The earthworm life cycle, like many others, starts with an egg. Within the egg, a young earthworm develops until it is ready to hatch. The egg is encased in an egg casing called a cocoon. The number of eggs within one cocoon can vary between species, ranging between 1 and 20 from earthworm species in the family Lumbricidae (but most species have just 1).
1. Cocoons tend to be ‘lemon’ shaped, but the specific shape varies between species. The amount of time that they take to hatch is also very variable and can vary by species, but also by environmental conditions. For example, for some species the cocoons may hatch quicker in warmer conditions than in cooler conditions and other species may ‘wait out’ undesirable drought conditions within the soil as the cocoon stage.
2. Hatchlings look just like mini-earthworms, they’re just smaller and paler. It can be easy to confuse these with potworms (Enchytraeidae), which are small segmented worms that are closely related to earthworms. As the hatchling feeds and grows it will gain the same colour as an adult earthworm.
3. Juvenile earthworms look very much like the adults but are missing the saddle (or clitellum).
4. Adults (or sexually mature) earthworms can be easily recognised through the presence of the saddle. Earthworms are hermaphrodite organisms, meaning that each earthworm has both male and female sexual reproduction organs.
5. Sexual reproduction involves two earthworms. The two earthworms produce a slime tube and grip onto each other using the tubercula pubertatis (located on the saddle). The slime tube provides the right environment for the two earthworms to exchange sperm, with each earthworm storing the sperm of it’s partner for use later. Because both earthworms are performing the function of both a male and female during sexual reproduction, they are known as simultaneous hermaphrodites. Following this sperm exchange the earthworms separate.
Asexual reproduction can also be undertaken by some species of earthworm. This involves a single earthworm producing young from unfertilised eggs and is known as parthenogenesis.
6. A mucus sheath is formed around the clitellum and is moved along the earthworm until it comes off the head end. Along this journey it picks up the egg(s) and the sperm of the earthworm that was mated with. This mucus sheath forms the cocoon and fertilisation of the egg occurs within the cocoon.