Worm Facts

Worms are members of the phylum Annelida (which means segmented worms), the class Oligochaeta (which means bristleless worms and includes earthworms), and the order Lumbriculida. Within the Order Lumbriculida, there are several hundred species of what we call earthworms, but less than a dozen are commercially important for worm composting. Earthworms range in size from just a few millimeters (tenths of an inch) to over three meters (10 feet) for the giant Australian earthworm.

Terrestrial (as opposed to marine) earthworms can be classified into one of three groups: litter dwellers, shallow-soil dwellers and deep-burrowers. The litter dwellers live in the thin layer of decomposing plant material (litter or floor compost) on the soil. In a forest, these worms are found just below the surface layer of leaves and twigs. The shallow dwelling worms, such as redworms, live primarily in the top 12 - 24 inches of the soil. These worms do not build permanent burrows, but tunnel randomly throughout the soil. Deep burrowing worms, like nightcrawlers, build permanent vertical burrows that extend deeply into the soil, sometimes as much as six feet. Deep burrowing worms are extremely important to the health of the soil. Their burrows and tunnels help to aerate the soil, while the organic material they pull into their burrows helps to improve the organic content of the soil.

An interesting fact is that most native species of earthworms in North America were wiped out during the last great Ice Age, which ended about 10,000 - 11,000 years ago. Most of the earthworms we see in our yards and gardens today were brought here by Europeans, as well as later settlers from other places around the world. The earthworms "hitched a ride" attached to the soil of plants carried by these settlers and spread quickly throughout both North and South America. A recent study in Canada found that only two out of nineteen species of earthworms identified were actually indigenous. Earthworms represent one of the few instances where invading species may actually have benefited an entire ecosystem.

THE BIOLOGY OF WORMS

The night crawlers (Lumbricus terrestris) and common redworms (Lumbricus rubellus) generally have a distinct, darker colored "head" end which does contain the primitive "brain" of the animal, and this tends to be the end of the worm that travels "forward" the most. The "tail" end of the worm tends to be more flattened than the head and lighter in color. Red worms (Eisenia fetida), a very common composting worm, are smaller, bright red and don't have a dark colored head end. Worms do have a proper top (dorsal) and bottom (ventral) surface, they are not just symmetrical tube-like organisms. The surface of the worm's skin is smooth and slimy, but also has many tiny bristles or "setae" protruding from it. These help the worm move and serve to anchor it in its burrows for self defense. The setae are part of the reason that robins have such a hard time pulling worms out of the ground. If you place a big night crawler on a piece of cardboard or paper, you can hear the setae scraping as the worm crawls!

Basic Worm Biology

Without going into a lot of details, here's a quick overview of worm biology. The body plan of an earth worm is basically a segmented tube. Each segment is a separate fluid-filled compartment surrounding a central digestive tract, or gut, which runs the length of the worm's body. Many of the worm's internal organs are also segmented, occurring as separate units in each segment, but there is also considerable specialization in the head end of the worm. The "brain", "hearts" and other major organ systems are clustered in the head end. Earth worms have no eyes, but they do possess cells which are sensitive to light. This is why when worms are brought out into the light, they squirm and thrash around. Worms also don't have ears, but they can feel vibrations in the ground. While earth worms don't have lungs, they do need oxygen to survive. Instead of lungs, worms have developed the ability to absorb oxygen directly through their moist skin, which is kept moist by mucous secreting cells. If a worm dries out, it will suffocate.

It is commonly believed that worms come out during a rain storm to avoid drowning, but that's not the case. Since earthworms absorb oxygen directly through their skin, they can survive for a long time in well oxygenated water. But why should earthworms crawl around on the ground surface, exposing themselves to predators? Some researchers suggest that earthworms may be looking for different kinds of food (lots of organic matter gets knocked loose during a rain storm), while others believe that they may be looking for potential mates. The "mating theory" has been documented by several scientists, although the results were not conclusive. It appears that earthworms can find and mate more easily in the moist 2-dimensional plane represented by the soil surface, than while plowing through the more confining 3-dimensional world beneath the soil. Several researchers have also noted that most of the worms crawling around the surface tend to be large, and presumably sexually mature, members of their respective species. They noted that if there were non-reproductive benefits to being above ground when it rains, worms of all ages would be present. So, rather than being a problem to overcome, the abundant surface moisture after a rain may actually represent an opportunity for worms.

Worms move by a process known as "peristaltic contraction". A worm's body is a fluid filled tube divided into separate segments. There are circular muscles that surround each segment and longitudinal muscles running from segment to segment for the length of the worm. Contraction of the longitudinal muscles shortens and widens the segments of worms body. Circular muscle contraction lengthens and narrows the segments. By alternating these processes in waves down it's entire body length the worms crawls forward or backward. Inside its tunnel, the widening of several segments serves to anchor that part of the body against the tunnel walls. The "leading end" segments are then elongated by circular muscle contraction (squeezing), pushing that end forward, and the "trailing end" is drawn up by longitudinal muscle contraction.

Life Cycle

Earthworms are hermaphrodites. The term "hermaphrodite" derives from the combination of the names of two Greek gods: Hermes, a male and Aphrodite, a female. Thus, a hermaphrodite is something with both male and female characteristics. With worms, each individual contains both male and female reproductive organs. However, each worm must still mate with another worm of its species in order to reproduce. When two worms mate, they lie alongside one another, and both transfer sperm to the other. Each will lay one or more cocoons which contain the worm eggs.

How long an earthworm can live in the wild isn't certain, but researchers estimate a normal lifespan of about 3 years. Captive earthworms (in a worm composting bin) have been reported to live as long as 10 years! Earthworms can eat many kinds of organic matter, including dead plant materials (dead leaves and other plant debris), soil micro-organisms (protozoa, nematodes, bacteria, fungi, etc.), and the remains of larger dead animals. They feed by swallowing organic matter or bits of soil containing organic matter. This passes through their gut and is finally deposited as castings (simply put - worm poop) which the worms pass out when they are at the surface. Presumably this helps them to keep their tunnels clean and open. Earthworms also act as food for a wide variety of soil predators. Just about anything that likes a bit of animal protein will eat worms: insects, fish, frogs, toads, snakes, mice, moles, gophers, raccoons, opossums, as well as most kinds of birds. For animals that weren't originally found in Southern California, worms now occupy a very important slot in the food web.