LAST UPDATED ON 2014-February-08.

Psychrometer. (Relative humidity gauge.) 71 Kb. ">FCIT

In this webpage we adopt the meteorological definitions for terms. Other sciences sometimes use slightly different definitions more suited to their requirements and concepts.


Note carefully that in this webpage we do not discuss REQUIREMENTS, but rather TOLERANCES.

Tarantulas do not REQUIRE any specific relative humidity, but rather exhibit TOLERANCES for a wide range of relative humidities.



Purists will argue that a volume of "space" (presumably a vacuum, see Space and Volume) cannot have a temperature, so a lot of this explanation is nonsense. And, they're right!

It isn't the space that increases or decreases in temperature, it's the water vapor in that space that's changing in temperature. But, if we hit you with all the details all at once, we'd lose you for sure.


In the factories where the consumer grade hygrometers and thermometers are manufactured (often squalid shacks in the ghettos of various Asian cities), quality control (QC) usually consists of someone on an assembly line occasionally glancing at a thermometer or hygrometer on the wall during their twelve or fourteen hour shift, and making a quick (½ second), cursory adjustment of the scale of the units that pass through their hands. They usually only concern themselves with the ones that they notice are way off. They don't worry about the changing temperatures in the factory, whether their "gold standard" was accurate or not (it was "calibrated" the same way!), whether there is a temperature or humidity difference between their gold standard and their workstation, whether the heat from the worker's fingers has artificially raised the temperature, or their breath the humidity.

Neither do they really care that they're all that accurate. After all, their product is shooting down the conveyor at the rate of several dozen per minute and they dare not fall behind or they'll lose the only paying job they ever had any hope of getting.

And, if the cardboard card holding the thermometer or relative humidity gauge claims some small limit of inaccuracy (e.g., 3%), why is there a 10° or more spread of temperature among the thermometers, or a 25% or more spread of relative humidity reported by the relative humidity gauges?

"HOUSTON, WE HAVE A PROBLEM." (With all due respect to the Apollo 13 astronauts.)

Relative humidity (a.k.a., "humidity," "rH.") is one of the most misunderstood topics in arachnoculture, but unlike with temperature, here there is a reason. The problem arises because

All of these factors force us to consider the problem as a sliding scale overlaying a spectrum of different shades of grey rather than in simple black and white. And, this serves as a cause for immense confusion and huge disappointment for the enthusiast who wants a cut-and-dried, quick-and-painless answer.


We will try to present this as simply as possible, and to sugar coat the difficult parts, but the enthusiast must meet us halfway. You must make a valid effort to try to understand the principles and issues in an effort to understand the problems and the solutions. "Ya gotta do your homework!" The good stuff never comes easy.


We need to lay the groundwork for the following discussion with some basic definitions.


A gaseous solution of molecules of nitrogen (about 78%) and oxygen (about 21%) flavored with traces of a few other gasses (about 1%) in dry air. Typically, this dry air may be further diluted with 1% to 4% water vapor, accounting for its relative humidity. (See Atmosphere of Earth for a more detailed treatment.) Another concept implied here is that these gasses are dissolved in each other very much like ethyl alcohol, water, and traces of flavoring are dissolved in each other to produce a mixed drink in a bar or tavern. The facts that air is a solution of gasses and your cocktail in a nightclub is a solution of liquids is largely irrelevant to this discussion.

Air pressure

The weight of all the air above a certain point. This changes greatly with elevation or altitude and slightly with the weather.

Concentration (of water vapor)

The amount of water vapor (See water load, below), usually expressed as a weight, divided by a volume (e.g., grams per liter, ounces per cubic foot, etc.).


No, we're not necessarily talking about stars and planets, space ships and space aliens. We're using this term to mean approximately the same as volume, below.


"A measure of the average kinetic energy of the particles in a sample of matter ..." (The Free Dictionary.) In our case, "particles in a sample of matter" are merely the individual molecules of water vapor. And, "kinetic energy" is merely the speed that these molecules are traveling.

"Molecules got speed?" you cry! Yes. But in any aggregation of molecules (e.g., a volume of water vapor) they are continually colliding and ricocheting against each other. And, we measure the average speed of the molecules and the vehemence of those collisions using a temperature scale. For more information the reader is directed to this Wikipedia article.


