D&D 5E (2014) Building a Better Ankheg

[Tonguez]

Let it burrow 30ft, advantage on grab from below, grapple and retreat and tunnel collape to have its hole fill with dirt leaving victims buried prone underground with the large hungry predator.

 

[Mistwell]

We are in session 6 of an ankheg mini adventure, and they're terrifying, complex and tough. Their acid spray is really nasty for a CR 2 creature, making AC useless against the spray. The burrowing usually leads to us being surprised. The tremorsense makes a lot of PC abilities nullified or a negative for the party. They grapple their targets well and with no additional action. Their HP is 45, which again for CR is no slouch.

These guys are scary and tough for a CR 2 creature.

 

[el-remmen]

These guys are scary and tough for a CR 2 creature.

No doubt. I guess I my issue is that I always imagined them more as a CR 5 or 6 creature, which admittedly is my own thing. But I think your post brings up the good point that the tactics it employs and making use of the environment are crucial to making it a challenge at any CR.