SI Vault
Bil Gilbert
August 27, 1979
For eons, the earthworm has survived floods, drought, hungry robins and even the dread worm hunters
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August 27, 1979

They Crawl By Night

For eons, the earthworm has survived floods, drought, hungry robins and even the dread worm hunters

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Individual crawlers are not particularly fertile, nor do they seem preoccupied with sex the way fruit flies are. One worm normally sheds eight or 10 cocoons a year, and from each only a single offspring emerges. However, because they do not worry about, or even know, which is male and which is female, every worm contributes to this reproductive effort and continues to do so for a long time. Fishbaits may live to be 20 years old and keep on making cocoons for most of this time—though not quite all of it. "Earthworms stop breeding sometime before they die," say C. A. Edwards and J. R. Lofty, co-authors of the Biology of Earthworms, a standard modern text on the subject. (The more this observation is pondered, the more astute it becomes.)

While nightcrawlers have their problems like everyone else, in many respects they are better able to cope with them than most. They are eminently edible creatures, being soft, stuffed with protein and apparently tasty, according to a scattering of human testimony as well as the responses observed in other species that occasionally get themselves a few mouthfuls. But even though a number of beasts might enjoy feasting on worms, surprisingly few are equipped to do so regularly. Shrews, moles, certain beetles and a few other diggers catch some, though not many in view of the total number of nightcrawlers. We may think of robins and other omnivorous birds as being serious worm feeders, but they only skim the surface, and there are just a few dawn and dusk hours when the light is such that worms care to surface and birds can see them there. (A note to the Great Ecologist: there seems to be a splendid opening for an owl-robin hybrid, a nocturnal worm-eating bird.)

The weather is of far more serious concern to nightcrawlers than predation, and many of their adaptations and activities are aimed at dealing with it. Water, which constitutes about 85% of a worm's body weight, is particularly critical. Worms do not drink as such but absorb moisture through their porous skins. To maintain their vital functions, they must operate in dampish soils where this osmotic action can occur. Being essentially thin membranes enclosing several drops of water, worms literally evaporate if they are stranded in hot, dry places. On the other hand, they can be troubled by too much water, too, because they need oxygen, which is also absorbed through the skin. They can wiggle about, breathing in their fashion, for some time in the well-aerated water of a stream or lake, but will quickly suffocate in dead or stagnant water.

Floods—worm-size floods at least—are frequent and fatal natural calamities for nightcrawlers, as anyone knows who has ever had to sweep off a tennis court covered with their slippery remains or missed a putt because a ball was deflected by corpses sprawled on a green. Most often this sort of mess occurs on a warm day after a rainy evening during which there was enough precipitation to fill worm tunnels with standing, unoxygenated water. So trapped, the nightcrawlers must get out and find high ground or drown. Once forced to abandon their familiar tunnels, they mill about in confusion. Something like trolls, who are reported to turn to stone if caught out in the sunlight, worms are usually doomed if morning finds them wandering around in the open. Many will be scoffed up by birds and other predators. Rising temperatures and drying winds will dehydrate the rest, leaving them like strands of stale spaghetti.

Nightcrawlers operate in a fairly narrow temperature range—optimally between 40 and 60 degrees. They run the risk of shriveling at higher temperatures and of freezing at lower ones. Still, extremes of heat and cold are less of a threat than the lack or excess of water, because they are at home in dirt, which is one of the best of all natural insulating materials. Like so many slippery columns of mercury in a thermometer, worms go up and down in their tunnels in response to temperature changes, surfacing on nice cool evenings and descending when the weather is bad for their purposes. During prolonged hot or cold periods, they will retreat to the bottom of burrows that may extend eight feet or so underground. There they will curl up in balls and wait for more clement conditions. If necessary, they can remain tunnel-bound, quiescent but alive, for four or five months.

