The occipital region in the basal Hymenoptera (Insecta): a reappraisal

The occipital region in representatives of all the 'symphytan' families and a few apocritans is investigated. A new character, the occurrence of occip ital 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 struct ure formed by the fusion of the apodemes for the profurco-postoccipital mus cles, forming a ventral connection between the tentorial arms, is present i n Cimbex, Cephoidea, and Syntexis. An external sclerotization, the hypostom al bridge, is present in Corynis, Cephoidea, 'Siricoidea', Orussoidea, and Apocrita. In Syntexis, the postoccipital and hypostomal bridges are continu ous. It is difficult to decide whether a hypostomal bridge is also present in the Pamphilioidea, or the external sclerotization in this superfamily ha s evolved independently. In the Siricidae, Orussidae, and some Apocrita, a postgenal bridge largely replaces the hypostomal bridge. The postgenal brid ge has evolved at least twice independently in the Hymenoptera.