One of the coolest scenes in the woods is when I go out with the pup after dark. I wear a headlight so I can find my way over and around the chainsawed tree trunks and busted branches — my eyes not being as good in the dark as Bowie’s, or the neighbors’ cats whose eyes I count glowing in pairs from their hunting posts in the Couple Acre Wood.
Recently I noticed something that had previously escaped my attention: tiny sparkles glittered in the beam of my headlight like chips of gemstones among the duff and the low shrubs revealed in my headlight’s beam. One night. I concentrated on the location of one such sparkle and slowly crept up to it.
And there it was, a Wolf spider, its eight eyes on the prowl for evening repast. The name makes them sound ferocious but Wolf spiders don’t eat people or howl at the moon, and they don’t weave webs that snag a human hair-do, though some weave funnels to guide unsuspecting prey to dinner. They eat flies, ants, fleas, ticks, and other insectly critters they can seize.
Everything we humans think we see is really invisible. Turn off all the lights in the living room, tie a blindfold over your eyes, and discover how long it takes to knock your shins on the coffee table. It definitely is there; but it is invisible. What we identify as a table in a brightly lit room is, in reality, a pattern of reflected light that out parents have told us to call “table.” No light, no sight, for most of us surface dwellers.
But certain mammals have eyes that shine in the beam of a light — it’s that “deer in the headlights” effect that makes the eyes appear as though the critter is pointing its own flashlight back at ours. That’s not far wrong.
It’s called the tapetum lucidum (Latin for “bright carpet”), essentially a mirror in the back of some nocturnal critters’ eyes that reflects light that has entered the eyeball from the front, and then is reflected off the eye’s back wall onto the retina.
The mechanism is bad and good: bad because the reflected image does not match perfectly with the direct one, resulting in a slightly blurred image — like a double-exposed picture when the camera is moved slightly between exposures — and good because without the second light image, the hunter might not see her prey at all in the otherwise dim light.
Wolf spiders have a similar mechanism. They have eight eyes arranged in three rows. The bottom row consists of four small eyes, the top row has two medium-sized eyes, and the middle row has two large eyes, each with a reflective coating that bounces the light and effectively gives the Wolf spider a better look at dinner.
Without a web, Wolf spiders must hunt in real time, and at night to avoid exposing themelves to critters that would like to dine on them. But darkness makes seeing potential targets difficult. Being able to use reflected light within their own eyes roughly doubles the brightness of the image, helping the spider find it’s next meal.
So the next time you’re wandering in the woods on a summer night, stop moving now and then and look in the flashlight beam, say 20 or so feet away from your feet among the low bushes and honeysuckle vines, for tiny sparkling jewels that glitter, then don’t, in the leaves and low shrubs.
You have discovered either a wolf spider on the prowl or a wood faerie dancing. (Or a Large Yellow Underwing moth, but that, as hs been said, is a dog for another bone, about which more later.) Either can be a rewarding experience.