General terms describing skeletal structure are defined and illustrated. Many of these terms have not been properly defined or consistently used previously in the Hymenoptera literature. Terms employed for specific structures are adjusted to fit the general terminology. New terms are proposed for certain structures of widespread occurrence in Apocrita.
The skeletal structure of Ibalia rufipes Cresson is described and illustrated with the aim of providing a foundation for phylogenetic studies of Cynipoidea. An Ibalia species was chosen for this purpose because Ibalia includes the largest species among the Cynipoidea and because it is considered to have retained many of the ground-plan characters of this superfamily.
Ibalia is also characterized by several specialized structures considered to be adaptations for exiting the tunnel of its wood-boring siricid host. Several features of the modified first flagellomere of the male antenna strongly suggest that it is the site of glands producing a sex pheromone.
The skeletal structure of I. rufipes is discussed in relation to that of other Hymenoptera. Many of the features described are reported for the first time in Cynipoidea or even Hymenoptera. Discrepancies between the results of this study and those of previous morphological studies are pointed out, and problems of interpretation are discussed.
It is concluded that study of internal structure is essential for a correct interpretation of external skeletal features, which are currently the most frequent character type used in systematic and phylogenetic studies of Hymenoptera.