ON dank and sunless afternoons in the west, I get pleasure from colours half-buried in the gloom, colours that glow with their own light. At the heart of dark thickets are blood-reds, purples, siennas and russets: rich, pre-Raphaelite colours of the sort enjoyed by medieval minstrels in berry-dyed doublets.
Or there are eerily phosphorescent colours: spooky greens of lichens on bone-bare branches, especially those of alders and willows standing with their feet in water. One imagines the greens coming off on one's fingers and glowing in the dark, like the luminous paint I daubed on my bicycle for the English black-out in World War Two.
Lichens aren't actually luminous, but there are some parasitic fungi that make timber glow in the dark. The "honey fungus", Armillaria mellea, deadly to trees and garden shrubs, gives a greeny glow to the dead wood. In the first World War, another bit of primal darkness, soldiers in the trenches fixed chunks of the wood to their helmets and bayonets to avoid nasty accidents at night.
Armillaria may also have played a part in one of my favourite Irish wildlife mysteries, that of the strange lights that regularly flew up and down in the gorge of the Blackwater River, Co Waterford, in or around 1911. In the Irish Naturalist, a Miss Mildred Dobbs of Cappoquin gave a most thorough report.
"Now when I see a light," she wrote briskly, " I stop to observe it, and if I see it suddenly swoop out over a river or travel in half a minute from the bottom to the top of a hill 400 feet high (and sometimes even high in the air above it, as I have seen on occasions), I know it is a bird and not a man with a lantern. "The most remarkable fact that strikes me each time I see one is the quality of the light. I expected to see a white phosphorescent light, not the reddish yellow it is. . . I have several times seen it reflected in the river when the bird has been over the hillside three-quarters of a mile from where I was standing."
Miss Dobbs may have been prompted by reports in Britain of luminous owls, seen floating like will-o'-the-wisps in Norfolk, where one of the birds "literally lighted up the branches of the trees as it flew past". Richard Mabey, discussing this since, thinks "the owls - if that is what they were - had probably picked up phosphorescence from roosting in the crumbling `touchwood' of a tree smitten with honey fungus." But neither the colour of the glow, nor its intensity, seems quite right for what Miss Dobbs saw.
My recent column about the "night-light" jellyfish, Pelagia noctiluca, which came ashore in great numbers on western beaches this autumn brought a letter from a reader in Portarlington, Co Laois, who regularly walks the dogs in the woods. "Twice recently we have seen a luminescent centipede. I suspect this is either a chemi- or bioluminescent effect, not fluorescence. The glow was white, and didn't seem to be refreshed by flashing a torch on it."
There are many kinds of "living light" in nature, produced by organs and tissues in worlds as different as the abyssal ocean depths and that of bacteria busy breaking down corpses.
Some, like the threads of honeyfungus, give out a constant glow, but in nearly all luminous animals and insects the light is intermittent: sometimes a warning, sometimes an advertisement (as in the bright dots and dashes of courting fireflies).
Whatever its function, the bioluminescence of animals (including insects) is the result of a process of oxidation, or combustion. When chemical energy is released with the help of oxygen, part of it is usually in the form of heat. But animal light wastes no energy and its luminescence remains cold, rather like the light from a fluorescent tube, or from one of the modern chemical "light-sticks" that lure fish to baited ocean fishing-lines.
The relevant chemical in animals was named "luciferin" 100 years ago by a scientist who ground up the light organs of the West Indian cucujo beetle, Pyrophorus, a creature so bright it was used in place of candles. Sometimes, granules of luciferin are scattered through the whole animal, as in the microscopic protozoan, Noctiluca. This makes plankton sparkle in the sea on summer nights: as Rachel Carson wrote, it causes dolphins "to fill the water with racing flames and to clothe themselves in a ghostly radiance'.
But in light-producing insects, luciferin is concentrated into definite organs, elaborately fed with oxygen through trachea in the body. In the "glow-worm" (actually a beetle, Lampyris noctiluca) and in the various species of firefly, the light flashes on with special brilliance in the breeding season. In glow-worms, the female has the brightest light: a pale greenish-blue, amplified by a reflector of minute crystals, it shines out from her bottom as she sits in the grass.
In the southern chalk-country of Britain, a hedgebank full of glowworms switched on at dusk is said to be a most impressive sight, but the Lampyris family is, alas, quite absent from the Irish list of beetles. And the European firefly, Luciola lusitanica, doesn't come west of the Rhone.
So we seem to be left with luminescent centipedes, not strictly insects at all, but arthropods, like wood-lice. The species that glow are some of the soil-dwelling geophilids, flexible and wriggly - one is actually called Geophilus electricus. They are phosphorescent, particularly in the autumn and when disturbed: the glow may be to frighten off their enemies.
An odd thing about the light of Lucifer, released from luciferin, is that it's entirely free from ultraviolet and infra-red rays and falls wholly within the very narrow, yellow-green band of visible light that looks brightest to the human eye. Which leads me to wonder: why don't animals produce their light in wavelengths that only they can see; and thus conceal it, as it were, under a bushel?
