BIOLOGY OF THE NIGHT CRAWLER (Please switch to the updated version of this article: Biology of Night Crawlers)

Description

The night crawler (Lumbricus terrestris) is a large worm, measuring up to 25 cm in length and up to 1 cm in diameter. They have a distinct, darker coloured "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 colour. Common garden worms (Aporrectodea spp. - I couldn't track down which species we have here) are smaller and don't have a dark coloured 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 few tidbits of worm biology, just a view things that came to mind and seem to be the kinds of things people most often ask about.

The body plan of an earth worm is basically a segmented tube. Each segment is a separate fluid-filled compartment surrounding the digestive tract (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 considerable specialization in the head end of the worm. The "brain", "hearts" and other organs are clustered in the head end. Earth worms have no eyes, but they do have cells which are sensitive to light. They do not have ears, but can feel vibrations in the ground. Earth worms don't have lungs, they absorb oxygen directly through their moist skin, which is kept moist by mucous secreting cells. If a worm dries out, it will suffocate.

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 (ring) 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.

Flattened "tail" end of a night crawler, moving "forward".

Worms can survive being cut in half! Well, for a little while, at least. It is usually only the head end that will regenerate some segments in the lower end and become a viable worm again. The lower end cannot regenerate a head. However, most often, when cut in half, worms die.

 Classification

The taxonomic classification of the night crawler is as follows. (Warning: Don't try to pronounce these names while chewing gum, serious lingual damage may occur.)

Kingdom: Animalia
Phylum: Annelida
Class: Oligochaeta
Order: Haplotaxida
Suborder: Lumbricina
Family: Lumbricidae
Genus: Lumbricus
Species: terrestris

Name

Night Crawlers get their common name because they do crawl around on top of the ground at night. They are also know as "dew worms", probably because they are found more commonly on nights when the ground is moist from a dew or rain.

The scientific name for night crawlers derives as follows:

Lumbricus: A Latin word for earthworm.

terrestris: Another Latin term, meaning "of or belonging to the earth".

So, Lumbricus terrestris is "earthworm of the earth". (I know that's a bit redundant, but at least it makes sense!) 

Habitat and Range

The night crawler is not native to Manitoba, nor to North America. It is a European species that was introduced to the new world with the advent of European settlement. It is most prevalent in the southwestern third of Manitoba, the agricultural region. Actually, it seems that all of Manitoba's earthworms are exotic species! There are very few native North American species of worms, and none of them are thought to have made it into Manitoba after the retreat of the glaciers with the end of the last ice-age. (I still find it hard to believe that Manitoba has no native earth worms! I'll keep checking around to see if I can verify this fact, and post an update later on.) I haven't been able to track down how many species of worms there are in Manitoba, yet. Ontario boasts 19 different species and North Dakota has 10.

Moist soils that are rich in organic matter are the preferred habitat of night crawlers. Proximity to human habitation is a major factor in the distribution and numbers of night crawlers. Remember, this is an introduced species, so it is most likely to occur where people have been active in working or altering the soil. Golf courses and farm fields near cities are some of the best places to find night crawlers. Forested lands along major waterways are also good places to find them. 

Populations

The populations of night crawlers will vary dramatically with soil conditions. It's thought that they require about 1500 cubic centimetres of soil each in order to thrive, that's equivalent to a cube of soil 12 cm on a side. If you laid such blocks out on a lawn, you'd have about 70 of them per square metre, so populations of night crawlers could be as high as 70 per square metre of lawn! However, published estimates tend to put their populations at a more modest 10-15 per square metre, still a lot of worms! The populations probably show a trend towards increasing numbers from spring until late fall. Presumably the hardships of winter take their toll on worm populations. How night crawler populations relate to the populations of other earth worm species is uncertain. Each species probably has its preferences for soil conditions and may dominate the overall worm population in its preferred habitat. 

Life Cycle

Night crawlers, and most other worms, are hermaphrodites. That is, each individual worm contains both male and female reproductive organs. However, the worms must still mate with another of their 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 capsules (like a cocoon for the eggs), from which will emerge one or two fully formed tiny worms. The familiar thickened "band" near the front end of most worm species is a structure called the clitellum (see above, also). It secretes the mucous and other substances that form the capsule containing the fertilized eggs.

A pair of worms mating. Photo by Jeanne Motley, Colorado, USA.

It's thought that night crawlers mate and lay eggs mainly in the spring and fall, when soil moisture levels tend to be higher. Each worm may mate and lay eggs several times each year, but they produce relatively few offspring per year, perhaps only 10-15 for each adult worm. It may take the tiny worms up to a year to reach full size and sexual maturity. How long they live after this in the wild isn't certain, best guesses are anywhere from 3-8 years, but captive worms have been know to live for 10 years! 

Food

Food for night crawlers consists of many kinds of organic matter. They eat plant tissue (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 (poop) which the worms pass out when they are at the surface. Presumably this helps them to keep their tunnels clean and open. 

Night Crawlers as Food!

What eats night crawlers, and worms in general? An easier question might be, what animals don't eat worms! Just about anything that likes a bit of animal protein will eat worms: insects, fish, frogs, toads, snakes, mice, moles, raccoons, birds (of all kinds!), bears; it's an enormous list. For animals that weren't originally found in Manitoba, worms now fill a very important slot in the food web. 

Habits

Night crawlers move up and down in the soil profile on a daily basis. They keep to a fairly well defined system of tunnels within their own home range.

Tunnel system of captive common garden worms.

