Temperature and humidity are the two things that kill the most captive invertebrates. Not predators, not disease, not old age. Conditions. Get these two parameters wrong and it doesn't matter how good your substrate is or how carefully you chose your species. The animal will die, usually slowly and without obvious symptoms until it's too late.
Why invertebrates care so much about conditions
Invertebrates are ectothermic. They can't generate their own body heat the way mammals do. Their body temperature matches their surroundings, which means their metabolism, digestion, growth rate, and immune function all depend on the temperature you provide. Too cold and everything slows down: feeding stops, immune function drops, and opportunistic infections take hold. Too warm and metabolic rate spikes, water loss accelerates, and the animal overheats.
Humidity matters because most exotic invertebrates come from tropical or subtropical environments where the air holds a lot of moisture. Isopods breathe through gill-like pleopods on their underside. If those dry out, the animal suffocates. Millipedes and many beetles rely on moist substrate for healthy molts. Jumping spiders are more tolerant of drier conditions, but even they need access to water droplets for drinking.
Temperature in practice
Most commonly kept tropical invertebrates do well between 20-26C. That's a comfortable range for a heated room in the UK, which is partly why these species work as pets here. But British houses cool down overnight and through winter, and that's where problems start.
If your house regularly drops below 18C at night, you need supplemental heating for tropical species. Heat mats are the standard tool. Attach them to the outside of the enclosure, on the side or underneath, and always run them through a thermostat. A heat mat without a thermostat is a fire risk and can cook your animals. It's not optional kit.
Place your thermometer inside the enclosure, at the level where the animals actually live. A thermometer stuck to the outside glass tells you the glass temperature, not the air temperature inside. For burrowing species like beetle larvae and millipedes, a probe thermometer pushed into the substrate gives you the most useful reading.
Temperature gradients
Ideally, your enclosure should have a slightly warmer end and a slightly cooler end. This lets the animal move to wherever it's comfortable. A heat mat on one side of the enclosure creates a natural gradient. The warm end might sit at 24-26C while the opposite end stays at 20-22C. That's a perfectly workable setup for most tropical inverts.
Avoid heating the entire enclosure uniformly. If every spot is the same temperature, the animal has no way to cool down if it needs to.
Humidity in practice
Humidity is trickier than temperature because it interacts with ventilation. More ventilation means humidity escapes faster. Less ventilation means humidity stays high but air stagnates, which promotes mould and bacterial growth. You're always balancing moisture against airflow.
Most tropical invertebrates thrive at 60-80% relative humidity. But that's a wide range, and species-specific needs vary. Cubaris isopods want the upper end, 75-90%. Armadillidium vulgare tolerates drier conditions than most isopods. Jumping spiders are fine at lower humidity as long as they have water droplets to drink from.
Measuring humidity
Buy a digital hygrometer. The cheap analog dials that come bundled with starter terrariums are unreliable. They're often off by 10-20%, which is the difference between "fine" and "everything is dying." A digital hygrometer costs a few pounds and gives you a reading you can actually trust.
Place it at substrate level for ground-dwelling species, or at mid-height for arboreal species. Humidity varies within an enclosure. The reading at the top, near the ventilation, will be lower than the reading at the bottom, near the damp substrate. Know where your animals spend their time and measure there.
Moisture gradients
The same principle as temperature gradients applies: give the animal a choice. Spray one end of the enclosure and leave the other end dry. This is particularly important for isopods, which need to move between damp and dry areas to regulate their own moisture levels. A fully soaked enclosure is as dangerous as a bone-dry one. Waterlogging drowns mancae (juvenile isopods) and promotes bacterial problems.
Managing humidity
The main tools are:
- Spraying with dechlorinated water (use a fine mist, not a jet)
- Moisture-retaining substrate (sphagnum moss is excellent for this)
- Reducing or increasing ventilation holes (fewer holes = higher humidity, more holes = lower humidity and better airflow)
- A water dish or damp moss corner for localised moisture in drier setups
Use dechlorinated or aged water. Chlorine and chloramine in tap water aren't great for invertebrates. Isopods are particularly sensitive because their gill-like breathing apparatus is in direct contact with moisture in the environment.
When things go wrong
Too dry
Signs: animals clustering in one corner (probably the dampest spot), visible dehydration (shrivelled, wrinkled appearance), failed moults (the old exoskeleton sticks to the animal because there isn't enough moisture to soften it), and in isopods, deaths without obvious cause. Mist immediately and review your ventilation. You may have too many holes, or the substrate may not be retaining moisture.
Too humid
Signs: heavy condensation on all surfaces, mould spreading across substrate and food, stagnant smell, and mite infestations. Grain mites (Acarus siro) thrive in warm, damp, stagnant conditions. They appear as a fuzzy white coating on food or substrate surfaces. They're not directly harmful to most inverts but they indicate conditions need fixing. Improve ventilation, reduce spraying frequency, and remove any mouldy food.
Too cold
Signs: animals stop feeding, become sluggish, stop breeding. Millipedes curl up and stay curled. Beetle larvae stop growing. In serious cases, immune function drops and infections follow. Add or adjust supplemental heating, always with a thermostat.
Too hot
Signs: animals actively trying to escape, clustering away from the heat source, dehydration. Overheating kills faster than cold. Move the enclosure away from direct sunlight (a south-facing windowsill in summer can easily exceed 35C) and check your heat mat thermostat is working.
The bottom line
Spend a tenner on a decent digital thermometer and hygrometer. Check them regularly. Adjust conditions before problems become visible, because by the time an invertebrate looks unwell, it's often past saving. Prevention is the only realistic approach to invertebrate health. There are almost no invertebrate vets, and most problems that kill inverts are environmental ones you can measure and fix yourself.