When I saw this small beetle on the back of the big Meloe blister beetle, I thought perhaps it was a parasite of the species. But I have since discovered, thanks to Bug Guide, the following fascinating information: "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)"

The tiny beetle, is a Fire-coloured Beetle (Pedilus terminalis)., I have since seen other, different beetles on the back of Meloe, perhaps another species of Pedilus.