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The Xyelidae are a comparatively species-poor family of sawflies comprising about 80 extant species in five genera worldwide.[2] Two genera and about 15 species occur in Europe.[3][4] The fossil record of the family is extensive, comprising more than 120 species[5] and including the oldest fossil Hymenoptera species dating back to the Triassic, between 245 and 208 million years ago. Xyelidae are to be regarded as living fossils since they represent one of the oldest lineages of insects and include still extant forms.

Xyelidae
Temporal range: TriassicHolocene, 227–0 Ma[1]
Xyelapusilla.jpg
Xyela, the type genus of Xyelidae, from British Entomology
Scientific classification e
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Hymenoptera
Superfamily: Xyeloidea
Newman, 1834
Family: Xyelidae
Newman, 1834
Genera

See text

The extant species occur in the Northern Hemisphere, especially in boreal regions of the Holarctic, though there are a few Oriental species. Considering additional fossil records from Australia,[6] South Africa[7] and Argentina,[8] the extant species display a relict distribution. The species of Xyelinae are associated with conifers (esp. Pinus and Abies), where the larvae feed on pollen or within buds, though larvae of Macroxyelinae feed on the leaves of deciduous trees (various Juglandaceae and Ulmus).

The family is characterized by the appendages of the head, which are remarkable in that the antennae and palpi are nearly leg-like in structure, with a long basal segment followed by a series of tiny segments, as in the tibia-tarsus.

ImagoEdit

Xyelidae represent small Hymenoptera. The prevailing number of species is 3–5 mm long, but species of Macroxyela and Megaxyela of East Asia and North America are larger measuring 10–15 mm. The imagines display a number of ground plan characters of Hymenoptera, which may be absent in more derived lineages of this order. Those include, for example, the absence of a wasp waist (thorax and abdomen abut without constriction), presence of cenchri on the metathorax to fix the wings at rest, presence of an antennal grooming apparatus on tibia and first tarsomere of the fore leg, and presence of a molar tooth on the mandible. Most intriguing is the morphology of the antenna which bears a long and thick third article followed by a number of shorter and more slender antennomeres. This so-called synantennomere 3[4] is the product from the ontogenetic fusion of several antennal articles, and it is unique among the extant Hymenoptera species. In Pleroneura, Xyelecia and most species of Xyela the maxillary palps are strongly enlarged and bear specialized setae on the distal articles. The wing venation is the richest among Hymenoptera: Only in Xyelidae the radial sector Rs furcates into the veins Rs1 und Rs2, while in other Hymenoptera Rs1 is absent. The females bear a more or less long ovipositor, which in some species of Xyela may be as long as the body. Morphology of the ovipositor and the ovipositor sheath are often decisive for the identification at species level. The penis valves of the males are densely setuous, which is a rare character state among the basal lineages of Hymenoptera. Females and males mate with the bodies directing in opposite direction. In Xyelinae the genital capsule of the males are revolved for 180° after disclosure from the pupal skin (strophandry). Macroxyelinae are orthandrous after emergence. They mate in the same position as Xyelinae, but the male genital capsule is rotated yet in course of mating ("facultative strophandry").[9]

LarvaEdit

Alike in many other sawflies, the larvae of Xyelidae are superficially similar to caterpillars of Lepidoptera ("erucoid" type of larvae). Larvae of species feeding inside plants are whitish, those of free-feeding species whitish green or yellow. Larvae of Megaxyela bear a conspicuous pattern of black spots (see plate 21 figure 3 in[5]) or they resemble bird droppings.[10][11] The roundish head capsule bears a larval eye (stemma) on each side, which is reduced in mining species, and short antennae comprising five articles. The thorax bears short legs comprising three articles. The abdomen bears prolegs on all segments. Opposite to Xyelidae, in larvae of all other Hymenoptera and Lepidoptera legs are absent at least on the first abdominal segment. In free feeding Xyelidae (Macroxyela, Megaxyela) the abdominal prolegs are conspicuous and consist of two articles, while in the mining species (Pleroneura, Xyela) they are reduced to inconspicuous transverse bulges.

PupaEdit

The pupa of Xyelidae represents the so-called pupa dectica, in which the antennae, legs and mandibles are free and mobile.[4] Actually, this stage represents the already developed ("pharate") imago which is still enclosed by the pupal skin. At this stage, the wings are still not expanded, and in females, the ovipositor and its sheath are still curving dorsally above the tip of the abdomen. Such a pupa dectica is capable to bite open the cocoon, to dig to the surface of the ground and to run around on the surface and to drink (,[12] figs 22-23 in[13]). The pupa dectica is a ground plan character of endopterygote insects. Outside Hymenoptera, it occurs in numerous other groups like Neuropterida, Mecoptera, Trichoptera and in the basal lineages of Lepidoptera.[14][15][16] The Hymenoptera excluding Xyelidae are characterized by an immobile pupa adectica.

