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