1. Department of Animal & Plant Sciences; University of Sheffield; Sheffield; S10 2TN; U.K
2. Department of Biosciences; University of Helsinki; PO Box 65 (Viikinkaari 1); 00014; Helsinki; Finland
3. Chemical Ecology Group, School of Physical and Geographical Sciences, Lennard-Jones Laboratory; Keele University; Staffordshire; ST5 5BG; U.K