hmm. actually, never mind adding a length cap. When I had the idea of adding a length cap I was thinking that the head of the worm would be that part that slowly degrades. But if the back of the worm is the part that degrades (instead of the head) , and each back segment disappears after 3 moves , then the player can only add up to 3 segments of the worm before the back segment pulls away from the payer's gun, making the lifespan of the worm 9 The player could still possibly add more segments to the worm, but only if it turns 180 degrees.
However, this does not solve your problem of having to make a new animation for the tail disappearing, and the head disappearing animation would only be seen on worms with 1 segment remaining.
Perhaps you could make the worm gun appear more often or sooner in the game.