Arizona’s desert millipede, known scientifically as Orthoporus ornatus, is one of the most recognizable creatures to emerge during the state’s summer monsoon season. These slow-moving, many-legged arthropods are harmless to people, and spotting one after a rainstorm is practically a rite of passage for Arizona residents. Here’s what you need to know about them, whether you’re curious about the one crossing your patio or trying to keep them out of your house.
What Arizona’s Desert Millipede Looks Like
The desert millipede averages about 4 inches long, though individuals range from 3 inches to over 9 inches. They’re dark brownish in color, sometimes with a yellowish tint, and their appearance varies slightly across different parts of their range. Each body segment carries two pairs of legs, which is the easiest way to tell a millipede from a centipede (centipedes have one pair per segment and move much faster).
Their head segment houses a pair of sensory organs at the base of each antenna called the organs of Tömösváry, which help them detect humidity and vibrations in their environment. This is part of what makes them so good at finding moisture in the desert. Despite the name “millipede” (thousand legs), most individuals have a few hundred legs at most.
Why They Appear During Monsoon Season
If you’ve lived in Arizona long enough, you’ve noticed these millipedes wandering across roads, sidewalks, and patios after summer rainstorms. Monsoon season, roughly June through September, is their most active period. The rain draws them to the surface from underground burrows, and they take advantage of the moisture to forage and find mates.
During the dry months, desert millipedes spend most of their time burrowed into soil where they can access residual moisture. They’re scavengers, feeding primarily on decaying plant material and other organic matter. In the desert ecosystem, they play a useful role by breaking down dead vegetation and recycling nutrients back into the soil.
Are They Dangerous?
Desert millipedes are not dangerous. They don’t bite, and they’re not venomous. Their only defense when threatened is to curl into a tight spiral and release a dark, foul-smelling liquid from glands along their body. This fluid is composed almost entirely of compounds called quinones, which make up roughly 87 to 95 percent of the secretion. The two main chemicals are types of benzoquinone, the same irritating compounds found in the defensive sprays of some beetles.
This secretion can irritate your skin, especially if it contacts sensitive areas like your eyes or the inside of your mouth. It may also leave a temporary brownish stain on your hands. If you handle a millipede and get the fluid on you, wash the area with soap and water. Avoid touching your face until you do. For most people, any skin irritation is mild and short-lived.
Keeping Millipedes Out of Your Home
After heavy monsoon rains, millipedes sometimes wander indoors, especially into garages, bathrooms, and laundry rooms. They’re attracted to moisture, so a damp interior is an open invitation. The most effective approach combines sealing entry points with reducing indoor humidity.
Seal Entry Points
Caulk foundation cracks at ground level. Install or replace door sweeps and weatherstripping, paying special attention to garage side doors. Seal gaps around utility line penetrations by packing steel wool into the gap, then caulking over it. Screen crawl space vents with quarter-inch hardware cloth. If your home has brick weep holes large enough for millipedes to squeeze through, cover them with fine mesh.
Reduce Moisture
Fix leaky pipes and faucets, even slow drips. Make sure bathroom exhaust fans vent outside (not into the attic) and that your dryer vent is properly connected. Address any standing water in floor drains, sump pump pits, or around HVAC units. If you have a basement or crawl space, running a dehumidifier to keep relative humidity below 50 percent makes the space far less hospitable to millipedes.
Manage Your Yard
Outside, pull mulch back at least 3 feet from your foundation. Remove leaf litter that collects against the house, and make sure downspouts extend at least 4 feet from the foundation wall. A gravel or bare-soil buffer zone around your home’s perimeter eliminates the moist, organic layer that millipedes travel through. Clear any stacked cardboard or paper from garage floors, since cardboard absorbs moisture and can even serve as a food source.
Natural Deterrents
Food-grade diatomaceous earth, applied as a light dusting along baseboards, foundation cracks, and door thresholds, can help. It works by damaging the millipede’s outer layer and causing dehydration, but it loses effectiveness when wet, so reapply after mopping or moisture exposure. Essential oils like cedar, peppermint, and tea tree oil show modest repellent effects when mixed (10 to 15 drops per cup of water) and sprayed around entry points, but the scent fades within one to three days, so frequent reapplication is necessary.
Keeping a Desert Millipede as a Pet
Desert millipedes are one of the more popular pet invertebrates in the Southwest, and their care is relatively straightforward compared to tropical millipede species. A 10-gallon glass aquarium with a screen lid works well for Orthoporus ornatus. The screen lid is important because this arid species needs higher ventilation than tropical millipedes, which are typically kept in sealed plastic tubs.
Substrate depth matters more than you might expect. Aim for 8 to 10 inches of substrate, or at least one to two times the length of your longest millipede. The substrate isn’t just bedding; it’s their primary food source. Mix organic, pesticide-free topsoil with at least 50 percent rotting wood and dried decaying leaves by volume. The wood should be soft enough to crumble apart in your hands, roughly the consistency of pulled pork. This mimics the decaying organic material they eat in the wild.
Unlike tropical species that need 70 to 80 percent humidity, desert millipedes do better on the drier side. Keep one area of the enclosure lightly misted so they can access moisture when they need it, but avoid making the entire substrate soggy. A shallow water dish provides additional hydration. You can supplement their diet with slices of cucumber, squash, or other vegetables, but the substrate itself should supply the bulk of their nutrition.
Desert millipedes are docile and tolerate gentle handling, which is part of their appeal. Just wash your hands afterward to avoid any residual defensive secretion, and avoid handling them roughly enough to trigger that curling defense response.