Xyelidae are to be regarded as living fossils since they represent one of the oldest lineages of insects and include still extant forms.

[5][6] Considering additional fossil records from Australia,[7] South Africa[8] and Argentina,[9] the extant species display a relict distribution.

This so-called synantennomere 3[6] 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 penis valves of the males are densely setulous, which is a rare character state among the basal lineages of Hymenoptera.

Larvae of Megaxyela bear a conspicuous pattern of black spots (see plate 21 figure 3 in[4]) or they resemble bird droppings.

[11][12] The roundish head capsule bears a larval eye (stemma) on each side, which is reduced in mining species, and short antennae comprising five articles.

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.

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.

[27] Lack of reliable, reproducible host data could cause inaccurate reports of monophagy or oligophagy in many taxa of Xyelidae, but at least some of the species included in Megaxyela and Pleroneura are truly oligophagous.

[22][28] After completing feeding, Xyelidae larvae dig into the ground and form an earthen cell, where they usually spin a cocoon and develop into a pupa.

All Hymenoptera recorded from the Triassic are classified as Xyelidae, while representatives of other hymenopterous families have been found no earlier than the Jurassic.

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.

[22] 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.