Double-legged Cylinders

by William H. Amos, Vermont, USA



A fast-running centipede gives people the willies, and rightly so when one encounters a giant venomous centipede skittering about in the tropics. Ask my wife about nearly stepping on a ten-incher in our cottage in Hana, Maui. In no uncertain terms you'll learn her opinion of these unlovely creatures—and of me: how upon being urgently summoned in the middle of the night, I trapped the monster in a wastebasket, took it outside, and released it unharmed on a lava rock wall. (My philosophical proclivities didn't make much headway with her on that occasion.)

But ask anyone about a millipede. They'll wait for a moment, hoping for a clue, then say, "Oh, yes, I know," remembering those long, cylindrical, heavily encased animals with a huge number of legs moving slowly and evenly from one place of refuge to another. Watching them crawl along, tapping the ground with their chemically-sensitive antennae, I am reminded of a blind man feeling his way with a cane. Or perhaps someone remembers their ability to curl up in a flat coil in a protective maneuver. Big tropical millipedes can fold themselves into a sphere the size of a squash ball. No one fears these odd and ancient creatures. There is no need to do so.

Millipedes are reclusive and shun light. They are most commonly encountered when a dead log's bark is stripped away, a rock is lifted, or forest litter is disturbed. Their short legs ripple in undulating waves, a miracle of coordination. Millipedes appear to be simple creatures, and that's where we're wrong. Their smoothly segmented external appearance belies a more complex interior, and in the history of life they gave rise to the most remarkable group of animals this world has seen. More of that later.

Most people looking closely at a millipede believe it has two pairs of legs to every segment, an appearance that gives rise to their identification as diplopods (double-legged). If we understood the evolutionary and embryological development of these creatures, we would know that a millipede (an arthropod or joint-footed animal) in no way departs from the architectural plan prescribed for every arthropod: thou shalt have no more than one pair of legs to each segment. In the case of a millipede, its long body consists of overlapping shields, each one covering two internal segments (revealed by doubled heart openings and pairs of nerve ganglia) and two pairs of legs. Somewhere in the distant past there must have been elongated ancestors of millipedes with just as many outer shields as internal segments, and that may have been enough of a problem (too many joints for efficiency's sake?) to reduce the number of outer plates. The impressive result is for a ten-inch-long, fully-grown tropical millipede to have 100 of these shields, therefore 200 pairs of legs!

When you get down to a millipede's level and try to understand how its legs move, you're likely to end up frustrated and rubbing your eyes. You simply can't take it all in, so slow motion films are the best way of seeing how this remarkable creature moves. There is an obvious rhythm of sequential waves of stepping movements passing from rear to front. Slowed up, when a millipede is walking normally, about a dozen legs compose a single wave of lifting and lowering against the surface, then pushing backward. If you try to prevent a millipede from moving forward and it doesn't reverse the direction of its leg-stepping to avoid the obstacle, you may find it very difficult. These animals are vigorous pushers capable of forcing their way into tight spaces. When added power (the original traction control!) is necessary, four times as many legs make up a wave, the exact number being determined by the amount of force necessary to get where it is headed.

Such a determined approach is mirrored by the overall cylindrical shape of a millipede: a blunt, rounded head; smooth hemispherical plates over the body; tightly packed legs, each with a sharp pointed claw. A millipede is not going to be denied going where it wants. Some flattened, robustly-armored species are even more persistent and forceful as they break up decaying wood and dense earth by jamming themselves into the smallest irregularities. The segmental plates of these flat millipedes project sideways like fenders to offer working space for their strong legs when they are in tight quarters. It's obvious that a strong animal able to push simultaneously with several dozen legs is a living bulldozer.

There are over 7,500 species of millipedes, but only a few are predatory, while most graze on small surface plants, or scavenge upon decaying vegetation that they moisten with saliva before chewing it up with sideways-acting jaws. In most animal circles a slow-moving herbivore is attractive food to predators, but if you've ever watched a toad snap up a millipede, you'll quickly see a highly disgusted toad who in its haste to spit out the victim, may stick a leg in its mouth to get rid of the awful taste as quickly as possible.