EcologyEdit

The larvae of all Xyelidae are phytophagous and associated with trees. Larvae of the comparatively species-rich Xyela live inside the growing staminate cones of pines (Pinus) and feed on the sporophylls and the pollen. The North American Xyela gallicaulis is exceptional in causing galls on fresh shoots of some pine species, inside which the larva feeds.[17] Larvae of Pleroneura feed inside young shoots of firs (Abies). Only the Japanese Pleroneura piceae is associated with spruce (Picea).[18] Larvae of Macroxyelinae are free feeders of deciduous tree species. The two North American species of Macroxyela feed on elms (Ulmus), the East Asian and North American species of Megaxyela on Juglandaceae like walnuts (Juglans), hickory (Carya) and wingnuts (Pterocarya).[19][20][21] For Xyelecia nearctica an endophagous life style and an association with firs is supposed.[22]

For many species of Xyelidae, only a single larval host plant is known. Monophagy is the prevailing life style in most species of Xyela.[23][24] Here, the proper date of oviposition is closely correlated with the development of the staminate cones of the host pines, which might prevent a host shift in many cases. For some North American species of Xyela the association with each several species of pines (oligophagy) has been reported in literature,[12] but supposedly such records are often based on taxonomic problems to separate the morphologically very similar species of Xyela properly.[4] Through molecular access, true oligophagy could be demonstrated recently for Xyela bakeri (associated at least with Pinus contorta, Pinus ponderosa and Pinus sabiniana)[25] and Xyela brunneiceps (associated with Pinus flexilis and Pinus strobiformis).[26] Lack of appropriate, reproducible host data might pretend monophagy or oligophagy in many taxa of Xyelidae, but at least part of the species included in Megaxyela and Pleroneura are truly oligophagous.[21][27]

After completing feeding, the larva digs into the ground and forms an earthen cell, where it usually spins a cocoon and develops as a pupa. Absence of a cocoon has been stated for Megaxyela togashii from Japan.[11] The imago emerges yet during next spring to mate. Subsequently, the female deposits eggs with the ovipositor. In most species of Xyela, the tip of valvula 3 of the ovipositor sheath bears a sensilla plate equipped with specialized sensilla trichodea and sensilla campaniformia, which is involved into the oviposition process.[13][24][26] In Pleroneura, opposite to practically all other Hymenoptera, the hard and conical oviositor sheath is used in addition to the ovipositor proper to penetrate the resinous buds of firs.[13] Megaxyela gigantea, alike most other species of Megaxyela, has strikingly long hind legs, which are used to fold a newly growing leaf, where the egg is fixed with glutinous material between on the upper sides of the folded leaf, providing a shelter for the egg.[28][20][11]

Many species of Xyelidae facultatively diapause for several years. In Xyela alpigena and Xyela obscura a diapause of at least two years is obligate. This strategy might have evolved to escape from the risk that well developed cones are unavailable for oviposition: Pinus cembra vs. Pinus mugo, hosts of those two Xyela species, are bearing cones very irregularly in the subalpine zone of the European Alps in course of subsequent years.[29][13][24]

Imagines of Xyela are pollen feeders of a variety of plants, which bear flowers with easily accessible pollen (e.g., Betula, Cercocarpus, Ostrya, Pinus, Purshia, Quercus, Salix[12][24]). The enlarged maxillary palps of most Xyela (and supposedly also of Pleroneura and Xyelecia) serve for the extraction of pollen from the flowers.[12][4]

Although being associated with often economically important tree species, Xyelidae are usually of only small significance as pest organisms. Pleroneura piceae damages the growth of Sakhalin spruce (Picea glehnii), since the larvae destroy the young shoots.[30] Larvae of Megaxyela major (and supposedly also of Megaxyela langstoni) feed on leaves of pecan (Carya illinoinensis) and are regarded as a pest of pecan plantations in the Southeast of the USA.[31][32][33][21]

Systematics and taxonomyEdit

Xyelidae represent the most basal lineage of Hymenoptera and very likely the sister taxon of all other extant Hymenoptera. This position results from phylogenetic analyses of both morphological characters and DNA sequences.[34] The great age of the family is supported by numerous fossil records. All Hymenoptera recorded from the Triassic are classified with Xyelidae, while representatives of other hymenopterous families have occurred earliest during the Jurassic. During the Mesozoic and the Tertiary, the Xyelidae obviously were much more species-rich and more widely distributed than today. Thus the comparatively few extant species can be regarded a relict group.

Extant taxaEdit

The Electronic World Catalog of Symphyta[2] provides a complete account of the valid extant genera and species, their synonyms, the concerning references to original descriptions and distribution data on the level of countries and provinces.

