I found two of this Schizura unicornis caterpillar devouring my apple. I have two apple trees, an 'Anna' apple and a 'Dorsett Golden' apple. Additionally, I've grafted a couple of branches of each onto the other. These caterpillars were munching on the 'Anna' branches of the 'Dorsett Golden' tree.
A unique feature of this caterpillar is the horn on the first abdominal segment. It's equipped with a gland that produces formic acid. They can spray formic acid several inches with that horn. Formic acid is what gives ants their generic name, Formica. Ants use it as a poison. If you've ever been bitten by a fire ant, you have experienced an injection of formic acid by an ant.