An oil beetle.
oil beetle definition by Webster's New World
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
any of various small blister beetles (esp. genus Meloe) which, when disturbed, produce an oily secretion from the joints of the legs
oil beetle definition by American Heritage Dictionary
Any of various blister beetles of the genus Meloe that exude an oily yellow substance from the joints of the legs when disturbed.