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Abstract

When the larvae of the caddis fly Apatania fimbriata (PICTET) are threatened drops of fluid appear on their head capsule. The excretions are produced in a prothoraic gland connected with an extrudable "cervical sac". The secretion was removed with a syringe and injected into the gas chromatograph together with the silylation agents. The secretion consisted mainly of fatty acids. The main components of the secretion are fatty acids with 12 and 14 C-atoms and with up to four double bonds. All together more than thirty different fatty acids have been analysed. In biotests synthetic acids (C6-C12) were used to test the reactions of potential predators in streams. Drops of the acids were released into the current some 10 cm upstream of the larvae. All tried to avoid the chemicals, shaking or nodding their heads, trying to clumb underneath stones or actively entering the drift. Following exposure larvae were paralysed and could not behave normally for at least a few minutes. 1/3 of Hydropsyche larvae, 2/3 of Rhyacophila larvae, and all Plectrocnemia larvae died within one day. In choice experiments performed in plastic boxes in the field Rhyacophila larvae preferred larvae of Agapetus fuscipes rather than Apatania fimbriata (n = 10, p <0.001, chi²-test). They selected Agapetus larvae when twice as many Apatania larvae were offered. When A. fimbriata and Drusus annulatus larvae were compared, larvae of the latter were preferred (n = 10, p <0.05, chi²-test). Larvae of the stonefly Dinocras cephalotes and the fish Cottus gobio did not discriminate between prey species. The fatty acids produced in a gland in the prothorax of Apatania fimbriata larvae act as an effective repellent against small predators in their natural environment. They are ineffective against large predators.