The invention relates to compounds of the general formula ##STR00001## wherein Ar.sub.1, Ar.sub.2, R.sub.4, R.sub.5, R.sub.6, R.sub.7, R.sub.8, W, X, a and b 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.