These are the civs (and intelligent races) at release, but I will make an intelligent copy of all creatures for adventure mode.
1 black ants
2 harvestmen
3 other detailed cave civs (not-playable), possible races:
bullet ant
carpenter ant
fire ant
honeypot ant
leafcutter ant
pharoah ant
bumblebee
yellow jacket
dry wood termite
damp wood termite
cone head termite
4 other tribal civs (not playable), possible races:
army ant
weaver ant
executioner wasp
honey bee
paper wasp
For the detailed cave civs, I think having at least one kind of
termite makes sense. They tend to compete with ants anyway, with some having developed specific weapons for fighting off ants (including soldiers with actual glue-guns for heads) that would make them interesting to deal with.
Bullet Ants could be interesting, as would yellow jackets. I'm not sure how a flying civ would work in game, but it would be "fun" to try and fight them lol. Maybe leafcutter ants to round out the three, since they're the ones that actually practice fungus farming in real life?
As for the tribal civs, I think executioner wasps are a must, if only for the name lol. Army ants would be interesting, though I'm not sure how you'd replicate their "roving through the forest in millions strong columns eating everything they come across" behavior. Maybe they could be a goblin analog, instead?
Can I interest you in
tent caterpillars? They're probably not social enough to count for a full on civ, but they might do the trick for a cave civ.
Can I also interest you in
Ambrosia Beetles, which I just found out about today and are considered to be the oldest farming creatures on Earth? Again, they might not count for a full civ (though they do have distinct queens and workers), but a cave civ might fit.
On the subject of beetles, have you heard of
ironclad beetles? They might make for a good addition, maybe to savage biomes? They're herbivores, but their exoskeleton is so tough people have to use powerdrills to punch through them. I can see them as large, neigh-invulnerable food thieves, simply waltzing into your nest and taking what they want.
EDIT: Thinking about it some more, maybe savage biomes include insects that are, well, "savage": particularly large, dangerous or weird bugs that wouldn't make sense in cave environments. Atlas moths, Hercules beetles, queen Alexandra birdwing butterflies (might just call them birdwing butterflies), goliath birdeaters, stalk-eye flies, scorpionflies, etc. Just the really big, scary things that don't really live in caves that you probably don't want to mess with as an ant. Some of them might break the 20 billion mark, but I can think you can exceed that with size multipliers, or you could fudge the numbers a bit.
Here are some examples just in case you were interested. I know you've been trying to figure out savage biomes, hope this helps.