By far the most common question I am asked at outreach events is “is it true if you cut a worm in half you get two worms?” and during the Earthworm Watch feedback survey I was asked: “Do all worm species survive to be two when cut in half?”
The short answer is no. Unlike some other types of ‘worm’ such as flatworms and nematodes (which are very distantly related to earthworms) if you cut an earthworm in half the parts will not become two worms. However, earthworms can survive if their tail end is cut off, and can regrow their segments but earthworms generally cannot survive if the front part of their body between the head and the saddle is cut as this is where their major organs are.
Different species of earthworm are better at regenerating their tails than others but all can regrow to some extent. Some earthworms may deliberately amputate their tail when grasped by a predator and escape while the end still moves and distracts their attacker. The compost worm Eisenia fetida accumulates waste in its tail which gives it a yellow colour. When they cannot store any more they amputate the tail which removes the waste.