Millipedes of Petroglyph

Pencil sketch of a Desert Millipede (Orthoporus ornatus).
Pencil sketch of a Desert Millipede (Orthoporus ornatus).

Ray Kriese, park volunteer artist.


A Fascinating Creature

Petroglyph National Monument is unique in many ways, but one of its most striking attributes is the seventeen-mile long volcanic escarpment. In addition to having thousands of petroglyph's on the basalt boulders, the escarpment is home to many plants and animals that may not usually be found this far north in the Chihuahuan Desert. The boulders strewn across the escarpment allow moisture and heat to be concentrated. This gives plants and animals the slight edge they need to be able to survive. One such animal is the millipede. Millipedes are known to exist in many different habitats from rainforests to sub-alpine mountaintops to deserts. The millipedes that live in these diverse habitats have adapted in order to survive the elements. Some have developed mechanisms to keep from freezing, to keep from being eaten or to keep from drying out in the heat. In whatever habitat they occupy, millipedes are fascinating creatures that have a long history and are an important member of the ecosystem.

Desert and Slate Millipedes

There are two species of millipedes at Petroglyph National Monument, the Desert Millipede (Orthoporus ornatus) and the Slate Millipede (Comanchelus chihuanus). These two species can be easily distinguished by size and color. Desert millipedes can grow to about 8 inches long and are a chocolate brown color. Slate millipedes are only about 3 inches long and are a dark gray (almost black) color.

The best time to see millipedes is during late July, August and September after rainstorms. This is the "monsoon" season in the southwest. The Chihuahuan Desert will receive up to half of its yearly precipitation during these three months. Millipedes need the extra moisture to be able to eat and process their food. Don't be surprised, however, if you see millipedes other times of the year, because as long as there is sufficient moisture and temperature, millipedes might be out and active.

What is a Millipede?

Millipedes are arthropods, not insects or bugs. An insect typically has three body segments and six legs. A millipede has many body segments and two pairs of legs per segment. If you want to know how many legs a millipede has, count the number of body segments and multiply by four.

According to fossil evidence, millipedes were one of the first organisms to walk on land. Fossilized remains of a millipede-like creature show that it was one of the largest invertebrates to ever walk on land. The creature was almost 2 meters long and 1/2 a meter wide (6 feet long and 1 1/2 feet wide) and looked very much like some modern day millipedes. Most millipedes look about the same and have similar body structure, but there are some millipedes that have hair, spines and other appendages. The body structure of most millipedes includes a calcified head for digging in soil, antenna 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.

Millipedes are detritivores, earth's natural recyclers. They feed on plants and animals that have died, which recycles nutrients back into the soil much faster than waiting for the plant or animal to decompose naturally. This is very important for the survival of living plants and animals because the sandy soils in Petroglyph National Monument tend to have very few nutrients. Waiting for plants and animals to decompose naturally can take years because of the dry conditions.

Millipede Petroglyphs?

Petroglyphs offer opportunities to think about how human inhabitants interacted with nature. For instance, spiral petroglyph's are commonly found in the monument. Some American Indians think they may represent a desert millipede in its coiled, defense posture while others think they may represent water. Since millipedes are most active during the rainy season, perhaps the connection is water. As you view the images, consider how they fit into the landscape and how they may have and continue to be important in American Indian culture.

Survival in the Desert

There are few ways for animals, insects and arthropods to survive in desert conditions. They can evade or avoid climatic conditions, they can find water or their bodies can adapt. Millipedes take advantage of all three of these strategies and if successful, can live up to 10 years.

During the summer and winter millipedes evade conditions by burrowing into the soil. This reduces the amount of water they loose through respiration and allows them to escape extreme temperatures.

To find water millipedes burrow into soils that are a little more moist than their surroundings, such as the base of boulders along the escarpment. Because they are surrounded by a moist environment this also reduces the amount of water they loose.

Finally, millipedes have gone through a physical adaptation. Their bodies, or exoskeletons, are covered with a waxy coating that holds moisture in and reduces water loss when they are active on the desert surface.

Watch your step

Millipedes are fun to watch. Keep in mind millipedes may be harmless but they are wild creatures and should be treated as any other wild animal. Millipedes do not bite, however, they do have natural defensive actions. If a millipede is disturbed or threatened it curls itself into a spiral. It may also secrete a liquid toxin from its secretory glands on the side of its body. This mildly toxic secretion is meant to discourage any natural predators from eating the millipede. Feel free to look at millipedes as they go about their lives, but please do not touch. Some humans may experience a mild to severe allergic reaction to the millipedes' toxic secretion in the form of a blister or rash.

Millipede or Centipede - Can you tell the difference?

Millipedes should not be confused with centipedes (Scolopendra Spp.). Centipedes have a brown and yellowish, flattened body with one pair of elongated legs per body segment. Do you remember how many legs per body segment a millipede has? Centipedes have one pair of longer antenna, millipedes have one pair of shorter antenna.

Centipedes bite. They have strong jaws that inject venom into their victims. A centipede bite is a very painful and unforgettable experience. The affected area will become increasingly painful, inflamed, and will secrete pus. You'll be sorry if you attempt to touch or handle this desert creature.

The sketches below will help you correctly identify a millipede from a centipede. Luckily, they have very distinct physical differences that help us spot them from a safe distance.

Pencil sketch of a Desert Millipede (Orthoporus ornatus).
Pencil sketch of a Desert Millipede (Orthoporus ornatus).

Ray Kriese, park volunteer artist.


Desert Millipede (Orthoporus ornatus)

Pencil sketch of a Centipede (Scolopendra Spp.)
Pencil sketch of a Centipede (Scolopendra Spp.)

Ray Kriese, park volunteer artist.


Centipede (Scolopendra Spp.)

Last updated: January 30, 2018

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