Butterflies and moths are often beautiful but their larval stage can look quite scary.
This scary appearance is intentional. The spines and horns warn off predators that would otherwise eat them. Unfortunately, this gets caterpillars into trouble with people sometimes.
I vividly remember a summer afternoon when I was about five years old. I was playing on our back patio when I suddenly noticed a very large, very scary caterpillar. I backed away from it, screaming and crying. My dad came over to see what was the matter, promptly killed the caterpillar and comforted me. "It's OK. It's dead."
Since that time I've learned to appreciate nature, but I'm still not wild about insects. I love butterflies and moths and would not kill their caterpillars (except, perhaps, gypsy moth caterpillars), but I prefer to keep a safe distance from the scary ones.
The scary parts are often harmless but without a caterpillar education I don't know when they aren't. The hickory horned devil (Citheronia regalis), shown above, is perfectly harmless but you have to be sure of its identification before you pick it up. Look at these photos!
Even with a caterpillar education, it's going to be a while before you see me doing this:
(This is a tobacco hornworm. For more photographs of caterpillars and moths, see Chuck Tague's photo galleries.)
(photos by Chuck Tague)