I've smashed quite a few that looked close enough that I didn't want to put my eye right down there next to it in the house. No confirmation on their fiddles from the resultant smush. Found a number of widows outside, none inside though.
Have never seen a rattler or a cottonmouth on my property, but have seen (and killed) a few copperheads. I had a cat killed by a copperhead/mesquite thorn combo infection (the mesquite thorn infection in it's paw migrated into the facial injury from the copperhead). Found a few of the garden variety scorpions in the mailbox and garden (harmless, but painful unless you're allergic, they say).
I'd take tenX the number of all of those in exchange for the fire ants.
How poisonous are adders, Ian?