Why you don't want to mess with these dudes:
When disturbed, the wheel bug (a type of assassin bug) can inflict a painful bite. The bite has been described variously as worse than stings from bees, wasps, or hornets. Barber (1919) and Hall (1924) described in detail the effects of such bites. In general, initial pain often is followed by numbness for several days. The afflicted area often becomes reddened and hot to the touch, but later may become white and hardened at the puncture area. Occasionally, a hard core may slough off, leaving a small hole at the puncture site. Healing time varies but usually takes two weeks and may take half a year.
Source: http://creatures.ifas.ufl.edu/trees/wheel_bug.htm
But these guys are considered beneficial, so just don't get mess with 'em.