Ants certainly can crawl backwards, as anyone knows who has seen an ant accidentally get something nasty on its antennae. However, most soecies of ants have little occasion to do so. Some insects can hardly crawl at all, also having little need, such as adult dragonflies. Mostly they fly wherever they want to go. I deny however, the claim that they actually cannot crawl at all. I have checked it a couple of times personally.

As for other insects crawling backwards, yes -- most don't, but as far as I can tell, most can. In particular, species that live in tunnels and tubes, such as the Australian "sand gropers" (Cylindrachetidae), and then there are mole crickets, wood-borers, larvae of Chironomid midges, and many more that do so routinely.