Despite all the playtime other huge bug-like creatures get in D&D, I don't think I've used giant ants in a game. There have been a lot of versions of this monster, but for these purposes, I'll be looking at BX's "Driver Ants." (B34) These suckers are 6' long picnic busters of the first order.
First off, they can turn up as a level 3 random encounter. This makes a certain measure of sense, as the colony might have tunneled into a dungeon corridor. In the wilderness, they are one of the most common random encounters! (Who knew??) Insects occur as a possibility on almost all the terrain types and driver ants have a 1:6 chance on the Insect Sub-Table of being the lucky bug (as opposed to 1:12 for all the other specific critters). Pet peeve: Should spiders and scorpions be on an "Insect" table? ;-)
Moving on, the random encounters yield either 2-8 or 2-12, depending. In the lair there are up to 24 "guards." Since these suckers are 6' long each, that's a lot of RAID. Now let's take a look at the stats:
Driver Ant (from Moldvay)
No. Enc: 2d4 (4d6)
Move: 180' (60')
First off, these guys have a decent AC and hit points. The mandible bite damage is pretty respectable, too. The scary part is that one in a fight, they IGNORE ALL MORALE "even trying to cross flames to reach their opponents." So no luck scaring them off. The eat anything edible in their path, so if a PC falls to them and the party can't get him back right away, any organic equipment (or remains) will be gone in short order.
This is my favorite bit right here. There is a 30% chance that the ants have –while digging their tunnels– been mining for gold! Yup! We got prospector ants here! Herodotus would be proud! If they are mining ants, somewhere in the nest is 1d10 x 1,000gp worth of gold nuggets. How cool is that? That's a great incentive to send PCs down into a maze of tunnels filled with giant insects if ever I heard of one.