I've seen several small bumble bees and occasional large bumble bees that all look very much alike - yellow hair on the front of their thorax, plus yellow hair on their abdomen, except for the last 2 segments, which are covered with black hair. Having searched through BugGuide.net, I'm quite sure that these are all American Bumble Bee (Bombus pensylvanicus). I'm guessing that the big ones are the queens and the small ones are the workers.
Then, just last Saturday, I saw this long individual, with the rear abdomen completely covered in yellow-brown hair. It looked like a different species to me, but after searching BugGuide.net and other identification sites, I've decided that it is the male American Bumble Bee.
Oh, and I thought this photo was interesting - I guess the wheel bug decided that the bumble bee was too big to tackle, because it did not attack despite obvious interest.