there are so many great species that prey just on ants in crazy ways. like the jumping spider that pretends to be an ant, wanders around near other ants till it gets one by itself then ambushes it. there are also parasites that permanantly alter ants behaviours in different ways, like making them climb up to the tips of grass blades so they get eaten by sheep (parasite can only reproduce inside a sheep, droppings get eaten by ants, so on.)
the best one i can think of for a megabeast encounter, which preys of leafcutter ants, is a kind of catterpiller which has a massive impregnible white shell like shield over it. its impervious to attack from the ants, who go berserk trying to get rid of it as it enters their nest, finds the hatchery, and eats all the ant larvae. then it spins itself a cocoon and is ignored for a while. when it pops up later as a moth, its covered in sticky stuff that the ants have major issues biting though. they go mental again as it finds its way out and eventually flies off.
THIS MUST BE MODDED IN!!!
i want a megabeast encouter that doesnt just die when it gets hit on the head by my first stonefall trap, or taken down by a random hunter. seriously, ive never ever had any kind of issue with any megabeast in DF, a moderately equipped army, or a single set of traps at the entrance, has been all ive ever needed. i mean, last time i was set on a tantrumming legendary farmer broke the dragons leg after it ate his pet cow, and it kept fainting before it could get to my cage traps so i shot it down with two crossbowmen.
BUT, something which is nigh on invincible that makes its way in, eats all your food and then spins a cocoon for a half year or so before emerging and escaping, only to have its offspring come back, would be hectic. i want to play this so bad.
seriously. the world of insects is so filled with pure ownage its hard to look around without tripping over something gruesomely wicked. how would you do weapons and digging tools? how would they reproduce, would the mayor act as a sort of queen? and bricks and things - i guess the mason class would make spitballs out of the soil. and what would the HFS encounter be about? im guessing you want to mold traits of many different ants into your playable civ.