Actually "volume of space." The three dimensional analog of "area," often expressed as a capacity (e.g., cubic meters, cubic inches, gallons, liters, etc.). In our definition it may be filled with some gas like air, water vapor, etc. or it may be a vacuum. In a hypothetical or generic sense it does not necessarily have to be contained within a vessel or container, or even exhibit a specific shape. (See Space above.)

Water load

The amount of water vapor held by a given volume usually expressed by its weight. See also Concentration, above.

Water vapor

Water in a gaseous state. Water vapor is just as properly a gas as is oxygen, nitrogen, methane, etc., and follows all the usual gas laws that you were supposed to learn about in high school physics.


Not being either physicists or climatologists, we can only offer a rather superficial answer to this question. Yes, we're "copping out!" Rather, we direct the interested enthusiast to Dr. Alistair B. Fraser's Bad Science webpages entitled Bad Meteorology and Bad Clouds, and used with great apologies to Dr. Fraser for any misinterpretations.

And, the reader is also referred to the webpage published by Steve Horstmeyer (Meteorologist, Cincinnati, Ohio, USA) entitled About Humidity... for a slightly different spin on the topic.

And, the interested reader might search the Wikipedia database for more authoritative or sophisticated definitions and discussions of the various terms used here.

Having said that, it is important that you get the notion out of your head that air has anything to do with relative humidity (both your high school science teacher and the weather person on your favorite, television, news program notwithstanding).

Or, at least almost nothing. In the real-world, at most, air in reference to humidity should only be considered as

The basic facts outlined above still hold!

(And, this means that the definition used on page 162 of The Tarantula Keeper's Guide, Third Edition is - Gulp! - wrong! This will be changed, we promise, in TKG4!)

The theory behind relative humidity describes the action of water vapor in a vacuum as well as it does in Earth's atmosphere. And, this theory is verified by 200 years of theoretical and practical experience in the laboratory, and over 40 years of real world experience in the vacuum of "outer" space. The presence or absence of air is nearly irrelevant. (But, remember our caveat above.) Air and water vapor may be dissolved in each other, but none of them has any special affinity or disaffinity for any of the others.

We fully understand that the concept that air is nearly irrelevant to relative humidity is in direct contradiction to everything you may have been told up till now. And, we also understand that the concept may be hard to "wrap your mind around," but we assure you that this new world view will be well worth the effort. Stay with us. It'll get easier as we go along.


Relative humidity is usually expressed as a percentage, and we all more or less have an instinctive understanding of what it means. But, the number of people who use the term without actually knowing its definition is surprising. So, for those of you who really want to know, we give a slightly formal but definitely non-technical definition of it here.

First we need to introduce two more concepts.

And, one simplistic definition is that relative humidity is the amount of water vapor actually held in a volume divided by the maximal amount of water vapor possible in that volume (and at that temperature).

For those of you with a background in Algebra (Yes, once in a while we utter the dirty word!), the relationship can be expressed mathematically as

Humidity equation

And that, ladies and gentlemen, boys and girls, is the only mathematical equation we intend to present to you in this dissertation!


In nature, the water load of a volume is constantly changing. Water precipitates out (Remember the weather forecaster's "Probability of Precipitation" or POP?) as fog, dew, rain or snow, for instance. And, this precipitation reduces the water load of the volume of space.

And, water evaporates from damp ground and standing bodies of water (e.g., ponds, streams, oceans), and transpires (a special kind of evaporation) from the leaves of plants. All these tend to increase the water load of the volume of space near them.

Did you notice the sneaky way we tacked on a reference to temperature in a couple of the sentences above? It seems that temperature is quite important where relative humidity is involved. The maximum water load of a volume varies with temperature. The theory behind this is a bit complicated, so we're going to "cop out" here too. If you're interested you might look it up on websites on Meteorology and Weather, or the Wikipedia database on the Internet. But, the basic rule is that as the temperatures rises, the maximal water load of a volume of space increases. And, as the temperature falls, the maximal water load of a volume of space decreases.

If you've been paying attention you should immediately see that we now have two subparts of relative humidity, the actual water vapor load, and the maximal water vapor load (which is influenced strongly by temperature). And, both of these change more or less chaotically. Chaos X chaos = chaos! (Oops! Was that a second Algebraic equation? We hope not!) And, that's why the National Oceanic and Atmospheric Administration spends millions of dollars a year to measure and predict relative humidity. The amazing thing is that sometimes they even get close.

