D e s e r t E x p o s u r e
November 2011
Creepy-Crawlies
Meet your desert nightmares: the millipedes, centipedes, scorpions and spiders.
by Jay W. Sharp
Of all the creatures that haunt your bad dreams, especially if you live in our Southwestern deserts, the millipedes, centipedes, scorpions and spiders, whether venomous or not, probably rank near the top of the list.
Even though all have limited mobility and range, the millipedes, centipedes, scorpions and spiders — like their arthropod kin, the insects — occupy widely diversified environments almost across the globe.
Like the insects, too, they all have jointed external skeletons. "Components of the skeleton meet (articulate) at joints, which allows one part of the body to move in relation to another," notes Barbara Terkanian in her article "A Vertebrate Looks at Arthropods," which appears in A Natural History of the Sonoran Desert. "Muscles spanning joints and anchored to different parts of the skeleton provide the power for movement."
The millipedes, centipedes, scorpions and spiders, however, do have distinguishing characteristics, and each of these creatures plays a distinctive role in the desert food chain.
The Millipede
A millipede, shaped much like your little finger, has a multi-segmented body, with two pairs of legs on each segment (hence the name "millipede" or "thousand feet"). It grows and sheds throughout its lifetime, adding a new segment and legs with each shed. Typically, the millipede ranges from tan to reddish-brown to black in color. "The body structure of most millipedes," according to the National Park Service's Petroglyph National Monument website, "includes a calcified head for digging in soil, antennae for sensing things like food, ocelli (simple eyes) for sensing light, mandibles for chewing food, a telson for waste excretion, and secretory glands for self defense."
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This desert-dwelling giant millipede is about eight inches long. (Photo by David B. Richman, NMSU) |
The millipede's ancestors marched near the head of the parade of the earliest animals to emerge from the sea and walk on dry soil. Some early millipedes, among the largest dry-land invertebrates in the fossil record, measured more than six feet in length and nearly two feet in width. Today, most species of desert millipedes measure only a few inches in length and less than half an inch in width.
The millipede takes refuge from the desert heat and drought in an underground burrow, sometimes appropriating an ant burrow, according to Floyd Werner and Carl Olson in Insects of the Southwest. It emerges to feed in the coolness of night or to celebrate in the desert's rejuvenation after a rainfall. It counts on the soil moisture within its burrow and the waxy coatings on its body to conserve water. According to Renée Lizotte's article in A Natural History of the Sonoran Desert, the millipede mother lays her eggs in concealed places, then abandons them, trusting that her offspring will be able to shift for themselves. Those that survive can live for 10 years or more.
While a millipede does not bite or sting, it does wind itself into a spiral shape when threatened, and it exudes a foul-smelling and evil-tasting toxic brown liquid from glands near the tops of its legs. The liquid can trigger a mild to severe allergic reaction in some people. Even if you are so inclined, it is best not to go around handling or eating millipedes.
In its place in the desert food chain, the millipede — one of nature's recyclers — feeds on just about anything that has died, according to the Petroglyph National Monument site. In the desert, where the dry climate tends to preserve organic matter, the millipede accelerates the process of decomposition, reintroducing nutrients to other organisms. Slow-moving creatures, millipedes have great patience and persistence; generations of millipedes may feed for many decades on the trunk of a single fallen tree.
The millipede's only real enemy, say Werner and Olson, is "the larva of a beetle called Zarrhipis. This is a slender yellow-and-black-banded larva up to two inches long, with a vicious bite and obvious venom. It searches down millipedes, kills them with a bite and eats its way down inside from the front."
The Centipede
While they may look rather similar, the centipede differs markedly from the millipede. It has a somewhat flattened, rather than a cylindrical, multi-segmented body trunk, with a single pair, instead of two pairs, of legs on each segment (hence the name "centipede," or "hundred feet"). The centipede's evolutionary predecessors, like those of the millipede, appear in fossil records hundreds of millions of years old.
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Centipede climbing down a rock. (Photos by Jay W. Sharp) |
The six- to eight-inch-long Giant Desert Centipede, the most conspicuous of our various desert species and one of the largest in the world, typically has a reddish-yellow body with a darker head and tail — a visual signal to the world that it can deliver a venomous bite. Its head comes equipped with sensing antennae, and, according to the Backyard Gardener website, "On the first segment behind the head, the centipede has hollow tubes [actually modified legs near the mouth], with openings at their sharpened tips that function as fangs. They are attached to venom glands, and are used to kill prey." As the Pestcontrol website says, at its tail, the Giant Desert Centipede possesses a "'psuedohead,' or false head, to confuse potential predators. Thus, if a predator unwittingly grabs the psuedohead, the envenomated true head is free to bite. The psuedohead bears elongated appendages very similar to the appendages on the head, rendering the centipede very symmetrical in nature."
Lacking the millipede's waxy body armor, which would help protect it from desiccation, the centipede simply avoids desert heat and drought. It seeks out the cooler and moister sanctuary afforded by burrows, caves and ground cover during the hours of daylight. It hunts in the coolness of the night. Like the millipede, the centipede mother lays her eggs in concealed places, but unlike the millipede, the centipede mother stays to tend her brood. She coils protectively around her eggs and her newly hatched young for days, grooming them, presumably to ward off bacteria and mold. The centipede may live for years.