Every experienced keeper has a list of things they did wrong early on. Most of us lost animals because of it. The frustrating thing is that nearly all these mistakes are avoidable if someone tells you about them in advance. So here's the list, based on years of collective "I wish I'd known."
1. Wrong substrate
This kills more invertebrates than anything else. Beetle larvae need fermented hardwood flake soil. Not potting compost, not garden soil, not wood shavings. They'll sit in the wrong substrate and slowly starve while looking like they're fine. By the time you notice they're not growing, the damage is done.
Millipede substrate must be hardwood-based. Pine, cedar, and spruce contain phenols that poison them. This applies to everything in the enclosure: substrate, decor, climbing branches. If it's softwood, it shouldn't be in there.
Isopod substrate needs a calcium source mixed in or available on top. Without it, moults fail. A failed moult in an isopod is fatal.
2. No thermostat on the heat mat
An unregulated heat mat gets hot. Much hotter than you'd expect. It can cook substrate, boil condensation into steam, and kill animals. Thermostats cost under a tenner and they're not optional. Plug the heat mat into the thermostat, set the temperature, and verify with a thermometer inside the enclosure that it's actually at the right level.
3. Trusting analog hygrometers
Those small round dial hygrometers that come with starter kits are unreliable. They can be off by 10-20%, which is the difference between "good humidity" and "critical dehydration" or "mould explosion." Digital hygrometers cost a few pounds and are worth every penny. Check yours against a known reference now and then.
4. Too much ventilation for humidity-loving species
New keepers often drill lots of holes in enclosure lids because they're worried about airflow. For species that need 70-80% humidity, this makes it nearly impossible to keep conditions damp enough. You end up spraying constantly, and the enclosure dries out within hours.
Start with fewer holes. You can always add more. Covering some holes with tape is a reasonable fix if you've overdone it, though it's not ideal long-term. The goal is enough air exchange to prevent stagnation, not a breeze.
5. Disturbing moulting animals
Moulting is the most vulnerable period in an invertebrate's life. The animal is soft, exposed, and physically unable to defend itself or move. Disturbing a moulting millipede, spider, or beetle larva in its pupal cell can be fatal. The old skin gets stuck, the new body doesn't harden properly, or the animal falls and damages itself.
If a beetle larva has stopped moving and appears to be in a compacted chamber of substrate, it's building a pupal cell. Leave it alone. If a spider is hanging motionless from its web sac, it's moulting. Leave it alone. If your mantid is hanging upside-down from the mesh ceiling and its skin is splitting, it's moulting. Leave. It. Alone.
6. Cohabiting species that shouldn't be together
Jumping spiders are solitary predators. Two in one enclosure means one spider. Mantids are the same. Even if you've "fed them loads," they'll eat each other. Cannibalism in these species is predatory behaviour, not a hunger response.
Some species do fine together. Isopods are colonial. Millipedes generally coexist peacefully. But don't assume everything can share space. Research the specific species first.
7. Using tap water for misting
Chlorine and chloramine in tap water aren't great for invertebrates, especially isopods. Isopods are crustaceans that breathe through gill-like pleopods, which are directly exposed to moisture in their environment. Dechlorinated water, water left out for 24 hours, or filtered water are better options. It's a small effort that removes a real risk.
8. Overfeeding protein to isopods
Isopods need some protein, but only a small amount. A bit of dried shrimp or fish flake once a week is plenty. Dump in too much protein and you'll get a grain mite (Acarus siro) infestation. Tiny white mites everywhere, coating the food and substrate surfaces. They won't kill the isopods directly, but they're a sign you've gone overboard. Cut back on protein, improve ventilation, and remove affected food.
9. Buying the animal before the enclosure is ready
This happens constantly. Someone sees a species they want at an expo or online, buys it impulsively, and then scrambles to set up housing while the animal sits in a temporary container. The enclosure should be set up, at stable temperature and humidity, before the animal goes in. Ideally at least 24 hours beforehand.
Animals that arrive into a proper setup do better than animals stuffed into a rushed temporary arrangement. The stress of transit is enough without adding bad conditions on top.
10. Panicking about white mould
White mould on decaying wood and leaf litter in a damp enclosure is normal. It's part of the decomposition process. Many invertebrates, particularly isopods and springtails, actually eat it. Don't strip the enclosure and start over at the first sign of white fuzz.
Green or black mould is a different story. That indicates stagnant, waterlogged conditions and poor ventilation. If you're getting green or black growth, increase airflow and reduce spraying.
11. No calcium provision
Isopods and millipedes need calcium to build their exoskeletons after moulting. Without it, moults fail. A failed moult in any invertebrate that sheds its exoskeleton is almost always fatal. Cuttlebone is cheap, lasts ages, and should be in every isopod and millipede enclosure.
12. Assuming "low maintenance" means "no maintenance"
Invertebrates need less daily attention than a dog. That's true. But "less" isn't "none." Colonies still need their humidity checked, food replaced, and conditions monitored. An isopod colony left completely alone for months will crash silently. You'll open the lid one day to find a dead colony and wonder what happened. Usually it's dehydration or calcium depletion, both entirely preventable with occasional checks.
Set a reminder on your phone if you have to. A quick check twice a week is enough for most species. Make sure the substrate is damp enough, food is available, and nothing looks off. Five minutes of attention prevents weeks of trying to recover a struggling colony.