Xyelidae

Family of sawflies

The Xyelidae are a comparatively species poor family of sawflies, comprising about 80 extant species in five genera worldwide, and is the only family in the superfamily Xyeloidea. The fossil record of the family is extensive, comprising more than 120 species Considering additional fossil records from Australia, South Africa and Argentina, the extant species display a relict distribution. The species in the subfamily Xyelinae are associated with conifers (esp. Pinus and Abies), where the larvae feed on pollen or within buds. The larvae of the subfamily 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.

Xyelidae are small Hymenoptera. Most species are 3 to 5 mm long, but species of Macroxyela and Megaxyela of East Asia and North America are larger, measuring 10 to 15 mm. The imagines display a number of ancestral characters of Hymenoptera, which may be absent in more derived lineages of Hymenoptera. Those include 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

Like in many other sawflies, the larvae of Xyelidae are superficially similar to caterpillars of Lepidoptera ("eruciform" 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) or they resemble bird droppings. 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. Unlike the larvae of all other Hymenoptera and Lepidoptera which lack prolegs on at least the first abdominal segment, larvae of Xyelidae have prolegs on all abdominal segments. 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.

Unique among the Hymenoptera, the pupa of Xyelidae are of the decticous and exarate types, in which the antennae, legs and mandibles are free and mobile. This pupal form is an ancestral character of holometabolan insects. In other orders, it occurs in including Neuropterida, Mecoptera, Trichoptera and in the basal lineages of Lepidoptera. The Hymenoptera excluding Xyelidae are characterized by immobile adecticous pupae.

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 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. Larvae of Pleroneura feed inside young shoots of firs. Only the Japanese Pleroneura piceae is associated with spruce. Larvae of Megaxyela major (and supposedly also of Megaxyela langstoni) feed on leaves of pecan and are regarded as a pest of pecan plantations in the Southeastern USA. The great age of the family is supported by numerous fossil records. All Hymenoptera recorded from the Triassic are classified as Xyelidae, while representatives of other hymenopterous families have been found no earlier than 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.