I don't know what is the proper way to rig the head of the venus flytrap so i can make it close or open. I also need to join it with the body (named "tallo") but I dont know how.
The way you rigged it is right. What you have to do is first select each opening and go into edit mode. now select any vertex from the back of the opening from where you want to rotate the thing. NOW press Shift+S and select cursor to selected. Exit edit mode and right click and go to set origin->origin to 3d Cursor. Now,rotating the thing makes it rotate from the back part. Parent it to the bone and things will rotate nicely.