When putting sprays of nettle or bramble into cages it is useful to cut long lengths, particularly of bramble, so that you can wrap it around the inside, ensuring that the top and bottom of the cage has some leaf material available - particularly the side of the cage which is nearest to the light source. This will ensure that larvae will find the foodplant. When the larvae have fed-up and reached maturity they will wander about the place, show little interest in food, and spend long periods being quite still. Prior to pupating they darken and shrivel slightly. Do not panic.
Preparation is all.
Larvae will spin loose webs and then bunch - usually in an upper corner of the cage frequently across the entrance. Individuals may pupate anywhere so check all material to be removed. May. It lasts for approximately ten to fifteen days. I have no evidence of cannibalism in this species - but when rearing the Garden tiger, Arctia caja, I have seen soft pupae eaten by the larvae if there is a scarcity of foliage. So make sure that there is plenty of the larval foodplant - just in case.