If your millipede has disappeared into the substrate and you haven't seen it for a week, don't panic. It's probably moulting. This is one of the most common sources of worry for new keepers, and nearly always turns out to be completely normal.
Why millipedes moult
Millipedes have a rigid exoskeleton made largely of calcium carbonate. Unlike our skin, it doesn't stretch. The only way a millipede can grow is to shed its old exoskeleton, expand while the new one is still soft, and then wait for it to harden. This process is called ecdysis.
Here's the bit that surprises people: millipedes don't just get bigger when they moult. They actually add new body segments and new pairs of legs with each successive moult. This is anamorphic development. A juvenile millipede has far fewer segments and legs than an adult of the same species. Each moult adds more.
How often do they moult?
It depends on the species, their age, temperature, and how well-fed they are. Young, fast-growing millipedes moult more frequently than adults. A juvenile Archispirostreptus gigas might moult every few weeks during its growth phase. Adults moult less often, perhaps once or twice a year, and the rate slows further as they age.
Warmer temperatures and good nutrition tend to speed up the cycle. Cooler temperatures slow it down.
What it looks like
You usually won't see the actual moult happening, because millipedes almost always moult underground. They'll burrow into the substrate, sometimes creating a small chamber, and shed their old exoskeleton there. This is why deep substrate is so important. Without enough depth to bury themselves properly, they may attempt to moult on the surface, which is risky. Surface moults are more likely to fail because the millipede is exposed to lower humidity and potential disturbance.
After moulting, the millipede's new exoskeleton is pale and very soft. Over the next week or two, it gradually hardens and darkens to its normal colour. You might occasionally find the old exoskeleton (called an exuvia) in the substrate, though millipedes often eat it to reclaim the calcium.
What can go wrong
Failed moults are the main risk, and they're almost always caused by one of two things: insufficient humidity or disturbance during the process.
If the substrate is too dry, the old exoskeleton doesn't separate cleanly from the new one underneath. The millipede gets stuck partway through, and there's not much you can do at that point. A partially shed exoskeleton can constrict the animal, prevent it from feeding, or leave sections of the body exposed and vulnerable to infection.
Disturbance is the other killer. If you dig up a moulting millipede, even accidentally while cleaning the enclosure, you can disrupt the process with the same result. Once a millipede has gone underground and you suspect it's moulting, leave it alone completely. Don't dig around in the substrate, don't rearrange the enclosure, and don't worry about not seeing it for a while.
How to support a healthy moult
Most of the work happens before the moult starts. Get the conditions right and the millipede does the rest.
Substrate depth. At least 10-15 cm for large species like A. gigas. Deeper is better. They need room to bury themselves completely and create a moulting chamber.
Humidity. Keep the substrate consistently moist, like a wrung-out sponge. If the surface is drying out, mist it. The lower layers should stay damp naturally if you've got enough depth, but check periodically. Never let the substrate dry out completely.
Calcium. The new exoskeleton requires a lot of calcium to harden properly. Make sure there's always a cuttlefish bone in the enclosure, and consider adding crushed eggshell or calcium powder to the substrate itself. A calcium-deficient millipede will produce a soft, weak exoskeleton that may not harden properly even under good conditions.
Temperature. Keep things stable. A sudden temperature drop during moulting can slow the process and increase the risk of complications. For tropical species, 22-26C is the general range to aim for.
Signs that a moult is coming
There aren't dramatic warning signs, but you might notice a few things. The millipede may eat less or stop eating entirely in the days before a moult. It may become less active and spend more time buried in the substrate. Some keepers report that their millipedes look slightly duller or paler just before shedding, though this is subtle and easy to miss.
If your millipede goes underground and stays there, the best thing to do is nothing. Keep the substrate moist, keep the temperature stable, and wait. Most moults take a few days to a couple of weeks from start to finish, including the hardening period afterwards.
After the moult
When your millipede reappears, it may look paler than usual. That's normal. The new exoskeleton darkens over several days as it fully hardens. Don't handle the millipede for at least two weeks after you first see it post-moult. The exoskeleton may look hard on the outside but still be soft enough to damage with handling pressure.
You might also notice the millipede eating more than usual after a moult, particularly calcium-rich foods. This is the body catching up on resources used during the shedding process. Make sure food is readily available when it resurfaces.
Moulting is completely routine, and with proper husbandry it goes smoothly the vast majority of the time. The millipedes have been doing this for hundreds of millions of years. Your job is just to keep the conditions right and resist the urge to check on them.