Back at the beginning of June orange tip butterflies were laying eggs on Jack-by-the-hedge (aka garlic mustard, aka hedge garlic) in our garden and since then the caterpillars that hatched have been doing pretty well.
In their early stages the tiny caterpillars are pale yellow but after a couple of weeks ....
.... they become grey-green and are quite well hidden when they align themselves with their host plant's seed pods. At this stage the prominent hairs on their segments are each tipped with a tiny droplet of secretion, which I suspect is something that repels potential parasites, like ichneumon flies. It doesn't deter wasps though, which I have watched carrying away small caterpillars. I once watched a wasp chew a larger orange tip caterpillar in half and then carry each bite-sized chunk back to the nest.
All the butterfly books mention the cannibalistic tendencies of orange tip larvae, and the way in which this butterfly only lays one egg per inflorescence, but this year I've seen several hedge garlic plants with two or even three caterpillars eating seed pods in what was the same inflorescence, where the butterfly must have laid multiple eggs. I wonder whether this was due to the late flowering of the host plant and the need for the butterflies to lay a lot of eggs quickly. It may be that they are more likely to do this in plants like hedge garlic that produce a lot of large, succulent pods, unlike the alternative and much smaller food plant lady's smock, where the food supply from each inflorescence is much less.
A caterpillar like this, that's approaching maturity, easily demolished one of these three inch-long hedge garlic seed pods in a day. Then, when they are fully fed, the larvae vanish, crawling away to pupate. We've had a thriving orange tip colony breeding in the garden for many years and I have only once found a pupa, suspended from a dead plant stem by a silken halter and extremely well camouflaged, with an outline that resembled a thorn.