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Chemical attraction and deception. Intra- and interspecific communication in Hymenoptera

  • 590 Tiere (Zoologie)


Communication is an essential process for intra- and interspecific interactions among organisms. It may or may not be advantageous for both the sender and the receiver of the signal. Honest signals may be seen in intraspecific sexual communication as well as in interspecific signals like aposematic coloration. However, deceptive signaling is a common phenomenon. While female sex pheromones have been extensively studied in several insect taxa, male sex pheromones have as yet received comparatively little attention despite their significance for honest mate assessment and mate choice. Chemical signals used for interspecific communication are called allelochemicals, and such signals have been described from numerous taxa. Males of a digger wasp, the European beewolf Philanthus triangulum (Hymenoptera, Crabronidae), establish small territories in the vicinity of female nests and apply a sex pheromone from a cephalic gland onto the territory substrate to attract receptive females. The chemical composition of the gland content is characterized using gas chromatography � mass spectrometry (GC-MS): The pheromone comprises a complex blend with (Z)-11-eicosenol as the main compound, other substances with functional groups and, an array of hydrocarbons. 3D-reconstructions on the basis of NMR imaging data and histological investigations reveal that the pheromone is stored in the postpharyngeal gland (PPG). The PPG is extraordinarily large and makes up about one quarter of the head capsule volume. The large amount of pheromone produced by the males suggests that the amount of pheromone produced is vital for the reproductive success of males and constitutes the result of strong sexual selection. Consequently, quantitative aspects of the pheromone may allow female mate choice according to a �good genes� model. In addition, the composition of the beewolf pheromone is demonstrated to vary qualitatively with, and, thus, to bear information about the geographical origin and kinship of males. This variation might enable females to choose their mate according to a model of �optimal outbreeding�. Complementing the investigations of the male sex pheromone, the territorial behavior of beewolf males and the spatial distribution of male territories with respect to female nesting sites is characterized. Spatial statistics reveal that male territories cluster around female nesting sites suggesting that the latter constitute �hotspots� for the formation of male leks. This implies that European beewolves exhibit a �hotspot� lek polygyny as the prevailing mating system. As a contribution to the scarce knowledge of male sex pheromone in insects, a new male-specific substance and the place of production is described for the cuckoo wasp Hedychrum rutilans (Hymenoptera, Chrysididae). The second part of this thesis deals with the interspecific chemical communication in insects. The significance of cuticular hydrocarbons (CHCs) for the interaction of females of the cuckoo wasp H. rutilans and its host, the European beewolf, is investigated. First, it is demonstrated that relatively large amounts of CHCs of beewolf females can be found in the material of female nest mounds. The presence of these nest-hydrocarbons is exploited by cuckoo wasp females as a kairomone for the localization and identification of the host nest. The cuckoo wasps subsequently enter the beewolf nest for oviposition, where they may be attacked by beewolf females. Chemical analyses reveal that cuckoo wasp females mimic the composition of the CHCs of their hosts, and behavioral assays suggest that the cuckoo wasp gains a great advantage from mimicking the composition of the CHCs of its host: the chemical mimicry significantly reduces olfactory detection by the beewolf females and therefore the incidence of host attacks inside the beewolf nest. Additionally, a possible second mechanism to evade olfactory detection by the beewolves is described: H. rutilans females have a drastically reduced amount of CHCs per mm² of the cuticle as compared to their hosts. This low amount of CHCs might be a quantitative means to escape olfactory detection and might represent a �chemical insignificance� strategy. However, the reduced amount of CHCs present on the cuticle might impose the risk of increased water loss on the cuckoo wasp. The beautifully iridescent cuticle of H. rutilans is investigated using Scanning Electron Microscopy, reflectance spectral analysis, and theoretical modeling. The analyses reveal a multilayer reflector in the exocuticle responsible for the coloration. Contributions of the multilayer to secondary functions like thermoregulation and mechanical strength are discussed.

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