Monday, October 29, 2012

Stars on Earth: How to find them, and how to make them shine

Amanda Spears is a sophomore in the Rubenstein School of The Environment and Natural Resources studying wildlife biology. She is (informally) a field naturalist in-training with a passion for birds.

Centennial Woods is absolutely breathtaking and brimming with life this time of year. . .during the day. But what about at night? Do we all assume that it rests with the sun to prepare for it’s early morning the next day? If you've been in the woods at night then you know that it’s just as awake at night as it is during the day, or rather it’s best described as a re-awaking. Owls, flying squirrels, deer, rodents, opossum, raccoons, frogs, and scores of insects use the absence of light to evade predators, and/or capture prey. Being in the woods at night is admittedly eerie, perhaps because you tend to feel the presence of life, something watching you, that uneasy feeling that you are not alone.

I’ve been out to Centennial Woods dozens of times, during the day of course. Recently, a few friends and I took advantage of a particularly warm evening and headed out for a night hike. We started at the main entrance off Carrigan Drive, and walked in quite a ways, stumbling upon a few bedding deer, and trying to hoot up a Barred Owl. No luck. However, sweeping our headlamps back and forth across the path, every once and awhile a small flash of light would catch our eyes, looking much like a bead of dew catching the morning sun - or in this case our headlamps. It had not rained recently, so what were these twinkling earthbound stars? Curiously we targeted one and walked slowly up to it, and what we discovered awed some of us, and frightened the rest . . .

it was a Wolf Spider!

This particular guy was hanging amongst the leaf litter and debris, perhaps patiently waiting for a meal to walk by. He looks a bit like a mini- tarantula, doesn’t he? So how does he produce that light? We’ve heard that spiders have eight eyes, and as it turns out, the arrangement of those eyes is a way to classify and identify certain spider families. Lycosidae, or Wolf Spiders, have 4 small anterior eyes and 4 large posterior eyes, giving them excellent eyesight to use for hunting, evading predators, and courtship. These eyes are also responsible for producing that glint: light from the headlamp reflects off the tapetum lucidum in the spider’s eyes. You’ve seen this elsewhere, maybe from a deer in your headlights, or even from your cat. The eyeshine from these animals is the same as this spider’s. 

Not all light that enters the eye is perceived by an organism. This is particularly problematic for nocturnal animals. The tapetum lucidum is a layer of cells in the back of the eye behind the retina that reflects incoming light back across the retina, allowing an organism to make the most of scant light at night. Not all the reflected light is absorbed by the retina on its second pass and so the glow comes from the light that "escapes" back out the eye. Those organisms that have a tapetum lucidum are, in most cases, nocturnal. Different colored reflections can be diagnostic for identifying animals at night.

So this Halloween, when you are out and about walking from this house to the next (for whatever reason), remember to bring your flashlight or, better yet, your headlamp. You may be lucky enough to catch a spider’s eye, or eight!

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