Ask ten keepers what humidity an arboreal tarantula needs and you will get ten numbers, most of them copied from a caresheet that copied another caresheet. The number is not the useful part. What matters is the gradient: damp at the bottom, drier at the top, with air moving between the two.
Why misting mostly fails
A spray bottle raises the humidity of the air for twenty minutes. Then it is gone, and you are back where you started, except now there are water droplets sitting on silk and cork where mould likes to start. Air holds very little water. Substrate holds a great deal.
Pour water down one corner of the enclosure instead, enough to make the lower half of the substrate damp without leaving standing water at the bottom. That reservoir evaporates slowly for a week or more and keeps the lower zone humid without soaking the spider's tunnel.
The hygrometer problem
A cheap hygrometer stuck to the inside of an enclosure measures a few square centimetres of air on one wall. Move it ten centimetres and the reading changes. Wet it once and many of them read high for good. If you want to know whether the enclosure is right, look at the animal and the substrate, not the display.
Signs the balance is right: the top surface of the substrate goes lighter in colour between waterings, the silk stays dry to the touch, the spider sits out rather than hugging the damp corner, and there is no persistent condensation on the panels.
Species that genuinely want more moisture
Some arboreals, particularly the Caribbean and South American pinktoes, come from genuinely humid forests. What they still do not want is stagnant air. Keepers lose more Caribena versicolor to closed, wet enclosures than to dry ones. Keep the base damp, keep the air moving, and accept that you will water more often than the person with a sealed box.
A simple weekly routine
- Refill the water dish. Every time you pass it, if you like.
- Once a week, pour water down one back corner until the lower substrate darkens.
- Once a month, look for mould on old prey remains and remove them.
- Never seal vents to raise a number on a gauge.
Winter, radiators and dry rooms
In a northern European winter, indoor air can drop below 30 per cent relative humidity. The enclosure will dry faster, so water more often rather than closing anything off. If a room is so dry that the substrate goes pale within two days, move the enclosure away from the radiator before you change anything about the enclosure itself.