Abstract
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 Philanthus triangulum, 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 Trachypus, which shows a life-history similar to Philanthus. We investigated whether Trachypus spp. also possess PPGs and embalm larval provisions. Using histological methods and 3D reconstructions we show that Trachypus boharti and T. elongatus possess PPGs that are similar to P. triangulum 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 Trachypus contain mainly unsaturated long-chain hydrocarbons. The gland of T. boharti additionally contains long-chain ketones. The hydrocarbons from the PPG of T. elongatus 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 Trachypus species under study possess PPGs that are very similar to the PPG of P. triangulum with regard to morphology, ultrastructure and chemistry. Moreover, we provide clear evidence that T. elongatus females embalm their prey, presumably as a means of prey preservation. The observed differences among Trachypus and Philanthus in gland size and prey embalming may have evolved in response to divergent ecological conditions.
Highlights
Pathogenic and competing microbes prevail everywhere that animals live
Trachypus boharti (Rubio) and Trachypus elongatus (Fabricius), for the presence, morphology, and ultrastructure of the postpharyngeal glands (PPG) by light microscopy, transmission electron microscopy and 3D-reconstruction of cephalic structures based on semi-thin sections
Morphology and ultrastructure of Trachypus PPGs The histological investigations revealed that females of both
Summary
Pathogenic and competing microbes prevail everywhere that animals live. This permanent threat has given rise to a remarkable diversity and complexity of animal adaptations to protect themselves, their offspring as well as their food against microbial attack [1,2,3,4,5,6,7,8].A solitary digger wasp, the European beewolf Philanthus triangulum Fabricius (Hymenoptera, Crabronidae), for example shows several intriguing adaptations to such kinds of threats. To protect the provisions and their brood against infestation by microbes, P. triangulum females embalm their prey with large amounts of hydrocarbons (HCs) [7,10,11,12,13]. This prey embalming considerably increases the total amount of HCs on the prey bee surface as well as the proportion of unsaturated and shorter-chain HCs [10,11,12]. A second benefit of the thick layer of HCs caused by the embalming is a reduction of water loss from the prey bees that helps the beewolf larva to resist desiccation under hot and dry conditions [12]
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