Breeding millipedes is one of those things that's either very easy or incredibly slow, depending on the species. Small tropical species like Trigoniulus corallinus will reproduce readily if conditions are right. Large species like Archispirostreptus gigas breed so slowly in captivity that most keepers wait years for results, and some never see any offspring at all.
Sexing millipedes
Before anything else, you need to know which are male and which are female. In most millipede species, males have gonopods: a pair of modified legs on the seventh body segment used for transferring sperm during mating. In practice, this looks like a gap or indentation on the underside of the animal where you'd expect to see legs.
To check, gently turn the millipede over and count forward from the head. Look at the seventh segment. Males will have either visibly modified legs or a noticeable gap. Females have a uniform row of walking legs without this modification.
In some species, there are other differences. Males of certain species are slightly slimmer than females, or have a different body shape around the head and anterior segments. But gonopod presence is the reliable method.
Getting them to breed
The honest answer is that you mostly just provide good conditions and wait. Millipedes aren't animals you can push into breeding by manipulating photoperiod or temperature cycles the way you might with reptiles. If you have a healthy group of mixed-sex adults in a well-maintained enclosure, mating will happen when the animals are ready.
That said, a few things seem to help. Stable, warm conditions (22-26C for tropical species) with consistently high humidity (70-85%) provide the baseline. Good nutrition matters: a substrate rich in decaying hardwood and calcium, supplemented with fresh vegetables. Some breeders report that a slight temperature increase or a period of increased misting can trigger mating activity, but this is anecdotal rather than well-documented.
Mating itself involves the male curling around the female and transferring sperm using his gonopods. You might see this at the substrate surface, particularly at night when the millipedes are most active. It's not aggressive, and there's no need to intervene.
Egg laying
Females lay their eggs in the substrate, typically in a small chamber they construct underground. The eggs are small, round, and pale. In some species, the female coats them in a mixture of substrate and secretions. Depending on the species and temperature, eggs take anywhere from a few weeks to several months to hatch.
You won't usually see the eggs unless you're specifically digging for them, which you shouldn't be doing. Leave the substrate undisturbed during and after egg laying. If you're doing a substrate change, be very gentle and watch for tiny eggs or newly hatched young.
Pedelings: the early stages
Newly hatched millipedes (pedelings) are tiny. For a large species like A. gigas, the pedelings are only a few millimetres long and have very few body segments and legs. They're pale, fragile, and live within the substrate, feeding on decaying organic matter and absorbing calcium.
With each moult, they add more segments and more pairs of legs. This anamorphic development is gradual. A juvenile A. gigas might take 3-5 years to reach adult size. Smaller species develop faster: T. corallinus pedelings can reach maturity within a year under good conditions.
Keep the conditions stable for growing young. Humidity is particularly important for pedelings, as they're more vulnerable to desiccation than adults due to their smaller body size and thinner exoskeletons. Make sure there's plenty of calcium available in the substrate, not just on the surface where tiny juveniles may not reach it.
Species-specific breeding notes
Archispirostreptus gigas: Notoriously slow breeders in captivity. Females may not lay for months or even years after mating. Clutch sizes are relatively small. Growth to adult size takes 3-5 years. If you want to breed this species, be genuinely patient. Some long-term keepers have had success by maintaining a stable group of 4-6 adults in a large enclosure and simply waiting.
Epibolus pulchripes: Somewhat easier to breed than A. gigas, though still not fast. Females lay in the substrate, and development takes a couple of years to reach adult size. More likely to breed successfully in a group than as a single pair.
Trigoniulus corallinus: The easiest commonly available species to breed. They reproduce quickly and in reasonable numbers. If you have a group in a warm, humid enclosure with good substrate, you'll likely see young appearing within a few months. Useful for building up colony numbers for bioactive setups.
Practical considerations
Think about what you're going to do with the offspring before you start. Small species can produce enough young to overwhelm a single enclosure. Large species grow so slowly that you'll be maintaining juveniles for years before they're adult-sized.
Separating juveniles from adults isn't strictly necessary for most millipede species, as adults don't prey on young. The main risk is competition for food in overcrowded conditions. If you notice a lot of young, consider splitting the group into multiple enclosures or offering them to other keepers.
Substrate quality matters even more when breeding. The young live in and feed from the substrate almost exclusively during their early moults. A substrate that's depleted, dried out, or calcium-poor will stunt growth and cause failed moults in juveniles long before adults show any problems.
Captive breeding of millipedes is worth doing if you're the patient type. It reduces demand for wild-collected animals, and watching tiny pedelings grow into full-sized adults over months and years is genuinely satisfying in a way that's hard to explain until you've done it.