“Urticating hairs” is actually a bit of a misnomer – urticating bristles is more correct, since “hairs” grow from follicles and are only found on mammals.  The hair-like bristles found on tarantulas are only superficially similar to hair, they don’t grow from follicles, and in fact they differ greatly in terms of structure, shape, and purpose.

Tarantulas have bristles all over their bodies, and different types are used for different purposes.  Some are used for sensing vibration, some tarantulas have stridulating bristles used to create sounds (the loud “hissing” sound created by some species is created in this way) but the type of most concern to tarantula keepers are those used for defence – the urticating hairs found on the opisthosoma (abdomen) of many new world species.

The term urticating comes from urtica, the Latin word for “nettle”.  These barbed bristles can be kicked or flicked off the abdomen by the tarantula’s rear legs, causing a cloud of these tiny hairs which cause irritation, discomfort and pain when they embed themselves in the skin or eyes of a would-be predator.

The bristles don’t appear at birth, but each time the tarantula moults, new ones are added.  They are loosely attached so that they easily break off when “kicked”, and are covered in barbs.  A number of different types are known, and these have different arrangements of barbs which cause varying degrees of irritation on the skin or mucous membrane (such as inside the nose or throat).  As they are kicked off, the tarantula may develop a “bald patch” on its abdomen, but this will be renewed at the next moult (see picture of a B. Smithi with a clearly defined bald patch)

Anyone working with new world tarantulas possessing urticating bristles must take precautions and care when working with these animals or their enclosures.  Some tarantulas shed bristles as territorial markings, so even if the tarantula is not present, there may well be loose bristles on the substrate or webbing which can still cause problems.

In general, urticating hairs are a minor problem so long as precautions are taken.  For most people, a few hairs on their skin will only cause a minor irritation; some itching which may continue for a few hours.  A more serious problem however is if a person suffers an allergic reaction to the bristles, or if they get into the eyes.  Unfortunately there is no way to know for sure if you’ll be allergic if you’ve not come into contact with them before, so take extra care the first time dealing with any new world species known to kick hairs.

If urticating bristles get into the eyes they can cause a lot of pain, and real damage.  In serious cases they can embed themselves into the cornea causing severe pain and long term problems which will require medical attention, so eye protections is advised.

What if I get urticating hairs on my skin?

If you do get hairs on your skin, wash the area thoroughly with plenty of running water.  Monitor the area and try not to scratch.  A solution of 2–2.5% hydrocortisone cream applied to the affected area may help relieve the symptoms, and antihistamine tablets such as those taken by hayfever sufferers have been reported to alleviate the symptoms by some keepers.  For most people, the irritation will subside over a few hours, but if it appears to be worsening or lingering, or if it’s accompanied by swelling or severe redness then seek medical advice (see picture, an allergic reaction to urticating bristles on the skin.  Image sourced from Wikipedia)

What if I get urticating bristles in my eyes?

This is potentially more serious.  Start by washing the eye out with lots of fresh running water, then I would suggest seeking medical advice.  With luck none of the hairs will have embedded into your eye and though sore, it should clear up relatively quickly.  If they have embedded though your doctor will be able to advice on treatment (typically a treatment of topical steroids).

How to avoid problems

The best form of treatment is prevention, right?  First of all, know whether the tarantula you’re dealing with has urticating hairs by researching the species.  Most of the new world Ts that are commonly kept by beginners do have urticating bristles, so unless you’re absolutely sure, assume they do!

Keep an eye on your tarantula’s body language.  You can often tell when a tarantula doesn’t want to be disturbed, and you can often clearly see when they kick hairs.  My B. smithi will often kick hairs as soon as the lid comes off her enclosure, and when that happens the best bet is to keep your distance.  Don’t try and handle or move a T which is flicking, don’t lean over the enclosure or get your face too near, and don’t breath in right over your T or the enclosure (remember, even if the tarantula isn’t present there may well be loose bristles in the enclosure!)

I would highly advise wearing glasses or some sort of eye protection when working with new world tarantulas, particularly those you’ve not worked with before, or which are known to be “flicky”.  Take care when doing cage maintenance, and consider wearing gloves when changing substrate etc.

Don’t rub your eyes or touch your face while working with Ts, and wash your hands straight away afterwards.  You don’t want to transfer loose bristles from your fingers to your eyes!  And finally, be aware that old exuvium (moulted skin) can still possess urticating hairs, so treat them with care in the same way you would a live tarantula!

Sources and further reading

Treating urticating hair reactions – Article on Arachnophiliac.info

Got a pet tarantula? Then wear eye protection – Article on Phys.org

Photo of urticating hairs from T. blondi in a human eye – Rick C. West