Caterpillar

What's this curiosity that I found peeking out from beneath a leaf on one of my plants?

It's only a caterpillar. But what an odd caterpillar. First time I've ever seen one of this shape. I wonder what the mature insect looks like? Can only be a butterfly or a moth, this much I know.

In fact, there were two of them making a feast of my plant's leaves

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