A little while back, we received a classic question about worms, one that most of us have probably been hearing since we were quite young: if you cut a worm in half, will it survive? (A common variant of the question: when you cut a worm in half, will you be left with two living worms?) The reader specifically asked about the regeneration of worms that are cut in half in the course of gardening (accidentally, we should add), so it is likely our reader is wondering about earthworms, that ever-present companion to gardeners (at least if your garden is healthy). So, the precise question we are interested in is whether an earthworm, when cut in half, will regenerate.
The answer to this question depends on what the reader means by “regeneration.” If he is wondering (along with so many other people) whether an earthworm that is cut in half will turn into two living worms, the answer is “no.” Of course, this makes sense, as even though the ends of earthworms may look the same, they are in fact different. Earthworms have anterior and posterior ends that serve different functions (food goes in the former and waste out the other), so when you cut an earthworm in half, you are not leaving behind two worms that are merely smaller; rather, you are leaving behind two incomplete halves of a worm. For this reason, an earthworm will often not survive being cut in half. So, cutting a worm in half leaves you one dead worm, not two living worms.
However, you will not always kill an earthworm by cutting it in half. If enough of the anterior end of the worm remains, then the back end of the worm can regenerate, although it is sometimes slightly smaller in diameter and a lighter color. The anterior of the worm needs to have a clitellum and several segments following it for this regeneration to be possible. In other words, about half of the worm (the front half) needs to remain for a worm to survive being cut in half. If you cut a worm too close to its head, it won’t be live.
So, the short answer to our reader’s question is “it depends.” If enough of the anterior half of the worm is preserved, it can regenerate and survive. If it is cut too close to its head, it won’t. When you cut a worm in half, the most you will be left with is one living worm that can regenerate and one piece of dead worm. You will never be left with two living worms.