Caterpillar Identification

[Smithmel]

I live in a slightly wooded suburban area outside of St. Louis,
Missouri, and today my brother and his friend found a caterpillar that
I have never seen before.  The friend accidentally brushed up against
the caterpillar and immediately experienced a painful tingling in his
arm and got a red rash with several large, raised bumps that subsided
after about 15 minutes.  The caterpillar is approximately one inch long
with no obvious segmentation. The body is new-leaf green with an oval
shape on its back ringed with white and grey-brown in the middle.  The
head,
abdomen and tail are the same grey-brown color.  On each side are six
tufts of hair.  The head has tufted antennae, and various tufts around
the face.  The tail is a mimic of the head with two bright yellow spots
that resemble eyes.  It was consuming a leaf when the boy brushed  up
against it.

Can anyone tell me what kind of caterpillar it might be?
Any help would be greatly appreciated as I am very curious.
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