Blister Beetle strikes again
DermAtlas.org is a great resource for derm. pics.
BB had the opportunity to make the Dx but the old man had to clean it up.
The toxic substance is cantharidin, there are many of these beetles (2000) but if you look at their pictures on the net you will recognize some that you have seen in the yard.
They are so toxic that in Neb. this year they have infested hay and if horses eat enough of these critters(in the hay) they get sick and can even die.