But either way, the part that is cut off will not regrow into an earthworm.
Earthworms have segments, and if you cut off x-number of segments, for many species, that number of segments will grow back. Researchers have experimented in many different ways with this interesting regenerative aspect of earthworms. Some researchers have cut off the same segments of an earthworm several times to watch the segments grow back every time. Some poor worms have been cut thirty times! And they kept regrowing those segments.
The lengths researchers are willing to go to explore this unusual ability of earthworms border on the repulsive. In her book, The Earth Moved, Amy Stewart wrote:
This phenomenon has led some researchers to experiment with transplanting heads or tails from one worm onto another. Like circus animals, the worms oblige and continue to perform. You can cut a tail off and suture it to the head of another worm, and within a couple of weeks, the intestines and nerves will join together and work properly, even if the two ends are rotated at a forty-five-degree angle to one another and then joined. You can take a head from one, a tail from another, and the middle section from a third, suture them all together in the correct sequence, and get one complete worm.
Strange and macabre, but hey, it's research, right? Back to our original question: No, you cannot increase the earthworm population by cutting worms in half. But give them the right food, and they will multiply mightily.