Fig. 2. Identification of rumen volatiles that stimulate olfactory receptor cells of ticks. Cattle rumen volatiles were analysed after collection on a porous polymer by gas chromatography coupled with electrophysiology recordings from olfactory receptor cells in wall-pore olfactory sensilla on the first leg tarsus of Amblyomma variegatum and Ixodes ricinus. (A) Flame ionisation detector (FID) response of the gas chromatograph to the separated rumen volatiles. (B) Enlarged sections of A where butanoic (A₁), isopentanoic (A₂), pentanoic (A₃) and hexanoic (A₄) acids, 2-nitrophenol (P₁), 4-methyl-2-nitrophenol (P₂), 4-methylphenol (P₃), indole (I₁) and 3-methylindole (I₂) eluted and induced electrophysiological (EL) responses from olfactory receptor cells of A. variegatum and I. ricinus (trace for A. variegatum presented). The receptor cell response profiles in B were generated by summing the frequencies of the action potentials of the responding sensory cells (frequency to voltage conversion). U indicates an unidentified stimulus. (C,D) The spike trains generated in responding (from the arrows) A. variegatum (C) and I. ricinus (D) receptor cells to 4-methylphenol, butanoic acid and indole eluting from the gas chromatographic column. Receptor cells from both tick species also responded to acetic acid, propionic acid, phenol and 2-methylphenol (data not presented). The same carboxylic acid and phenolic volatiles identified in rumen odour of cattle were also recovered in the odour of roe deer rumen. Sensillum identity (DI and DII types) is according to a classification by Hess and Vlimant (1982).