So, it turns out that humidity in the real world is a very slippery eel indeed. And, while we can measure it, trying to make sense of our measurements can be quite frustrating.

But, before we discuss relative humidity and our tarantulas we need to make a point about the accuracy of any relative humidity measured by the casual enthusiast. The instrument to be used may be a psychrometer as pictured at the top of this page, or any of various other styles of relative humidity gauge. If manufactured with great care, calibrated properly, and regularly checked for accuracy against some "gold standard," these may be entirely acceptable. Unfortunately, most often none of these is true, and the mass produced models sold in almost all pet shops are absolutely ludicrous. You are directed to QC? We Don't Need No Stinkin' QC! for a discussion of the problem.

Furthermore, these instruments go out of calibration distressingly easily. A slight knock against a cage wall, or even the relaxing or deterioration of the sensor (in the cheaper models often just a piece of hair!) with time can lead to profound errors. The Internet forums are replete with postings by enthusiasts wondering why their humidity gauge always reads 26% or 85%. The answers are short and simple: "It's cheap," and, "It's broke!"

The message here is that no matter what the label says or what the pet shop personnel have told you, you cannot trust your relative humidity gauge as far as you can spit unless you've made a concerted effort to calibrate it against some gold standard.

And, where might one find such a gold standard or get a humidity gauge calibrated?

A government weather service office in your city.

A college or university physics or meteorology teaching or research laboratory.

A "heating, ventilation, air conditioning" (HVAC) company.

Could you do it yourself?

Yes, and it's not terribly complicated. It requires working with several specific salts and requires some patience and some familiarity with normal chemistry and physics principles and laboratory procedures. But, the levels of these are hardly more advanced than what most high school courses would teach. King of the House provides a very good description of the process.

Is it worth the effort?

In the overwhelming number of cases, no. Almost all enthusiasts can get by without ever owning a humidity gauge or worrying about humidity. See below.

Further, if your humidity gauge is reporting the wrong humidity, it can cause you to kill you pet.

Very advanced enthusiasts who are conducting controlled experiments whose results may be affected by uncontrolled humidity (and these are rare birds indeed!) may find a use for a humidity gauge. But in this case, the experimenter should DEFINITELY NOT work with a consumer grade instrument, but should procure a professional, industrial, or research grade instrument instead, and recalibrate it often.

Bottom line: Whenever a pet shop tries to sell you a humidity gauge, point at the clerk, laugh hysterically, and immediately proceed to the next item on your shopping list.


Now that we've cleared up a little of the science of relative humidity we need to examine how living organisms, specifically tarantulas, interact with it. Again, we're going to simplify this as best we can without outright lying, and sugarcoat the rest. But, you'll have to meet us half way by at least trying to understand the principles. As before, you might search the Wikipedia database for more authoritative or sophisticated definitions and discussions of the various terms used here if you wish.

We are now at the point where we need to examine tarantula anatomy and physiology in a little more depth than the superficial treatment given in most tarantula books.


Because life in a cage is so much different than life in nature, knowing a tarantula's natural state is not very useful in determining an "optimal" relative humidity in captivity. In fact, trying to mimic nature too closely can actually be outright dangerous to your pet. For a detailed discussion of this premise, read NATURAL IS BETTER, Another Myth Perpetuated by Tarantula Enthusiasts. You can press the "BACK" key to return here.


By terrestrial invertebrate standards tarantulas are quite massive. And a lot of that mass, perhaps 80% or more, is water. But, the tarantula doesn't need all that water. So, tarantulas typically carry around a substantial amount of extra water, almost literally water in the bank. And surprisingly, a tarantula can lose a substantial amount of its stored water and still live and function, although not necessarily entirely normally, especially at the extreme. And, after being given the opportunity to rehydrate themselves, few show any signs of illness, or wear and tear.

As with humans, tarantulas can become a little dehydrated with no bad effects whatsoever, and as opposed to humans, can tolerate some substantial dehydration if it's corrected soon. The troublesome parts are determining where the limit is for the "substantial dehydration," and how soon it must be corrected before the tarantula dies. We shall discuss these at length shortly.


We all know that tarantulas, like most other arthropods, are enclosed by a layer of body armor called an exoskeleton. This exoskeleton defines the shape of the arthropod and all of its various external body parts. This covering is a very complex organ system, and we shall not diverge into a detailed discussion of it here. (The interested enthusiast might perform an Internet search using the search strings, spider exoskeleton and arthropod epicuticle among others.)