The tunnel is to a nightcrawler what the hive is to a bee or a shell to a snail—an absolutely indispensable structure without which life is poor, brutal and short. Perhaps no modern authority is more emphatic on the importance of the burrow than is Ray Edwards, the earthworm guru of Maple Park, Ill. Edwards is the author and publisher of a slim paperback, The Nightcrawler Manual, which so far as I am concerned is a modern natural history classic both for instruction and entertainment. A quiet, unpretentious man of 34, Edwards has evolved a unique literary technique that can perhaps be best described as writing at a shout. A passage from The Nightcrawler Manual demonstrates this forceful and effective prose style as it drives home some essential points about the home life of fishbaits: "Night-crawlers DON'T live in dirt! They are TUNNEL DWELLERS. Soil is merely the BUILDING MATERIAL they use to construct the tunnel. The difference between saying 'worms live in dirt' vs. 'nightcrawlers live in a tunnel' may appear slight. Let me assure you it is of the utmost importance that you make the distinction. Tunnels are the KEYS to the survival of crawlers. Understanding this fact is ONE OF THE KEYS which will lead to your success in working with them."

A nightcrawler is continually improving its tunnel, extending it downward—but seldom horizontally—and enlarging the diameter to accommodate increased girth as the worm grows. Considering the importance of this activity, nightcrawlers are not impressively equipped for excavation, lacking hard parts which might serve for scooping, shoveling or prying. They create tunnels by wriggling downward into crevices and moving earth from them by ingesting it. In a day they may swallow an amount equal to their own weight. This burden is then deposited on the surface as excrement, the mounds of castings which dot any wormy field. Properly speaking, worms cannot be said to eat dirt, because, except for minute quantities of nutritional minerals, dirt is taken in not for food but to provide grit for their gizzards. In this organ, just as in those of chickens and other birds, the abrasive matter serves to grind up organic foodstuffs which they collect on the surface.

Being slow, relatively weak burrowers, nightcrawlers cannot quickly construct a new tunnel if something happens to or they are removed from the old one. Thus, attempts to increase the worm population of a garden or lawn by throwing out a few handfuls of mature animals are not apt to be successful. Long before most of them can get back underground, they are taken by predators or dehydrated. Successful individuals spend their entire lives in, or at least with part of their bodies in, the same tunnel. Confusion about this situation is common and makes Ray Edwards very IMPATIENT, as he indicates in The Nightcrawler Manual. "Crawlers are NOT nomads. They have no way of finding the entrance to their tunnels once they leave. The worms you see CRAWLING have become separated from the only source of protection they know and are fighting a losing battle for survival. Crawling is NOT part of the NORMAL behavior of LIVING nightcrawlers. So why am I making such a 'big deal' about it? When you make an OBSERVATION of an ISOLATED EVENT, derive CAUSE and EFFECT relationships from that 'sighting,' then GENERALIZE your CONCLUSIONS, and APPLY your ASSUMPTIONS to the habits of ALL 'nightcrawlers'—serious problems arise which slant your thinking and fog your mind."

Edwards goes on, "It is ironic that an entire species of animals has been named in reflection of the behavior exhibited at the time they are about to die." He suggests that more descriptive names would be "night-stick-their-heads-outers" or "after-dark-comers-near-the-surface," but admits these may be a bit cumbersome for everyday use.

Though they never voluntarily leave their tunnels entirely, nightcrawlers do indeed spend a good bit of time partly extruded from them, and the acts which they perform in this position are vitally important. Mating, for example, occurs on the surface when two neighboring worms meet by leaning out of their tunnels. They surface tail-first to deposit the casting of earth which they have swallowed underground. In feeding, the process and position are reversed. Keeping at least a few tail segments securely wedged in the mouth of a tunnel, a worm stretches out and grabs bits of grass, leaves, twigs and almost any other sort of available organic matter in its muscular mouth parts. These foodstuffs are dragged underground, where they are either eaten and digested immediately or cached in galleries. For their size, fish-baits are immensely strong. While foraging, one of them can seize and carry back to its tunnel objects that weigh 60 times more than it does. Many of the faint background rustlings and scrapings to be heard on a still summer night in grasslands are the sounds of worms moving huge loads of food across the surface and down into their tunnels.

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