Head anatomy of Xyelidae (Hexapoda: Hymenoptera) and phylogenetic implications

External and internal head structures of Macroxyela ferruginea (Say) and Xyela julii (Brébisson) were examined. A detailed description is provided for Macroxyela. The results are compared to the conditions found in other basal hymenopterans and representatives of other groups of endopterygote insects. Hitherto unnoticed autapomorphies of Hymenoptera are the concavity of the posterior head capsule, the very dense, regular vestiture of hairs, the collar-like, strongly developed posterior tentorium, and a large epipharyngopharyngeal lobe. Microphagous habits and associated features (asymmetric mandibular molae, epipharyngeal brush, infrabuccal pouch) are possibly groundplan features of Hymenoptera and Endopterygota. A switch to more or less liquefied food took place early in the evolution of Hymenoptera. The sitophore plate and a constricted, elongated prepharyngeal tube are likely synapomorphies of Hymenoptera and Mecopterida. Monophyly of Hymenoptera excluding Xyelidae is supported by the reduction of the mandibular molae and epipharyngeal brush. These changes are likely related to modified feeding habits. Widely separated mandibular bases, the loss of the median labral retractor (parallel loss in Xyelidae), and the presence of a hypostomal bridge are potential apomorphies of Hymenoptera excluding Xyelidae and Tenthredinoidea. Monophyly of Xyelinae and Macroxyelinae, respectively, is well supported by the results of our study. There is conflicting evidence as to whether Xyelidae is monophyletic. The presence of a subdivided galea is a putative autapomorphy of the family. The presence of unsclerotised paraglossae with dense fringes of thin hairs and the presence of a muscle connecting the anterior tentorial arm with the posterior edge of the sitophore plate are features shared by Xyelinae and members of non-xyelid families.