However, the outer layer of the exoskeleton, called the epicuticle, is of particular interest to us. It contains a large amount of a wax-like substance that effectively waterproofs the tarantula. This wax-like substance covers both the body surface and also the surfaces of the setae (a.k.a., bristles). Not only does this prevent the tarantula's body surface from wetting easily with water, but it prevents the tarantula from losing precious water through its body wall. Once most tarantulas have developed this waxy layer they are more or less impervious to low humidity, high humidity, and a variable humidity. They're almost hermetically sealed against the outside world.

Most tarantulas. Over most of their bodies. Most of the time. But, like so many other aspects of biology, and characteristics of tarantulas, there are exceptions and qualifications. And, often much ignorance on our part.

Tarantulas can and do lose internal body water several ways.

And, two categories of tarantulas may lose large amounts of water uncontrollably under almost any circumstances except an inordinately high humidity.

Thus, it would appear that tarantulas are very nearly hermetically sealed against water loss to their environment. In fact, many desert species acquire very nearly 100% of their water needs through their food (liquid water as part of the prey's body mass plus a small amount of metabolically produced water). In a good year these tarantulas may be able to drink liquid water only once or twice, and may have to survive several years without liquid water altogether during a drought.

The rain forest species, of course, do not have this problem, and several such species (but not most) have apparently lost a significant portion of their water retentive ability. We term these the "swamp dwellers," and these are discussed at length in The Tarantula Keeper's Guide, Third Edition, to which the reader is referred for more information.

We humans have an inherent tendency to approach tarantula physiology and behavior from our own, singular, self-oriented point of view. Thus, we need to remind ourselves that human skin is nowhere near so impervious to water loss as the tarantula's exoskeleton. Our cuticle is thin and there is very little about its composition or construction that would prevent water loss. In fact, our skin possesses a multitude of small glands that secrete a water solution in copious amounts as a temperature control mechanism. Unlike tarantulas, we leak like a sieve!


In his World Spider Catalog, Dr. Norman Platnick lists more than 930 species of theraphosid tarantulas. Yes, we are painfully aware that there are many errors among these. (Which by the way are being worked on even as you read this, hopefully to be finished within the next century or so!) And, rest assured there are many more to be discovered and/or named. (Also a work "in progress."). But, we can use that as a representative if not factual number.

Considering that there are so many different kinds of tarantulas, one might assume that every tarantula should have its own preferred relative humidity. And, judging from the recommendations of the multitude of care sheets available on the Internet, we have good reason for doing so. But, as we shall see shortly, this is not necessarily true.


We recount the following partly as a means of refreshing the enthusiast's memory, and partly to bring together into a coherent story the collection of seemingly unrelated facts about this facet of the subject of humidity. This topic is also discussed in detail in Growing Your Own.

Egg Laying.

Female Grammostola rosea (Chilean rose tarantula) laying eggs. Note the deflated abdomen from the fluid loss. Photo by Ryan Nefcy, one of the photocontributors for TKG3. Used with permission. A tip o' the hat and a pat on the back to Ryan!

When the mother tarantula first produces an eggsac and lays her eggs in it they are suspended in a fluid that is primarily water. For the first little while after leaving the mother's body the eggs are completely wet. Subsequently this water is either absorbed by the eggs or dries away. In either case, this represents a significant water loss to the mother, and their opisthosomas ("abdomens" in the vernacular) are reduced in size and wrinkled. They have been seen to drink long and hard after producing an eggsac to replace the lost water.

Brooding and Incubation.

Within a very short time, probably less than a very few hours, the eggs absorb much of that water, and the remainder dries away. From that time forward until the babies "eclose" (emerge from the eggsac) they must be kept at a relative humidity somewhat higher than what we would consider a normal, ambient humidity to prevent the eggs from desiccating and dying. For most tarantulas this is usually 60% relative humidity or higher. However, the eggs of deep desert kinds often are resistant to desiccation at somewhat lower relative humidities, and those of some deep rainforest kinds often must be kept at significantly higher relative humidities. To retard the growth of bacteria and mold, however, the humidity in a brooding female's cage or an incubator should never be high enough to cause condensation. If this ever happens every effort should be made to increase ventilation and reduce the humidity to more moderate levels.


