Here are some ideas to improve one of my least favorite invertebrate monsters. Each is meant to represent a different mutant species, but they can be used in combination if the DM wants to really make the encounter difficult.

1. Their acid spit is replaced with a liquid that Transmutes Mud to Rock. Those who are in the monster's burrow and covered with soil must save versus breath weapons (reflex). Failure results in being rendered immobile and success means the target(s) movement rate and DEX scores are cut in half. In both cases, they have to be chipped out, which takes 1d4 hours (those encased) or 1d4 turns (those who escaped most of the effect).
2. Their cries of pain cause Confusion for 1d4 rounds. This even affects other ankheg, making combat much more chaotic. And yes, they can cry out every round they are struck or are suffering in some manner.
3. Their gaze is horrific, effectively a Scare spell. This means most of their prey will not resist being eaten.
4. Their exoskeleton is iridescent, and those who see it in bright light are exposed to a Hypnotic Pattern effect.
5. Their bite is somewhat like Horrid Wilting, but less damage. The fluids drawn out are turned into the ankheg's acid attack.
6. Their honey dew acts as a Sleep potion. Those smelling it must save or gain the compulsion to eat it.
7. They produce bubbles of mucus that line their tunnels. It is explosive, but only when exposed to bare flesh.
8. They produce blinding flashes of light, including infrared (i.e. there is a penalty to the save for those with infravision).
9. They have gills and can Raise Water to drown those who invade their burrows.
10. Their acid acts as Fool's Gold with a long duration (weeks). It turns pebbles in the burrow into "gold".
11. They produce silk that creates enchanting music when vibrating (physical contact or wind usually). It is very similar to the song of harpies.
12. Small animals are drawn to the ankheg's burrow, effectively giving it a permanent Summon Swarm spell.