What do cats, deer, and wolf spiders have in common?

One of the many podcasts that I listen to on a regular basis is called BrainStuff.  It is well written by Marshall Brain and only lasts 4 or 5 minutes.  There is one episode from October 2010 that really interested me. 

Marshall took a head lamp outside one night.  It was around his head and he turned it on and started looking around the yard.  There was a pile of dirt and it seemed to be sparkling.  Many, many points of light.  He could not figure out what it was.  So he got closer and found that it was many, many wolf spiders.  The light from his headlamp was reflected in their eyes.

It turns out that wolf spiders have the same thing that deer, cats, dogs, and raccoons have in their eyes.  It is called tapetum, a shiny layer behind the retina. Light passes into their eye, hits the retina, reflects from the tapetum and hits the retina again.  This makes things look much brighter, making it easier to hunt at night.  It also makes their eyes seem to glow when light hits it.

The light shining from the head lamp reflected off the spider's eyes and returned again to its source.  If Marshall had been holding a flashlight down by his waist, he would not have seen this reflection. 

(Hmmm.  This is a little like the last post, where light bounces off a corner cube reflector on the moon.)

Have you ever wondered whether a spider is poisonous?  The answer is no. So far as I know, there are no poisonous spiders. There are venomous spiders. Venomous means that the creature's bite or sting is toxic. Poisonous means that it is toxic if you eat it. 

Now you have heard something interesting.