Giant ant

Giant ants are the effect of nuclear fallout mutating regular ants. What they lack in individual fortitude, they make up for with relentless attacks and swarm tactics. Their exoskeleton isn't particularly thick, but it can turn aside weak punches and dull knives. Their mandibles can cut through most leather and plastic with ease, even snapping bone on a lucky shot. Giant ants become particularly aggressive when their queen is threatened.

Variations
A giant ant species found in Northern California and Oregon lives mainly in dark places, often underground. Areas where they appear include Arroyo and Broken Hills. A stronger variant called a "tough giant ant" also exists.

A larger species of giant ants, with horned-looking skin can be found on the East Coast (e.g. Springvale). They are better adjusted to daylight than their California cousins.

The fire ants are another variation of giant ants that is even more mutated than the "regular" giant bugs. Their name comes from their ability to literally breathe fire, similarly to the fire geckos from the Core Region.

A colony of friendly forager ants can be found in Shalebridge. Unlike most Giant Ants, this colony is peaceful towards humans. It is under attack by a hostile, more aggressive colony of rival Giant Ants. Players can help the forager ants by traveling into the rival ant nest and killing their Giant Ant Queen.

Another rare variation are giant albino ants, found in the Hoover Dam Scum Pits. Their exoskeleton is a very pale yellow that almost looks white, their eyes glow red like the rats, and their mandibles glow green and drip a green, acidic fluid. These ants live in a nest back in the office area and, luckily, are not prolific. But they are hungry, and they do come out fairly often to catch and drag a rat or leech back into its dark lair.

Appearances in games
Giant ants appear in Fallout 2 and Fallout 3. A friendly giant ant called Mr. Pinch is used as a mount by the Puppet Man in the official Fallout 3 Penny Arcade comic.

Fallout 3
There are three variants of Giant ants in Fallout 3: Workers, soldiers and queens. There are only 2 of the latter in the Capital Wasteland, in the Corvega Factory and Shalebridge.

In Shalebridge you can also find a colony of friendly Forager Ants which you can help defend against the attacks of their larger brethren.

Behind the scenes
Like many other creatures in the Fallout series, the giant ant draws a large amount from the background lore of old black and white cinema where creatures were mutated into enormous versions of their former selves in unbelievable ways by nuclear radiation. They have appeared in the weekly Penny Arcade comic with the one shown carrying the Vault 77 Dweller which has been nicknamed Mr. Pinch. They are very large and have no special abilities other than the ability to nip you with their pincers, which are crippling. They are normally found by bogs and will charge the player when he steps on it. Presumably, the bogs are a byproduct of ant nests and the ants are roused by the noise above them. So naturally, they rush to defend their colony.

Also the sound effect when they are killed is made by styrofoam. I suppose it adds to the "Ewww!" factor of it.