sexes (Fig. 1b, h and Supplementary Figs 2e and 3b), suggesting that the adults do not feed. The antennae have a scape and a pedicel, both of which are large, and a long flagellum with the main section annulated, comparable in critical details with those of nymphomyiids 14 (Fig. 1h and Supplementary Figs 2b, c, e, i and 3b). The thorax of strashilids shows the typical subdivision of the thorax of true flies (Fig. 1a, g and Supplementary Fig. 2h). The male wings, large and membranous, are fringed with numerous long setae (Figs 1i, j and 3a, and Supplementary Figs 5a, e, f and 7b) and are similar to those of nymphomyiids, a primitive lineage of aquatic flies. The wings bear a 1 State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China. 2 CNRS UMR 7205, CP 50, Entomologie, Muse ´ um national d’Histoire naturelle, Paris F-75005, France. 3 Division of Entomology, Natural History Museum, University of Kansas, Lawrence, Kansas 66045, USA. 4 Department of Ecology & Evolutionary Biology, University of Kansas, Lawrence, Kansas 66045, USA. 94 | NATURE | VOL 495 | 7 MARCH 2013 Macmillan Publishers Limited. All rights reserved ©2013