After the babies emerge from the eggsac and through the next several instars they must still be protected from humidity that is too low to prevent desiccation and death. This results from the fact that the babies have not yet fully developed epicuticles with a water retentive waxy covering. The development of a fully functional epicuticle may occur as early as the 5th instar in deep desert kinds, or may never develop completely in the few kinds that we colloquially call the "swamp dwellers."

Baby Bottles
There are very few ventilation holes so as to hold in the humidity. The canning jar on the left contains an Aphonopelma seemanni, Costa Rican zebra tarantula, that is large enough to be housed in an adult cage. See Growing Your Own for more information.

The most common way of achieving this is to keep them in rather closed containers on damp substrate. Keeping them this way ensures a higher, constant humidity around them. Here is an example of some baby containers.


At some time the baby's epicuticle develops its water retentive properties to the point where it is no longer in severe danger of desiccation and death at a somewhat lower relative humidity. What that might be and how to recognize when the tarantula is having problems is discussed below.

However, this acquisition of a wax layer usually requires only one or two instars to complete, and probably varies somewhat with each kind of tarantula, so its timing is vague. When attempting to advise enthusiasts about the care of tarantulas during this growth phase we usually assume that the babies have acquired the ability to develop a functional wax layer by the time they reach a diagonal leg span (DLS) of about 1.5" (38 mm), mostly to ensure plenty of developmental time for that to happen. But, in fact, most all kinds of baby tarantulas will have developed a functional wax layer earlier than that with some deep desert species doing so as early as their 5th instar, perhaps even earlier. It's just better to wait a little longer and be more safe than sorry.

At that point in their development all that remains is to encourage the baby tarantula to develop an epicuticle that is appropriately impervious to water loss. This is done by moving the tarantula into a slightly larger container if necessary, supplying an adequate water dish, increasing ventilation slightly, and allowing the substrate to gradually dry out over the next two or more molts. The baby tarantula automagically responds to the drier conditions by producing a more impervious wax layer.

Spiderlings and Adults.

These are tarantulas with leg spans of about 2.5" (6 cm) or larger. These have developed a relatively thick waxy layer and are pretty much impervious to sub-optimal humidity. For about 95% or more of the tarantula species we use an arid cage with a water dish. (There are a few species that require a damp and humid cage their entire lives, the so-called "swamp dwellers." Examples are Theraphosa blondi - goliath birdeater, and members of the genus Hysterocrates. See TKG3 for a more thorough discussion.)

Once the tarantula has completed the adjustment process described under "Tweens" the spider is kept according to its preferred care regimen (e.g., arid species, arboreal, obligate burrower, etc) for the remainder of its life.


Have a Drink on Me.

When most tarantulas are subjected to arid conditions, their first response is to drink more water. But, within a few weeks to a few months they develop a thicker, more impermeable layer of the wax-like substance on their epicuticle to reduce water loss to the absolute, bare minimum. Many of the desert species are notorious for requiring little or no liquid water whatsoever, surviving almost entirely on the water held by their prey and from metabolic water (water produced de novo as food is metabolized for energy).

Decades of experience by literally tens of thousands of enthusiasts has amply demonstrated that within one or two molts almost all tarantulas larger than fifth or sixth instar babies, even most of the rain forest species, can not only survive but thrive in an effectively desert condition.


A major fact that some enthusiasts have trouble dealing with is that most tarantulas are so adaptable that they can survive and even thrive in a much wider range of conditions than is found in their native habitat. Good examples of this are the Brachypelma. These come from coastal plains to deciduous tropical forests to outright rain forests in Central America. But, those of us who keep and breed them find that they do as well, if not better, in a cage that's kept bone dry but with a water dish. And, they can tolerate, if not thrive in temperatures significantly lower than in their native habitats. This adaptability is almost surely one of the reasons tarantulas are such survivors.

Which Ones Have Trouble Adapting?


As stated above, babies younger than about their 5th or 6th instar (occasionally somewhat older) have not yet developed the wax layer that protects them from dessication. Given a little time and a little nudge, almost all will do so without missing a heartbeat.

Swamp Dwellers.

At least the wild caught individuals of a few kinds of tarantulas have a great deal of difficulty with dessication in a lower relative humidity. Among them are members of the following genera.

