I saw this insect the other day in front of my office in Dartmouth. The head and thorax looked a lot like an ant, but the abdomen is pretty crazy. That end looks more like a beetle. There are two tiny shapes on top of the abdomen that look like wings or wing covers. And the thing was a shiny blue-black.
Looking it up in Patti's Peterson Guide, the closest I could come up with was that it was a "blister beetle" but the description wasn't quite right and the drawing was a little off. However, a search on the 'net shows pictures of blister beetles that exactly match my little guy.
Apparently, they contain a substance called "cantharidin" which can cause skin irritaiton and blistering. When ingested, it is a poison similar in toxicity to cyanide or strychnine.