The invention relates to compounds of the general formula (I), wherein R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5, R.sub.6, R.sub.7, R.sub.8, R.sub.8', R.sub.9, W, a, b and n have the significances given in claim 1, and optionally the enantiomers thereof. The active ingredients have advantageous pesticidal properties. They are especially suitable for controlling parasites on warm-blooded animals. ##STR00001##