Wheel Bug (aka Assassin Bug)

From Wikipedia: The bite of a wheel bug is very painful and may take months to heal (sometimes leaving a small scar), so caution is highly advised when handling them.

Wheel bugs are highly regarded by organic gardeners because they consume a variety of insects and their presence indicates a healthy, pesticide-free ecosystem. "They're the lion or the eagle of your food web," Dr. Michael J. Raupp, an entomologist at the University of Maryland, notes. "They sit on top. When you have these big, ferocious predators in your landscape, that tells me that this is a very healthy landscape, because all these other levels in your food web are intact."