Morphology, Chemistry and Function of the Postpharyngeal Gland in the South American Digger Wasps <i>Trachypus boharti</i> and <i>Trachypus elongatus</i>

<div><p>Microbes pose severe threats to animals as competitors or pathogens and strongly affect the evolution of life history traits like parental care. Females of the European beewolf <i>Philanthus triangulum</i>, a solitary digger wasp, provision their offspring with paralyzed honeybees and embalm them with the secretion from large postpharyngeal glands (PPG) that contain mainly unsaturated hydrocarbons. This coating changes the physico-chemical properties of the prey surface, causes a reduction of water condensation and retards growth of mold fungi. Here we examined the closely related South American genus <i>Trachypus</i>, which shows a life-history similar to <i>Philanthus</i>. We investigated whether <i>Trachypus</i> spp. also possess PPGs and embalm larval provisions. Using histological methods and 3D reconstructions we show that <i>Trachypus boharti</i> and <i>T. elongatus</i> possess PPGs that are similar to <i>P. triangulum</i> but somewhat smaller. The ultrastructure of the gland epithelium suggests that the gland content is at least partly sequestered from the hemolymph. Chemical analyses using gas chromatography / mass spectrometry revealed that both the cuticle and PPGs of <i>Trachypus</i> contain mainly unsaturated long-chain hydrocarbons. The gland of <i>T. boharti</i> additionally contains long-chain ketones. The hydrocarbons from the PPG of <i>T. elongatus</i> occurred on prey bees excavated from nests in the field but not on conspecific control bees. While the embalming only slightly elevated the amount of hydrocarbons on prey bees, the proportion of unsaturated hydrocarbons, which is crucial for the antifungal effect, was significantly increased. The <i>Trachypus</i> species under study possess PPGs that are very similar to the PPG of <i>P. triangulum</i> with regard to morphology, ultrastructure and chemistry. Moreover, we provide clear evidence that <i>T. elongatus</i> females embalm their prey, presumably as a means of prey preservation. The observed differences among <i>Trachypus</i> and <i>Philanthus</i> in gland size and prey embalming may have evolved in response to divergent ecological conditions.</p>