Archispirostreptus gigas is the millipede most people start with, and for good reason. They get properly big, they're calm enough to handle, and their care isn't particularly complicated once you've got the substrate sorted. That said, there's a difference between keeping them alive and keeping them well. So here's the detail on what they need.
Size and lifespan
Adults commonly reach 25-30 cm in length, with some specimens exceeding 30 cm. They're heavy, solid animals. In captivity, they typically live 7-10 years with proper care, which makes them a longer commitment than a lot of people realise when they pick one up at an invert show. Think about where you'll be in a decade before buying.
Enclosure
These are big millipedes and they need a proportionally sized enclosure. A glass terrarium or plastic storage box of at least 60x40x40 cm works for a pair. If you're keeping a group (which they tolerate well), go larger. Floor space matters more than height, since they're ground-dwelling, but some vertical space lets them climb over decor and each other.
Ventilation should be moderate. Avoid full mesh lids that let humidity escape. A few rows of holes or a partially ventilated lid keeps the air moving without drying things out. If you're using a glass terrarium with a mesh top, cover part of the mesh with cling film or a piece of acrylic to retain moisture.
Substrate
Substrate is the single most important part of A. gigas care. They eat it, burrow in it, moult in it, and lay eggs in it. Get this wrong and everything else becomes secondary.
Mix roughly 50% well-rotted deciduous hardwood (oak and beech are the standards), 30% coco coir, and 20% organic topsoil or leaf mould. The hardwood component needs to be properly decomposed, soft enough that you can crumble it between your fingers. Fresh or partially rotted wood isn't suitable.
Depth: 15-20 cm. These are large animals that burrow deeply, especially before moulting. Shallow substrate restricts this behaviour and can lead to incomplete or failed moults.
On top of the substrate, add a thick layer of dried deciduous leaves. Oak and beech are standard. The millipedes eat these from the surface. Replace them as they get consumed and broken down. This leaf litter layer also helps maintain humidity at the substrate surface.
Calcium supplementation is essential. Scatter crushed cuttlefish bone or limestone chips across the surface. Millipedes need calcium for their exoskeleton, and without it, moults go wrong. You'll see them actively grazing on cuttlebone, especially after or before a moult.
Never use softwood in any form. No pine, no cedar, no spruce. The aromatic phenols in softwoods are toxic to millipedes. This applies to substrate components, enclosure decor, and any wood you add for climbing structure. Deciduous hardwood only.
Temperature and humidity
Aim for 22-26C. Most UK homes are a bit below this in winter, especially at night. A heat mat on the side of the enclosure (not underneath) with a thermostat is the standard fix. Side mounting avoids drying out the lower substrate layers where the millipedes spend much of their time. If your room stays above 20C consistently, you may not need supplemental heating at all.
Humidity should sit around 70-80%. Mist the enclosure every day or two. The substrate should feel damp when you press it, like a wrung-out sponge. Not dripping wet, not dry and dusty. If the surface is drying out quickly, your ventilation might be too aggressive. If you're seeing standing water or soggy patches, ease off on the misting and improve airflow.
Feeding
The substrate and leaf litter are the primary diet. Beyond that, offer fresh vegetables a few times a week: cucumber, courgette, sweet potato, carrot, and mushrooms are all taken readily. Remove uneaten food within 48 hours to avoid mould and grain mite problems.
Don't overfeed fresh food. It's supplementary, not the main course. The bulk of their nutrition comes from the decaying wood and leaves in the substrate. If you're offering fruit, keep it occasional. Fruit attracts more mites and moulds faster than vegetables.
A small amount of protein now and then is fine. Some keepers put in a few fish food flakes or a piece of dried shrimp weekly. Millipedes do consume small amounts of animal protein in the wild, though they're primarily detritivores.
Handling
A. gigas tolerates handling well. Let them walk from one hand to the other rather than gripping them. Support the full length of the body. If they curl up, they're startled. Set them back and try again later, or just leave them be.
Wash your hands afterwards. A. gigas produces benzoquinone defensive secretions from ozopores along the sides of their body. These can stain skin brown and irritate your eyes and mucous membranes if you rub your face. It's not dangerous in normal circumstances, but it's unpleasant and completely avoidable with basic hygiene.
Health and problems
The most common issue is dehydration. A millipede that looks dull, moves sluggishly, or has a slightly wrinkled body surface needs more humidity. Mist the enclosure and check your substrate moisture. If caught early, they bounce back quickly.
Mite infestations happen. Tiny white grain mites (Acarus siro) thrive in the same warm, humid conditions your millipedes need. A small background population is normal and harmless. If numbers suddenly increase, you're probably overfeeding fresh food or the substrate is too wet. Cut back on food, remove any mouldy items, and improve ventilation slightly.
Failed moults are the most serious problem and usually result from calcium deficiency or insufficient humidity. There's very little you can do once a moult goes wrong. Prevention through consistent calcium supplementation and stable humidity is the only realistic approach.
Wild-caught specimens sometimes carry nematode parasites, visible as tiny white worms in the substrate or on the animal. This is one of the reasons captive-bred stock is preferable.
Breeding
It happens, but slowly. Females lay eggs in small chambers deep in the substrate. Offspring are tiny and pale with very few segments. They grow by moulting, adding new segments and legs with each moult (anamorphic development). It takes years for them to reach adult size. If you keep a mixed-sex group in good conditions, you'll likely see offspring eventually, but don't expect rapid population growth.