By night, night crawlers can be found on or near the soil surface, but in daylight they retreat down well below the soil surface, except when it rains. Everyone is familiar with the phenomenon of worms coming out when it rains, but the folklore of worms emerging to avoid drowning in their burrows doesn't hold water. They absorb oxygen directly through their skin, which must be kept moist in order to do so. Worms can survive for a long time in well oxygenated water. So, rather than being a problem to overcome, the abundant surface moisture after a rain may actually represent an opportunity for worms. But why should a seemingly helpless invertebrate want to crawl around on the surface, exposing itself to all manner of predators? What benefits are to be had? Well, precisely what they are doing crawling around on the soil surface at night or after it rains isn't entirely clear -- at least, I wasn't able to find anyone willing to provide a definitive answer. Perhaps they find different kinds of food this way or it may be easier for them to find mates. The "mating theory" -of why worms come out after a rain - suggested by some authors supposes that it is far easier to find a mate, and perform the mating, in the moist 2-dimensional plane represented by the soil surface, than it would be to do so while plowing through the 3-dimensional world beneath the soil. I even thought of something to add weight to this theory. I've never come across tiny worms crawling about after a rain, they all seem to be large, and presumably sexually mature, members of their respective species. If there were non-reproductive benefits to being above ground when it rains, you'd think that the young worms would be out, too.

What do worms do in winter? Good question! I'm sorry, but there isn't a lot of information out there on worms in general, and I couldn't find anything about their over wintering habits. I'll keep looking and post an update. My guess would be that they operate as normal, except they avoid the soil surface once the temperatures dip below freezing. One author suggested that worms are slightly freeze tolerant (to about -3C), but they probably avoid freezing by venturing deeper into the soil. 

Economic Importance

At first glance, you wouldn't think that night crawlers, or worms in general, would have significant economic importance. Guess again! Some people earn "big money" from worms. Other people earn a lot of money thanks to the actions of worms. And worms can even cost people money, too.

Worm harvesting is a significant source of income. Although I wasn't able to track down any information about professional harvesting activities (if any) that occur in Manitoba, there is a significant "worm industry" in southern Ontario. The value of night crawlers is as fish bait and the chief market for Canadian worms is the avid fisher population of the United States. Apparently, U.S. production doesn't meet it's own needs for bait worms!

In Ontario, commercial pickers purchase the rights to collect worms at night on agricultural lands and golf courses. In 1980, 370 million worms were exported from Canada, with a Canadian export value of $13 million and an American retail value of $54 million! More recent unofficial estimates put the Canadian export value at closer to $50 million annually. A container of night crawlers, about a dozen per bucket, from your local bait shop, which probably originated in southern Ontario, will still cost you $2.00 today, about 15% more than they cost back in 1980. Tomlin estimates that a good worm picker back then could earn as much as $300 per night, based on a price of $24 per 1000 worms, and as much as $12,000 for the worm picking season, from April to late October.

With that kind of money on the line, you'd think that raising night crawlers in captivity might be the way to go. But, alas, according to Tomlin, night crawlers don't lend themselves to vermi-culture; though this hasn't stopped con-artists from offering "home based night crawler raising franchises" to the unwary!

In a less direct sense worms earn a lot of other people a lot of money, too. Everyone who earns money from the fertility of the soil owes a little to worms of one sort or another. So what exactly do worms do that is so great for the soil? Well, their burrowing helps to churn the soil making it more porous and improving air and water infiltration. They eat, and help to break down organic matter of all sorts; they compost it! Worms produce copious amounts of urine containing urea, which is rapidly broken down into available nitrogen for plants. Their castings (poop) help to neutralize soil pH, make more basic minerals and nutrients available to plants and stimulate microbial populations in the soil. Worm castings can amount to as much as 30 tonnes per hectare per year! That's a lot of poop, and a lot of soil enrichment! Worms enhance this vital resource, all at no charge to the farmers and gardeners who grow our food.

So, now you think that worms are wonderful and can do no wrong. Unfortunately, there is a down side to everything. Though, all in all, worms are a positive boon to society, they do play one nefarious role. They can knock a 747 out of the air! (Yes, there was an old lady that swallowed a fly...) Think back to what I mentioned about the habit of worms to crawl around on top of the ground after a rain. And how they become particularly visible when they become stranded on bare concrete or asphalt, like on airport runways. And what eats worms? Birds! And what happens when jet airplane engines suck in birds or birds slam into cockpit windows?

Airports take bird strikes, of which there were more than 50 reported cases at Winnipeg Int'l Airport in 1997, and the factors that lead to them very seriously. Unfortunately, most airports consist of large grassy fields with paved runways occupying a small portion of the ground. After heavy rains, out come the worms and in come the birds, particularly large, flocking birds like gulls, which are already attracted to the safe open spaces that airports represent. Add fine dining to the safe roosting sites that airports represent and it's no wonder that large numbers of gulls and other birds are found there. So, every year airports around the world spend a lot of money trying to get rid of worms, or at least trying to keep them off the runways. Application of the fungicide, Benomyl, is one means used to suppress worm populations around airports. Worms are sensitive to a wide array of pesticides and chemical pollutants. (Dumping such nasty stuff on your lawns does a disservice to the worms and can be counter productive to your goal of a "healthy" lawn.)

There are even people out there spending money researching ways to reduce worm castings on "amenity" grasslands. Can you imagine! Trying to reduce the presence of worms in fancy-dancy lawns so "one doesn't have to see or tread on worm poop". What's the world coming to?

Final Thoughts

It's time to stop thinking of earthworms as just fish bait, or as "ooky" things in the garden. Aristotle called them "the intestines of the soil". They are a vital component of the ecology of our soils and an important rung in terrestrial food webs. Night crawlers and all the other species of earthworms are fascinating critters and worthy of our respect.

Interested in observing night crawlers yourself or in your class room? Then follow this link to find out how: Night Crawlers in the Class Room.