The European species can be identified with help of „The Western Palaearctic Xyelidae“ of Blank (2002),[13] all Eurasian species of Xyela with Blank et al. (2013).[24] The North American Macroxyelinae were revised by Smith & Schiff (1998),[19] the North American species of Xyela by Burdick (1961),[12] the North American species of Pleroneura by Smith et al. (1977),[27] the East Asian species of Megaxyela by Shinohara (1992),[20] the East Asian species of Pleroneura by Shinohara (1995),[18] and the species of Megaxyela of the World by Blank et al. (2017).[21]

Extinct taxaEdit

The Electronic World Catalog of Symphyta[2] provides a complete account of the valid fossil genera and species, their synonyms and the concerning references to original descriptions.

Genus without classification into a subfamily of Xyelidae

Archexyelinae

Macroxyelinae

Madygellinae

Xyelinae

  • Genera without classification into a tribe of Xyelinae
    • Aequixyela Wang, Rasnitsyn & Ren, 2014, 1 fossil species
    • Cathayxyela Wang, Rasnitsyn & Ren, 2014, 1 fossil species
  • Liadoxyelini
  • Xyeleciini
    • Microxyelecia Rasnitsyn, 1969, 1 fossil species
    • Uroxyela Rasnitsyn, 1966, 1 fossil species
    • Xyelecia Ross, 1932, 1 fossil species in addition to extant species
    • Xyelites Rasnitsyn, 1966, 2 fossil species
  • Xyelini
    • Enneoxyela Rasnitsyn, 1966, 4 fossil species
    • Eoxyela Rasnitsyn, 1965, 5 fossil species
    • Spathoxyela Rasnitsyn, 1969, 2 fossil species
    • Xyela Dalman, 1819, 7 fossil species in addition to extant species, including Xyela (Pinicolites) Meunier, 1920
    • Xyelisca Rasnitsyn, 1969, 1 fossil species
    • Yanoxyela Ren, Lu, Guo & Ji, 1995, 1 fossil species

ReferencesEdit

  1. ^ Wang, Yan-hui; Engel, Michael S.; Rafael, José A.; Wu, Hao-yang; Rédei, Dávid; Xie, Qiang; Wang, Gang; Liu, Xiao-guang; Bu, Wen-jun (2016). "Fossil record of stem groups employed in evaluating the chronogram of insects (Arthropoda: Hexapoda)". Scientific Reports. 6: 38939. doi:10.1038/srep38939. PMC 5154178. PMID 27958352.
  2. ^ a b c Taeger, A., Liston, A.D., Prous, M., Groll, E.K., Gehroldt, T. & Blank S.M. 2018: ECatSym – Electronic World Catalog of Symphyta (Insecta, Hymenoptera). Program version 5.0 (19 Dec 2018), data version 40 (23 Sep 2018). Senckenberg Deutsches Entomologisches Institut (SDEI), Müncheberg.
  3. ^ Taeger, A., Blank, S.M. & Liston, A. D. 2006: European Sawflies (Hymenoptera: Symphyta) – A Species Checklist for the Countries. Pp. 399-504. In: Blank, S.M., Schmidt, S. & Taeger, A. (eds): Recent Sawfly Research: Synthesis and Prospects. Goecke & Evers, Keltern.
  4. ^ a b c d e f Blank, S.M. 2002: Biosystematics of the extant Xyelidae with particular emphasis on the Old World taxa (Insecta: Hymenoptera). Dissertation, Freie Universität Berlin.
  5. ^ a b Taeger, A.; Blank, S.M.; Liston, A.D. 2010: World Catalog of Symphyta (Hymenoptera). Zootaxa 2580: 1-1064.
  6. ^ Riek, E. F. 1955: Fossil insects from the Triassic beds at Mt. Crosby, Queensland. Australian Journal of Zoology 3: 654-690.
  7. ^ Schlüter, T. 2000: Moltenia rieki n. gen., n. sp. (Hymenoptera: Xyelidae?), a tentative sawfly from the Molteno Formation (Upper Triassic), South Africa. Paläontologische Zeitschrift 74(1/2): 75-78.
  8. ^ Lara, M. B., Rasnitsyn, A. P. & Zavattieri, A. M. 2014: Potrerilloxyela menendezi gen. et sp. nov. from the Late Triassic of Argentina: The Oldest Representative of Xyelidae (Hymenoptera: Symphyta) for Americas. Paleontological Journal 48(2): 182-190.
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  34. ^ Sharkey, M.J., Carpenter, J.M., Vilhelmsen, L., Heraty, J., Liljeblad, J., Dowling, A.P.G., Schulmeister, S., Murray, D., Deans, A.R., Ronquist, F., Krogmann, L., Wheeler, W.C. 2012: Phylogenetic relationships among superfamilies of Hymenoptera. Cladistics 28 (2012) 80-112. doi:10.1111/j.1096-0031.2011.00366.x
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  39. ^ Vieillot, L.P. 1807–1809 Histoire naturelle des oiseaux de l'Amérique septentrionale, contenant un grand nombre d'espèces décrites ou figurées pour la première fois. Chez Desray, Paris, tome 1: [7] + [i]–iv + [1]–90, tabs 1–57, tome 2: [4] + [i]–ii + [1]–74, tabs 58–124. doi:10.3931/e-rara-7221
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