The reason for such revulsion can be found in tiny pores running laterally along the length of the millipede, one to each side of every covering plate. When a millipede is stressed out, it exudes a bunch of unpleasant substances through these pores, including bitter alkaloids and forms of quinine, even hydrogen cyanide in some species. A single nasty experience is all it takes for most frogs, toads, lizards, snakes, and small mammals to disregard a millipede meal from then on. Even birds, which can't taste much and have almost no sense of smell, seldom bother to pick up a millipede. When threatened, a few kinds of millipedes—to make absolutely sure no predator is going to get them—shoot out a spray of toxic fluid up to a foot away. If such fluids secreted by large tropical millipedes come into contact with human skin, a painful burning and severe rash results.

Mating techniques differ widely between separate groups of millipedes. In one type, when a male nears a female, he signals his interest both by sound and by touch: he chirps and bumps his head on the ground, taps her last segment with his antennae, then climbs on her back, clinging to her with special leg cushions to keep her calm. He transfers sperm collected in sacs on his seventh pair of legs to her reproductive opening. Immediately after mating, the female's last segment—the sensitive one he first touched to allure her—now sends an entirely different message and she scurries away out of his reach.

The males of one family of millipedes have discovered second childhood in the procreative process. After mating, a male molts and reverts to an immature state incapable of sexual function, goes about his everyday business for a while, then molts again, this time as a newly restored reproductive individual in search of a mate. Among this group of millipedes such rejuvenation goes on repeatedly and insures an over-supply of males for every female—reassurance for her, but pretty chancy for the eternally hopeful and youthful guys.

A lot of millipedes build nests of one sort or another that can be architecturally complicated with chimneys and chambers and seals to the opening. Others secrete substances that they make into little cups, one for each egg. When nest or cups are ready, a female lays her eggs, each fertilized at that moment by sperm held in the sac she received earlier from the male.

When a young millipede hatches, it reveals how important its kind has been to the history of life on earth. Millipedes have an ancient origin and appear almost as they do today in rocks 300 million years old. That closely related kinds show up on many continents tells that they evolved and flourished long before the great land masses separated and moved to their present locations. The role millipedes have played on Earth is this:

A just-hatched baby millipede has a head with a pair of compound eyes, a pair of antennae, and lateral jaws. Each of its first three segments supports a pair of legs, and the following four none. This could be a description of a basic insect. A great zoological mystery has always asked, where did insects come from? They appeared in planetary history almost "overnight" in a geological sense. There is a phenomenon in the animal world known as neoteny, or the attainment of sexual maturity when still in a physically juvenile stage. (Human origins are one of many suspected examples, a subject for another article.)

If an immature millipede somehow could escape its evolutionary fate of becoming a long sequence of segments by reproducing in a basic, generalized condition, then all kinds of new opportunities for specialization might be possible. When I said they "gave rise to the most remarkable group of animals this world has seen," it is simply that insects are biologically the most successful animals ever to have lived. By "success" I mean only that they are more diversified, more abundant, more adaptable, more capable of survival in every terrestrial habitat, than any other kind of creature. There seems to be no other explanation for their springboard leap into the world than owing their beginnings to the humble millipede, an animal still here to remind us of origins.

When next you see a millipede, don't recoil in distaste or fright. After all, they do us no harm and their 7,500 species throughout the world are more evidence of the almost infinite diversity of life on earth. I have no idea how many different kinds I've encountered, but of all the millipedes I've seen in the world, the rarest (an unknown species) were several blind, translucent, colorless specimens half a mile down in a lava tube in Hawai'i. They fed upon decaying roots of o'hia trees that penetrated the ceiling of this huge cave, having evolved there long ago after being separated from their surface dwelling ancestors. Still unstudied (as far as I know), other habits and the life cycle of this isolated population remain mysteries. The most impressive millipedes have been tropical species I've found in Asia and Central and South America. When you encounter one a foot long and as thick as your thumb, look but don't touch. You shouldn't even bend over to look closely at several unhappy specimens placed in a container, for in their dismay at being confined they can produce enough hydrogen cyanide gas to make you woozy.

Comments to the author Bill Amos are welcomed.

1997 William H. Amos

Bill Amos, a retired biologist and regular contributor to Micscape, is an active microscopist, naturalist and author. He lives in northern Vermont's forested hill country colloquially known as the Northeast Kingdom.

Editor's notes: Other articles by Bill Amos are in the Micscape library (link below). Use the Library search button with the author's surname as a keyword to locate them.


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