This is a Wheel Bug (Arilus cristatus), easily identified by the crest on its back that is shaped like a cog wheel. It is one of the true bugs, related to stinkbugs and assassin bugs. Wheel Bugs are ferocious predators, eating all sorts of insect pests. This makes them welcome in most gardens. The bug pounces on its prey, stabbing it with its sharp beak. It then injects a mixture of enzymes which paralyze the prey, and liquify it so the Wheel Bug can drink the liquid. They are not aggressive towards people, but if handled, they can give a very painful bite, which can take a long time to heal.