Ugh, the joke is NOT that the scorpion has a poison tail. Good lord, I can't believe anyone who reads PBF would think something so banal (not to mention it makes the last frame superfluous).
The joke (I hesitate to explain it, because jokes like frogs die on the examining table) is that the scorpion realizes he killed his friend but no one else seems to realize it. You can take it a bit further in some different directions.
It really isn't funny ha-ha, but most PBF comics aren't. They're more like comedic tragedies. See Bee, Preach Skate.
That said, no thumbs up for stumbling a lone image. Stumble the comic page here.