Roughly 3,000 species of earthworms have been discovered all across the globe in a variety of habitats. You'll find them on the surface sometimes, or just under; but they live their lives mostly underground, improving the quality of soil and helping to shape the ecosystem. Despite their differences, all earthworm species have similar life cycles.
Do It in the Dirt
Earthworms are hermaphroditic, meaning they have both male and female sexual organs. However, except in very few species, this doesn't mean they can fertilize their own eggs. To reproduce, they must mate with another of their species. They line up their clitellums -- thickened sections of their bodies that contain their reproductive organs -- and exchange sperm packets. Sheaths form around their clitellums, which dry and fill with albumin.
The Morning After
Once earthworms have mated, they go to some effort to fertilize them. The worms wriggle out of their hardened sheaths leaving eggs from the earthworms' ovaries and sperm from the recent mating. Once the worms work their way out of their sheaths, the sheath ends seal up to form cocoons, inside which the sperm fertilizes the eggs and the worm embryos grow.
A Long Time Coming
Depending on several factors -- including species, temperature and soil conditions -- the eggs will be ready to hatch as soon as three weeks later or as late as five months. A cocoon may hold one to 20 juvenile earthworms; the number varies according to species and external conditions, though usually no more than a few will emerge. The young look just like adults of their species, except much smaller. It takes between 10 and 55 weeks for them to mature to their full adult size.
All Grown Up
Once earthworms have reached adulthood -- that age varies according to species -- they're sexually mature and ready to produce the next generation. They can produce between three and 80 cocoons each year. Those species who live deeper underground tend to produce less, as they're better protected and more likely to hatch without disturbance. Some earthworm species can live to be 8 years old.
Ryan McVay/Photodisc/Getty Images