If you see this caterpillar, give it a wide berth.  Contact with its spines causes pain as severe as a wasp sting.  These were found "the hard way" by friends.  The caterpillars were on Sweet Corn leaves in their backyard garden, but note that the species will feed on a wide variety of plants.  Note too that it's common to find them in numbers on the same plant and on both upper and lower leaf surfaces: if you see one, there are probably more lurking about.

 

Saddlebacks are in the Slug Caterpillar family Limacodidae.  All have abdominal prolegs reduced to flatttened suckers.  They move in a slug-like manner through an undulating motion of the lower body surface.  (You can see the motion in a related species here: http://www.performance-vision.com/airline2011/airline-summer-11l.htm )

 

The head is usually tucked under the front of the body (at right in these photos) and is rarely extended enough to be seen.  In addition to protection from birds and mammals, the spines - especially those projecting laterally - may deter (or impale) predatory insects like ants.

 

The well-armed head end (though the head is not visible).

 

 

 

The back end looks like an evil clown character.

 

In this 2008 phopto, you can clearly see pale rings at the base of each spine.  These are venom-filled reservoirs.  On one of the spines in the photo (upper middle), you can just make out what looks like a surface groove, presumably the means by which venom is channeled along the length of the shaft.  I'm not sure if capillary action keeps the spines always armed, or if the venom is released when the spines are bent by contact.  (I supect the former.)

 

One of my 2008 Saddleback caterpillar photos made the cover of the January, 2010 issue of the Journal of the American Academy of Dermatology.

 

Two articles by Eric W. Hossler, MD, from the "JAAD", the Journal of the American Academy of Dermatology, used my 2008 Saddleback photos and discussed stings and rashes caused by various Lepidoptera.  Interesting stuff!

Caterpillars and Moths - Part I: Dermatological manefestations of encounters with Lepidoptera

Caterpillars and Moths - Part II: Dermatological manefestations of encounters with Lepidoptera