I noticed the purple worm has both a bite and a sting attack. The anatomy of the bite is obvious enough, but I'm wondering what part of the worm delivers sting attacks. Is it the tail end (in which case, can it not use its stinger when only the front half has emerged?), or some spikes in/around the mouth, or some third option I haven't considered?
Being a worm, I'm guessing it's something at either end, other than massive size there doesn't seem to be a lot going on in between.
The monster is fully emerged or it is not emerged. A GM can say it only half comes out, but then he has to make up rules for cover. A GM might also say that since they burrow so easily that the ground has no affect on the tail smacking you in the face.
Basically if the GM has the monster only half-emerge it is us to him how to decide if the tail can still hit you freely or not.