Reply to Meg
These are the caterpillars of the large white butterfly. Unlike moth caterpillars, most of which burrow into the soil to make their silken cocoons, butterflies choose a stout grass stem, or some other secret spot above ground and make a chrysalis against it. They will often crawl several metres away to find a safe and sheltered spot. You can sometimes find them at the bottom of fences or under window wills.