Urticating Hairs

What do tarantulas, a handful of moths, and some of your garden plants have in common?

Urticating hairs.

Stinging nettle urticating hairs (Jerome Prohaska) CC BY-SA 3.0

More accurately called urticating bristles, because only mammals have hair, these protrusions are an effective form of defense for several reasons. Being sharp and pointy, they are no fun to run into. In some cases, urticating bristles can be forcibly ejected at an enemy, thwarting attack. Very often, these bristles are coated with chemicals that can cause burning or itching sensations.

If you’ve ever gotten ahold of stinging nettles, you know what I’m talking about.

The word “urticating” is Latin for “nettles.” Nettles are characterized by pointy, hollow trichomes that grow next to a gland which produces bothersome chemicals. Adding insult to injury, the tips of urticating hairs often break off in your skin, making matters worse.

In most cases, urticating bristles are painful but not dangerous. There are exceptions.

Australia’s gympie gympie (Dendrocnidae moroides) causes stings that are described as a cross between being electrocuted and burned with hot aid. The gympie gympie has been known to kill people, horses, and dogs. And there is a tree in New Zealand, called the ongaonga, or tree nettle, that killed a person who walked through a thick patch. All because of urticating bristles and the associated toxins. Yikes!

Buck moth caterpillar (Judy Gallagher) CC BY 2.0

Urticating caterpillars

Several species of moth and butterfly caterpillar have protective urticating bristles. These include:

• black-wave flannel moth (Megalopyge crispata)
• buck moth (Hemileuca maia)
• hag moth (Phobetron pithecium)
• io moth (Automeris io)
• puss caterpillar (Megalopyge opercularis)
• saddleback caterpillar (Sibine stimulea)
• spiny oak slug (Euclea delphinii)
• stinging rose caterpillar (Parasa indetermina)
• white flannel moth Caterpillar (Norape ovina)

As for tarantulas, you're on your own.

Tarantula showing patch of urticating hairs due to light exposure (Sarefo) CC BY-SA 3.0

First aid for stings

Sadly, there isn’t much you can do about these painful stings. Adhesive tape may help remove the urticating bristles from your skin. Wash thoroughly with soap and water to remove at least some of the irritating chemicals. Applying an ice pack may help, too.

Personally, I make a paste out of baking soda and a little water. It seems to work better than anything else. Antihistamines are not helpful.

Fools may tread where angels fear to go, but gardeners go where they have to, to care for their plants. Before reaching into a space blindly, take a moment to put on your gloves.