Look out, here comes the Spider-Ant! Actually, it's an ant-mimicking jumping spider (Myrmarachne formicaria), but close enough. It's native to Africa, with relatives in many places.
In all seriousness, though, there are a number of jumping spiders who have ants as alter-egos. They span at least two genera, and are native to most tropical parts of the world. A few live in temperate regions. Yes, they eat insects, like most other spiders. Despite awesome mimicry, this is not cannibalism. There are an amazing amount of these spiders. Cue mass-entry guilt.
For those of you who know nothing about arthropod anatomy, here's a little primer. Insects have three body segments and six legs. Spiders have two body segments and eight legs, plus two pedipalps. A number of tricks are used to pull off that visual trickery, including raising the forelegs like antennae and making the pedipalps look eerily close to an ant's head. It's a darn good job.
Why the spiders do this is something of a mystery. Always suspecting the worst of a venomous creature, scientists first thought that spider-ants were infiltrating anthills by going in undercover. This turned out not to be the case for many, but not all, ant-mimics. Unbeknownst to most humans, ants taste bad, and can also sting with formic acid. Not a bad thing to disguise oneself as.
Of course, this won't protect them from humans who hate ants, too...and there are quite a lot of those. You fooled nobody with A Bug's Life, Disney - ants are crafty little buggers who wind up in my bathroom every spring. Yeah, disguising yourself as an ant won't work well all the time.