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 adaptated 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.
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.
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.
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.
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.