Breeding
Some species are easy to breed such as most of the ones we sell. Others are more difficult. If you have both genders, have mature animals, care for them properly, and have a deep, nutritious substrate: you'll likely find baby millipedes like I just did! It really is that easy. It's hard to wait, but the millipedes do the work. All you can do is try to provide them with ideal conditions--do so and they should reproduce. Some millipedes, such as A. gigas, typically only breed once a year as adults whereas others breed constantly. Also, eggs can take several months to develop, so you must have patience. Put the adults in a setup, then wait. If you don't have babies within 2 years (you will need to change the substrate or move the adults to a fresh cage during that time) you're probably doing something wrong. Millipedes won't breed if the animals are not mature, healthy, and all the same gender (females can store sperm, but males obviously cannot lay eggs.)
A layer of oak leaves on the surface of the substrate may encourage Archispirostreptus gigas to reproduce, but these millipedes are notoriously difficult to breed (they do not readily lay eggs) and for this reason they are becoming extinct in the US hobby.
A layer of oak leaves on the surface of the substrate may encourage Archispirostreptus gigas to reproduce, but these millipedes are notoriously difficult to breed (they do not readily lay eggs) and for this reason they are becoming extinct in the US hobby.
Different species cannot inter-breed, but they might try...
The video below shows a male flameleg millipede trying to mate with a female Philippine blue millipede. The blue millipede is not receptive. Watch what happens: