From the pages of 'The British Tarantula Society Journal'

Humidity For Tarantulas

Dr. Robert Bustard


Humidity appears to be the most misunderstood factor in the tarantula keeping equation. Many people with small collections - and this must include most beginners - cannot justify heating a space for their tarantulas which is perhaps ideal. So they (mostly) resort to heat mats.

I find from my correspondence that invariably beginners place the heat mats underneath the tank. This results in a number of problems fully dealt with in Alan Stillwell in an admirable, two-part article "So you want to keep a tarantula?" in 'Insect World' Vol. 1 parts 1 & 2 (1995). Here we are only concerned with the humidity aspects which, as Alan Stillwell correctly states, can be overcome by fixing the heating pad to the back cover or side of the tank.

I always recommend a peat base to the tank. Clearly for tarantulas this has to be a minimum of half an inch (12mm) deep, and more usually at least an inch (25mm) deep. The instructions with heat mats clearly state that you should not use a covering layer of "more than a thin (5mm) thickness of base medium." As pointed out by Alan Stillwell, if you use Vermiculite that is an even more efficient insulator (preventing the heat from rising into the tank).

The effect of a heat mat below even slightly moistened peat is that it will drive the moisture out of the peat. The resultant water vapour will, on reaching the cooler lid of the container, condense to drip back into the substrate. This constant recycling of the moisture contained in the peat substrate will ensure that the humidity in the tank is a constant 100% which is totally unsuitable for your tarantula irrespective of it's habitat preferences. When a tank has been set up and allowed to stabilise, if there is a lot of water condensing on the sides of the tank, then the humidity therein is 100% (exceptions are where the room temperature has suddenly dropped or is greatly below tank temperature).

Having set up your tank it is a good idea to allow the peat substrate to at least partially dry out at one end and only spray about one half of the tank. This will allow the spider to choose it's substrate humidity by shuttling across the tank. I have found this technique very useful when rearing baby Theraphosa blondi, which invariably choose to moult at the dry end.

Ventilation is also invaluable. It will reduce the humidity (which, if it falls too far can be increased by more spraying). Most importantly, it will provide air movement which is so crucial for the successful keeping of many arboreal species which will not flourish in (too) humid, stagnant air. I tell beginners that you should be able to grow a plant in arboreal spiders' tanks. If the plant dies from 'damping off the air is too humid/stagnant for the tarantula.

I have recently written a detailed humidity guide for beginners. At Ann Webb's suggestion, when revised, I will offer it to the Journal for a lot of our losses are due to too high humidity, often combined with too stagnant air. This is fatal for eggsacs.

Again I find that many people have little idea of humidity levels and the compact mini-temperature and humidity gauges can prove invaluable.

I would fniish by underlining a comment of Alan Stillwell in his excellent article referred to above. If put along the back of the tank the heat mat should not cover the whole length of the tank (one half or so should be adequate) so that the spider can move to a cooler area if it becomes overheated. A sheet of polystyrene should be placed behind the heat mat to direct the heat forwards into the tank.

Copyright 'The British Tarantula Society', 1996,2007


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Last Updated: February 12, 2007