They make a loud chattering noise with their mandibles and are better adjusted to daylight than their California cousins. There are four variants of these ants: workers, soldiers, queens, and the mutated forager ants.
Ant nectar can occasionally be looted from their corpses and refuse piles (found in their lairs).
Ants usually attack in droves of 3-5 head on. They don't try to flank enemies, but will surround them if they get close enough. Their mandibles can be devastating to lower level players, but they are not much of a threat to higher level or well equipped players. If their antennae are crippled, ants will frenzy and attack everything around them, including their own kind. The Entomologist perk will increase damage against giant ants and other insects by 50%.
The mother of the ant brood, this variant is roughly four to five times larger than your average giant ant. When threatened, her wings will buzz about and she will hiss at the enemy. She will then proceed to spit an acidic saliva at the enemy in an attempt to ward it off.
Ant queens are not very fast, though they do seem to notice you from a distance and spit with surprising accuracy. The queens are generally considered to be one of the "boss" characters in Fallout 3, as they have extremely high health (about 1/2 that of a super mutant behemoth) making them one of the few creatures in the Capital Wasteland that can survive a Nuka-grenade blast.
There are only three queens in Fallout 3: in Marigold station, Shalebridge and the Corvega factory. The Marigold ant queen is the mother of the fire ants that have been harassing Grayditch. She was mutated by Doctor Lesko in an attempt to create smaller ants with a chemical mutagen that would be injected into the queen ant, but instead fire-spitting giant ants resulted (for a more detailed description see the fire ant page).
The following is based on Fallout 3 cut content and has not been confirmed by canon sources.
A flaming version of the globs of spit, exclusively for the Marigold fire ant queen.
End of information based on Fallout 3 cut content.
Invader ants are a species of ant found in Shalebridge that, as their name suggests, are invading the colony of the forager ants (see below). They have the same health as a worker ant, so they can still pack a punch for low level players.
These ants are friendly and won't attack the player. They are small for mutated ants and live in the colony located in Shalebridge tunnels, which is under attack by a hostile, more aggressive colony of rival giant ants. The Lone Wanderer can help the forager ants by traveling into the rival ant nest and killing their queen. The growth of the colony can then be stimulated by injecting the egg clusters with stimpaks. These forager ants are the same as any friendly non-player character; they will become hostile if attacked. If shot, one of the others won't attack you unless they are about two feet away. They are extremely small for regular wasteland ants, even smaller than radroaches.
pcps3xbox360 A glitch may occur where once you kill the fire ant nest guardian and leave, the Marigold ant queen will glitch/crawl through a wall near the exit and stand in your way, trying to attack you. with some effort you either can jump over her legs to the side and escape (or, of course, kill her). [verified]
ps3 There is a glitch that may occur when you get close to the first defeated nest guardian where the queen will teleport to your location. [verified]