This creature is great. It takes advantage of the thixotropic properties of sand to drop its victim waist deep into sand, pops up to the surface, covers it in acid, and then eats it. Heavier victims will sink faster. The victim doesn't know what is going on until it is sunk into the sand and the larvae comes up. That sounds awful for adventurers that might think that they just stumbled into a patch of quicksand, and react by trying to stay still to avoid going deeper. The surprise of seeing the larvae come up should add to the experience, especially if they feed in groups. This monster will be a lot of fun to encounter.