Structure, chemical composition and putative function of the postpharyngeal gland of the emerald cockroach wasp, Ampulex compressa (Hymenoptera, Ampulicidae).

The postpharyngeal gland (PPG) plays a major role in the social integration of ant colonies. It had been thought to be restricted to ants but was recently also described for a solitary wasp, the European beewolf (Philanthus triangulum). This finding posed the question whether the gland has evolved independently in the two taxa or has been inherited from a common ancestor and is hence homologous. The latter alternative would be supported if a PPG was found in more basal taxa. Therefore, we examined a species at the base of the Apoidea, the solitary ampulicid wasp Ampulex compressa, for the existence of a PPG. Both sexes of this species possess a cephalic gland that branches off the posterior part of the pharynx, is lined by a cuticular intima and surrounded by a monolayered epithelium with the epithelial cells bearing long hairs. Most of these morphological characteristics conform to those of the PPG of ants and beewolves. Chemical analysis of the gland content revealed that it contains mainly hydrocarbons and that there is a congruence of the pattern of hydrocarbons in the gland, on the cuticle, and in the hemolymph, as has also been reported for both ants and beewolves. Based on these morphological and chemical results we propose that the newly described cephalic gland is a PPG and discuss its possible function in A. compressa. The present study supports the view of a homologous origin of the PPG in the aculeate Hymenoptera.