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Danaus plexippus
We think this is a monarch caterpillar but it was very small and not on milkweed.
On leaves of a vine plant by the fence of our pond area, in our school yard.
Actually, the vine visible in the photo is Sandvine, also known as Honeyvine milkweed. It's a common host plant for monarchs, especially later in the summer.
This looks like a Monarch. Please update your spotting to reflect the ID so your spotting can be considered complete. Here is a link that you can use to compare. http://bugguide.net/node/view/540
Looks like Monarch - if it were a Queen it would have an extra pair of 'horns' after the front pair.