This small beetle was on the back of the much larger Meloe blister beetle. Bug Guide has this to say about why small beetle is riding on the back of the big one: "Adults of many genera, notably Pedilus, are attracted to cantharidin (males seek out blister beetles, climb onto them and lick off the cantharidin the blister beetles exude and use the blistering agent to impress a female of their own species who then mates with them, whereupon most of the cantharidin is transfered to the female in the form of a sperm packet. The eggs the female subsequently lays are coated with cantharidin to protect them from being eaten before they hatch. -Jim McClarin, 26.iii.2006)"