Inter- and intraspecific chemical cues are known to be dominant factors in aquatic ecosystems, as they overlay trophic food webs as a third dimension. Very often their chemical identity is unknown. It has long since been known, that predators release chemical cues (kairomones) that prey use as a predictor of an increased predation risk. I was able to identify and synthesise the kairomones released by the predatory phantom midge larvae Chaoborus . This composition of chemicals is involved in Chaoborus digestive processes which explains why they are released despite the disadvantage for its emitter. The knowledge of the active agent of course now facilitates the elucidation of the signalling cascade underlying predator perception.