The occipital region in the basal Hymenoptera (Insecta): a reappraisal LARS VILHELMSEN Accepted 22 December 1998 Vilhelmsen, L. (1999) The occipital region in the basal Hymenoptera (Insecta): a reappraisal. Ð Zoologica Scripta 28, 75±85. The occipital region in representatives of all the `symphytan' families and a few apocritans is investigated. A new character, the occurrence of occipital sulci, is described. The absence of occipital sulci and corresponding internal ridges is a putative synapomorphy for the Xiphydriidae, Orussoidea, and Apocrita. Independent loss has occurred in the cimbicid genus Zaraea. The evolution of sclerotizations between the occipital and oral foramina in basal Hymenoptera is discussed. A postoccipital bridge, an internal structure formed by the fusion of the apodemes for the profurco-postoccipital muscles, forming a ventral connection between the tentorial arms, is present in Cimbex, Cephoidea, and Syntexis. An external sclerotization, the hypostomal bridge, is present in Corynis, Cephoidea, `Siricoidea', Orussoidea, and Apocrita. In Syntexis, the postoccipital and hypostomal bridges are continuous. It is difficult to decide whether a hypostomal bridge is also present in the Pamphilioidea, or the external sclerotization in this superfamily has evolved independently. In the Siricidae, Orussidae, and some Apocrita, a postgenal bridge largely replaces the hypostomal bridge. The postgenal bridge has evolved at least twice independently in the Hymenoptera. Lars Vilhelmsen, Zoological Museum, University of Copenhagen, Universitetsparken 15, DK-2100 Kùbenhavn, Denmark. E-mail: lbvilhelms@zmuc.ku.dk Introduction With the exception of the Xyeloidea and most Tenthredi- noidea, all Hymenoptera posses a sclerotization between the occipital and oral foramina. Ross (1937) was the ®rst to describe the considerable variation in the con®guration of this sclerotization among the `symphytan' superfamilies. Rasnitsyn (1969) provided additional information. It is evident from these earlier treatments that it is dif®cult to interpret the evolution of the different sclerotizations and the transformation series possibly linking them. This is highlighted when trying to make sense of them in a phylo- genetic context, as has been attempted by Rasnitsyn (1988) and Vilhelmsen (1997). The discrepances in interpretation are evident when comparing these papers (Ronquist et al., 1999). In addition, the terminology employed by different authors is not always clearly de®ned and hence somewhat confusing. In this paper, I have undertaken a more detailed investi- gation of the occipital region in the `Symphyta'. This has been done in order to try to resolve the differencies in interpretation and terminology between earlier treatments. Furthermore, a couple of hitherto overlooked features providing phylogenetically relevant information are described. Material and methods Whole mounts. Both ethanol preserved and pinned speci- mens were examined, the latter when the former could not be obtained. Specimens were dissected with scalpels and eyelens scissors; some were macerated in KOH prior to examination. They were stored in glycerol and examined under a binocular dissection microscope. SEM. After removal of mouthparts, heads were dehy- drated in an ethanol-benzene series. They were freeze- dried, mounted with double-addhesive tape on stubs and coated with gold prior to examination. Histological sections. Only specimens preserved in alcohol, Pampels ¯uid or Bouins ¯uid were used. Specimens were dehydrated in butanol and embedded in paraplast. Sagittal sections of 7±20 mm thickness, depending on the size of the specimen, were prepared using a microtome. The sections were mounted on slides; they were stained with a Q The Norwegian Academy of Science and Letters . Zoologica Scripta, 28, 1±2, 1999, pp75±85 75