However, enthusiasts are discovering that the captive bred and raised (CB) individuals of those genera do not require the same oppressive humidity of the rain forest as do their wild caught brethren.

By their very nature, CB swamp dwellers become acclimated to life in a cage almost from their "birth." This initial period is the most plastic phase of their development, allowing them to make the required myriad of tiny adjustments in their anatomy and physiology in the normal course of their growth. They go through the same baby-care-regimen to adult-care-regimen transition as other tarantulas, and consequently develop a slightly more impermeable epicuticle than their wild brethren from the get-go. They needn't go through a lengthy and risky acclimatization process at the same time as they recover from bad care and shipping shock.

But, even with CB individuals, the process is neither perfect nor complete. As adults they still often must be kept in a somewhat elevated relative humidity to survive very long, and that exposes them to infection and infestation, although probably to a lesser degree than wild caught individuals. And, if the enthusiast has one of the CB swamp dwellers and it begins to look or act sick, the VERY FIRST thing to do is get the tarantula into an ICU as soon as possible. And, while the tarantula is receiving treatment, its cage and care regimen should be re-evaluated and corrected before returning the spider to its home.

A growing number of advanced enthusiasts are reporting success with acclimatizing wild caught swamp dwellers to a somewhat dryer care regimen if this is done slowly and carefully enough. However, these may be clear exceptions by gifted hobbyists, and not universally reproducible results. Do not risk the rent money on such a project!

Many enthusiasts consider members of the genus Haplopelma to fall into this group, but these authors have kept H. minax, H. albostriatum, and H. lividum under effectively arid conditions with little problem, and generally view such reports with much scepticism. This is reinforced by photos and reports of the genus in the wild. (But note that H. lividum is classified as an Obligate Burrower. See TKG3 for more information.) For the most part we consider this matter settled.

Always an Exception

There are a very few tarantulas that cannot tolerate the wet conditions of the swamp dwellers, but still need a little help with humidity. These are an intermediate group with not enough members to establish a separate care regimen, and their care requirements tend to be rather vague. At the time of this writing these authors are only aware that members of the genus Megaphobema fall into this group, but there may be others. And no, 450 of the 930+ species of named tarantulas don't belong here! Almost all other tarantulas (exceptions noted above) will adjust quite happily to an arid cage.



One kind of tarantula among all of them seems to need a little special understanding and care. That one is the Chilean rose tarantula, Grammostola rosea. It wouldn't be a big issue except for the fact that this is currently the single, most commonly available kind found in the pet industry. And therefore a lot of novices are unknowingly faced with the challenge of properly caring for a new tarantula that came from the pet shop with "issues." In response to this problem we have written a care sheet specifically for Chilean roses (as much as we abhor care sheets in general!) and posted it on our website.

We assure you that this is not your typical witless care sheet!

ALL enthusiasts keeping a Chilean rose tarantula, from the rank beginner to the book writing aficionado are strongly urged to read this care sheet at least once. THERE WILL BE A TEST ON FRIDAY!

We've already covered many of the practical aspects of humidity and tarantula care above. But here we address some additional, specific topics on the subject.


There is at least one very good reason why we keep tarantulas in arid cages: With persistent, high humidity comes the probability of infection and infestation. So, we keep our cages dry to delay or prevent disease and the untimely death of our tarantulas.

We cannot stress this fact strongly enough. In nature, for every tarantula that dies there are 100 waiting to take its place. For free!

In our homes, each pet tarantula costs us a lot of time, effort, and money. And, one would hope, we have come to value each pet tarantula not only as a valuable possession, but also as a weird, little friend. And so it is in both our's and the tarantulas' best interest not to jeopardize our little buddy with some hair brained or ignorant care scheme.

As evidence in support of our basic premise, many (though certainly not all) wild tarantulas living in excessively humid habitats build their burrows more or less horizontally into hillsides or slopes in the ground. In fact, several species of putatively arboreal tarantulas (e.g., Psalmopoeus and Tapinauchenius species) and many other spiders preferentially build their burrows into steep banks and cliff faces. Thus, rainwater and dew would tend to flow out of and away from the burrow's entrance, presumably to avoid flooding during a downpour.

And, several other species (e.g.,Haplopelma lividum (cobalt blue tarantula) and Aphonopelma mojave (putatively A. sp. "Hualapai")) build extensions of silk and/or soil protruding above the entrances to their vertical burrows in nature and often in cages, presumably to prevent excessive water from entering during a downpour. In all these cases, the end result would be a drier burrow, and a reduced probability of infection and infestation.

