Can someone ID the larva in this photo?

[Gerald Mines]

Hi everyone,

A friend sent me a photo of some caterpillars he found.  I was only able
to identify the green one as being a Spicebush Swallowtail larva.  I
could not identify the darker caterpillar with what appears to be a horn
on the posterior.  My friend was unable to tell me what it may have been
feeding on so I don't have any host plant information to pass along. 
The caterpillars were found near Palestine, Texas, USA.  Here is a link
to the photo:

	http://www.tconl.com/~mines/Images/MVC-005X.JPG

Thank you,

Gerald Mines
Omaha, Nebraska
http://www.tconl.com/~mines