Wednesday, June 9, 2010

Adirondack Entomology: Appreciating Jumping Spiders

I like spiders. They are clever, they are colorful, they are beneficial. Spiders come in a stunning array of sizes, shapes and colors. Some build webs to catch their food, others go fishing, while still others hunt by ambush. With the exception of a few truly venomous species, which most of us will never encounter, there is really very little about spiders to dislike. Still, many people find them creepy and go through life squashing any spider they meet. It’s a sad state of affairs, but some folks simply refuse to see anything beautiful in spiders.

Today I’d like to introduce you to one of our more interesting spiders: the zebra jumping spider (Salticus scenicus). This is a tiny spider, measuring 4-8mm if it is a female, 4-6mm if it is a male. It has black and white markings that make us think of zebras, and hence the name. But what really draws me to this spider are its eyes.

Like all jumping spiders, the zebra jumper has excellent vision. This isn’t just because it has eight eyes, but because two of those are huge, face forward, and have moveable retinas. What this all boils down to is binocular vision that can track moving objects.

If your head was held immobile, you could still move your eyes to watch what is happening around you. The jumping spiders cannot move their eyeballs, but they can move the retinas in those two large front eyes. This comes in very handy if you make your living stalking and pouncing on your food.

Jumping spiders, especially these little zebra jumpers, are famous for watching things, like those who are observing them. Give it a try the next time you see one of the little fellows. All you have to do it lean in close and it will turn its body and move its head so that it’s looking directly at you. You can tell when it’s looking at you by noticing the color of the eyes. As the retinas move, the eye color changes. When it is totally black, you are being watched.

Next, hold your finger a foot or so in front of the spider’s face. Move it around. Odds are, the spider will focus on your finger, tracking it with its eyes and moving its head to keep it within sight.

As mentioned, these spiders have eight eyes, which are arranged in a line around the spider’s head, kind of like Geordi LaForge’s visor. The two large ones on the front are flanked by two smaller ones that are also forward facing. The remaining two pairs are placed further back along the sides of the spider’s head (technically, this is the cephalothorax, which is a body unit that combines both the head and the thorax). The end result is that jumping spiders have peripheral vision that enables them to see all the way around their bodies. Sure wish I could do that!

Jumping spiders are diurnal, and you can likely find them near, or even in, your house. Look for them on sunny days hanging out on walls, fences or plants, where they will be hunting. When another spider or insect comes along, the jumping spider sizes it up. Most prey items are smaller than the spider, but the zebra jumper has been known to take down mosquitoes, which are up to two times its own size. Like a cat, these spiders slowly sneak up on their prey from behind and then pounce, immobilizing the meal with a quick bite. (Yes, they are venomous, as all spiders are, but their venom won’t hurt you, assuming they bit you, which they are highly unlikely to do since you are way too big for them to eat.)

Safety is always a priority, though, or it should be, and jumping spiders follow this rule, too. Before leaping after a potential prey item, a jumping spider will anchor itself to the surface on which it is standing. This is done by gluing a strand of silk to the surface. This is about the only time these spiders spin out silk, for they don’t build webs. (The other occasion which calls for silk-spinning is when the female makes her egg sac.) Now, should the spider miss its prey or tumble out into space, it is tethered to a solid object and need only climb back up its silken ladder to safety and a new attempt to catch some food.

Having spiders in our houses is really kind of nice, when you think about it, for they consume all the other insects that also live there. Don’t think you have insects in your house? Well, that could be because the spiders are doing their jobs. So the next time you see a spider in your house, think twice about killing it. Even evicting it isn’t a solution, for it will likely find its way back in. And, just in case you were wondering, most spiders found in your home are species that have evolved over thousands of years to live in human abodes. If you chuck them outside, they will just return, for su casa es mi casa is their motto. And really, for the services they render, they ask only for very little: a dark corner in which to set up housekeeping. It’s worth it in my book.

Photo: Zebra Jumper (Salticus scenicus). Photo by Olaf Leillinger for WikiCommons.

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Ellen Rathbone

Ellen Rathbone is by her own admission a "certified nature nut." She began contributing to the Adirondack Almanack while living in Newcomb, when she was an environmental educator for the Adirondack Park Agency's Visitor Interpretive Centers for nearly ten years.

Ellen graduated from SUNY ESF in 1988 with a BS in forestry and biology and has worked as a naturalist in New York, New Jersey, and Vermont.

In 2010 her work took her to Michigan, where she currently resides and serves as Education Director of the Dahlem Conservancy just outside Jackson, Michigan.

She also writes her own blog about her Michigan adventures.



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