But does the opposite condition, excessive aridity, necessarily have an opposite effect? For the most part we can say, "Yes." Most terrestrial tarantulas originating from arid and semi-arid regions build more or less vertical burrows whether they are on level ground or slopes. And, the few species that build such turrets or embankments around their burrow entrances live in areas that, though classified as arid or desert, suffer torrential downpours, so-called "monsoons," during some seasons of the year.

The basic principle of a dry cage is recommended as a default care strategy for tarantulas in TKG2 and TKG3, and also discussed in Mighty Mites in specific reference to controlling mite infestations. Rest assured that it also is relevant to controlling Collembola (springtail) infestations as well as the vast majority of nematode, fungal, and bacterial infestations and infections. (Note that there is growing support in favor of encouraging springtail populations in damp cages as "sanitation engineers." But, the practice has not been used long enough or widely enough for us to endorse. Experienced enthusiasts may wish to explore this practice. Novices are strongly encouraged to follow the care regimen outlined here, in the sister webpages, and in TKG3 for at least the first year rather than experimenting from a state of abject ignorance.)


Our Basic Philosophy.

In the foregoing paragraphs we have discussed:

We supply water dishes because we're keeping these tarantulas in an alien environment, and we're not very good at recognizing when they really, REALLY, REALLY badly need a drink. And, water dishes are so cheap, and so easy to maintain, that there's really no excuse for not supplying one.

When Should You Worry?

There are instances when the relative humidity around your pet tarantula is of concern.

What's the Game Plan?

This begs the questions, "How do you know if your tarantula needs help with relative humidity?" and "What do you do about it?" We boil the matter down to these rules of thumb.

If you suspect that your tarantula needs a higher humidity, do not bother misting or spraying it. Instead, cover the cage to dramatically reduce ventilation, and install a larger or a second water dish if necessary. All this is discussed at length in Misty-eyed Misting and TKG3.

The white stripe down the length of the cover is Magic Disappearing Tape, that didn't disappear in the camera's flash, holding a sleeve of nearly invisible, plastic food wrap in place around the grid/cover to retard almost all ventilation. Brachypelma emilia - Mexican redleg tarantula.

When It's Too Soggy.

Stuff happens. Sooner or later, for whatever reason, you're going to end up with a cage that's too wet. Now what do you do? Basically, your options are limited, there are only two alternatives:

Increase ventilation and allow the cage to dry out by itself.

This strategy has the serious limitations that it may take too long to work, and changing your care regimen may put the tarantula at risk (e.g., by changing to an unproven, possibly non-escape-proof cover).

Clean the cage and set it up de novo.

While this strategy works better, it's major drawback is that it requires a major cleaning operation, possibly at a most inopportune time. (And, there is never a really GOOD time for this sort of thing!)

Plan "A."

Experienced enthusiasts commonly institute the practice of keeping one or more empty, spare cages set up and stored away for crisis situations. These are kept bone dry and covered to keep household dust out. Whenever required (e.g., a suspected or proven mite infestation, or a too wet cage), the spare cage is immediately pressed into service, dampened if necessary (for babies, intermediates and swamp dwellers only!), and the affected tarantula is moved into it forthwith. At the earliest opportunity, the old cage is cleaned, set up again, and stored away dry for the next crisis. (And, there is always a next crisis!)

Time for the Bitter Truth!

For babies, this may only mean keeping a few extra pill vials in a dresser drawer. But, for keeping a Theraphosa blondi this may mean keeping an extra 50 gallon aquarium in the closet! Our only comment is that if you wish to keep large, expensive, problematic species, you should be prepared for large, expensive, problematic solutions to their issues. If this isn't possible, perhaps you'd better console yourself with one of the hardier, less demanding species, or a plastic geranium, instead.


The layman has only a very vague understanding of the term average. For instance, it can be used to mean that you added together all your data points and divided the sum by the total number of data points. Or, sometimes it is used to mean merely the commonest value in a set of values.

And, which values are you considering? The day's highest? The day's lowest? Values taken according to a religiously followed schedule? Or only when you thought to do so? We strongly suspect that no thought went into their collection at all, and most such values are merely guesses on the part of the "expert!"

There are huge problems with using such a method for trying to characterize relative humidity.

