The saddleback caterpillar is the larva of a species of moth. 

Saddlebacks have a green body that is brown on both ends with a white-ringed brown spot in the center.  The spot resembles a saddle, thus giving these critters their common name.  There are horns on the front and rear of the body that are covered with urticating hairs which can secrete an irritating venom.  Coming into contact with these hairs can result in a painful sting.  Redness and swelling are common at the site of the sting and can last for several days. 

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There are about eleven families of moths and butterflies whose larvae, and even some adults, have urticating hairs.  The hairs serve as a defense system against predators.  After being stung once, a potential predator has probably been sufficiently scared away!  Plants can have these stinging hairs, too.  If you have ever brushed up against stinging nettles, you have likely felt how unpleasant these hairs can be.

The photos of the saddleback caterpillar below were taken at the Waterloo Mills Preserve.  There were several caterpillars in the vicinity when this photo was taken, one of which had just stung one of our staff members, inspiring today's post.