Witches' whiskers

As I wandered through the ancient woodland of Ty Canol yesterday I stopped to admire the silvery strands of Usnea florida, the lichen strands that in places thickly beard the low-hanging oak twigs. I picked up some tufts that were lying beneath, where sheep graze (for it is traditional woodland pasture, the open tracts of grass allowing light to reach the trees and encourage lichen growth) in order to set up this image with an old sheep skull I had at home.

First, notice the unusual structure of the lichen with the little discs like tiny sunbursts. These are the apothecia that appear when the lichen is in reproductive mode and make it instantly recognizable. It's highly sensitive to atmospheric pollution and thus is found only in the cleanest air.

So why the skull? Well, in mediaeval times usnea, from the Arabic word for moss (not then distinguished from lichen) was the apothecaries' name for a macabre but highly prized ingredient of the medicine cabinet: 'skull moss', the mould or mildew which supposedly grew upon an unburied human skull, and preferably that of a criminal 'hung in chains'. This loathsome mould, scraped off, was worth its weight in gold as a remedy for infections, nosebleeds, epilepsy and head wounds. Gibbets and battlefields must have offered good foraging to those who weren't averse to a bony rummage.

I didn't have a human skull handy so I've substituted the sheep's - and I bet there was not a little 'skullduggery'* of that sort back when flogging usnea guaranteed a quick profit. In fact there was a recipe for cultivating skull moss if you could not find the official version.
'They take the moss of large meadow stones, gathered in the month of April; this, when gently dried, they reduce to a gross powder in a glass mortar, sprinkling it with Malmsey wine... till it has acquired the consistence of a thick poultice. Then with a knife they spread this preparation very thin on the cranium of a carcass broken on the wheel. As it becomes gradually dry, they spread more of it on the cranium, which in the open air they expose to the rays of the sun, removing it when rains come on. This they repeat till the plant begins to ?ourish, and afterwards gather from it an Usnea not inferior to that which grows spontaneously from the skull.' (Rather like spreading yoghourt on your plant pots to achieve that rustic look although with the additional chore of picking up a human carcass, not normally available at garden centres.)

In reality I don't imagine this lichen would grow on bone but there are a couple of images here and here which show what the stuff might look like in situ and you can read much more on the subject here.
Usnea moss is still sought after for use as a medicinal tincture and a tea but it's not advisable to remove it from the branches where it grows; it's already endangered and has disappeared altogether in some countries.
Witches' whiskers was the name that came out of a competition to invent popular names for species without them, skull moss having fallen out of favour as a handle.

*Skulduggery, as it's usually spelled, has no connection with skulls according to this.