Thus, some care sheet presenting an "average" relative humidity for any given species of tarantula based on some measured values in its native habitat is nearly worthless information simply because it's meaning is so vague.


Another concept that has been used is optimal humidity. It's supposed to be the best humidity at which your pet tarantula should be kept.

In fact, tarantulas are so plastic in their ability to adjust to different levels of long term humidity, that the concept of optimal humidity is probably completely bogus.



What on Earth could catching a monkey possibly have to do with tarantulas?

We thought at first that this was an urban myth. But, after doing a little research on the 'Net we found this video. The video is in French, but it very clearly demonstrates in a humorous way the principle behind a tarantula not being able to pull out of its old exoskeleton. All you need do is turn the sound down and watch what happens.

Now, did this help you understand?

There is a continuing myth that high humidity is necessary for a successful molt among tarantulas. Theory and experience suggests that this may be at least partly true for baby tarantulas because they lack the wax layer in the epicuticle of their exoskeleton to waterproof them. And, this may be an important reason for keeping baby tarantulas in mostly closed containers on damp substrate - to keep the humidity high around them.

But, because of their wax-coated exoskeletons and the water stored within their bodies, spiderlings and adult tarantulas have plenty of water to get them through a molt. This is evidenced quite convincingly by the fact that the inside of the cast-off exoskeleton of a freshly molted tarantula fairly glistens with moisture in the form of the exuvial fluid. They are literally bathed in moisture as this fluid fills the exuvial space between the old and new components of the developing exoskeleton.

Then why do some tarantulas have difficulties molting? There are probably a number of reasons. But, we suspect that the major one is that the tarantula is too weak or old to be able to get out of the old exoskeleton before the new one begins to harden. When that happens, the supporting rings of chitin around the joints, for instance, can no longer deform enough to slip through the solid, older rings and the tarantula becomes trapped. It's a monkey trap pure and simple.

Too many tarantulas owned by too many enthusiasts have molted too many times over too long a time span to leave much doubt. This is just another fallacy perpetuated by enthusiasts!



All the foregoing would seem pretty much cut and dried, but it really isn't. There is still a lot we don't understand about the interaction of humidity and tarantulas. And, advanced enthusiasts are intentionally violating selected aspects of the official party line for the purposes of advancing that understanding.


The strategy was to develop by whatever means, a system for caring for tarantulas that was pretty much bullet proof. And, that we have done, and that's what we've presented here, in the sister webpages, and in TKG3. But now that we've accomplished that, advanced enthusiasts are carefully testing variations of those parameters with different kinds of tarantulas. There are a number of reasons for doing this. Among them are:


Lastly, there is one very important aspect of arachnoculture that depends heavily on violating the rules: breeding tarantulas. There are a few kinds of tarantulas that breed readily in captivity, Avicularia avicularia (Guyana pinktoe tarantula) and Brachypelma albopilosum (curlyhair tarantula) for instance. But, there are many tarantulas that have proven very difficult to breed in captivity, and enthusiasts are exploring these species' sensitivity to, or need for special conditions outside those recommended here to promote their breeding.


It should be noted, however, that these are advanced enthusiasts who are trying to answer specific questions or achieve specific goals, not rank novices "doing their own thing" out of ignorance. The novice should follow the rules at first. Learning how to properly care for your newfound pet is the important goal for now. The time for experimenting will come later, after you've gained much experience with tarantulas and learned and understand experimental methods, and the risks and benefits of experimenting with your pets.



We found out that few laymen and a lot of professionals really don't understand relative humidity. We also found out that almost no arachnoculture enthusiasts (that would be you and me) truly understand relative humidity in respect to their tarantulas. Except that, because you've read this "little" essay, you're among the very few elite who may be getting close to an understanding. And we've gained a much better understanding of tarantula biology in general and physiology in particular.


On the one hand you know much better how to care for your tarantulas. But, on the other hand, you're now going to be held to a higher standard. There will be no excuse for you making the simplistic mistakes, and passing along the simple minded and often brain damaged myths that you once might have. And, you'll always be double-checking what you're about to do or say to make sure YOU aren't committing some major, relative humidity blunder!

Enjoy your little 8-legged buddies!



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Copyright © 2011, Stanley A. Schultz and Marguerite J. Schultz.
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This page was initially created on 2011-June-25.
The last revision occurred on 2014-February-08.