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Ectal mandibular glands of insects are exocrine glands localized in the anterior–lateral region of the head, close to the base of the mandibles. In social wasps, the gland is composed of secretory cells and a reservoir into which the secretion accumulates. At the time of emission the secretion flows onto a specialized cuticular area on the outside of the base of the mandibles. Secretion of Polistes dominulus is emitted only when wasps are greatly disturbed or in the presence of predators, and its function seems to be mainly defensive. Morphometric studies did not reveal any size differences between the glands of the queens and those of the workers. GC-MS analyses of the glands identified 32 compounds, mainly acids and aldehydes in the range C2–C18. The overall odor, caused by the mixture of aldehydes, is distinct. Workers do not respond strongly to the odor. The secretion probably serves as a warning signal to vertebrate predators.