This is why the insecticide for fleas is so persistent. Flea eggs can remain viable for years, and have the remarkable ability to hatch almost instantly when a target host moves close enough. A persistent toxin allows most if not all of the fleas to hatch out, then die out, thereby cleaning out the stock of unhatched eggs. In my case, having treated the cat with a suitable parasite killer (frontline I seem to remember), I never saw another flea, and that's in about 11 years.

A few months later the wolf spiders came back, and they have done a very good job of getting rid of everything else that's smaller than them (which is almost everything other than myself)

pca