Not seen this before
Well I've seen tussock moth caterpillars before but not seen them do this. This one was/is on the underside of 2 beech tree leaves stuck together with silk. The first time I photographed it I notice an odd black line across it's back as if it had a split there. When I disturbed it some more whilst photographing it, the line widened into a gaping mouth and then slowly closed again.
Note the animation is actually playing backwards, it opens quickly and closes slowly.