April 8th, 2006
spider eyes
As some of you may have noticed, I take a lot of photos of spiders. I do my best to identify as many as possible, but I’m just a rank amateur when it comes to IDing arachnids. I figure I’m doing okay just to get spiders into the the right families. Beyond that, I do my best, but often don’t get too far. There’s a lot to know and only a few good identification guides. The truth is, it’s not all that necessary for me to know the exact species of a spider in order to enjoy watching and learning more about it — so I try not to get too hung up on the IDing thing.
One thing I have learned after a few years of insect photography is that spiders have very interesting eyes. Most spiders have 8 eyes which vary in size and placement depending on species. So, knowing something about spider eyes can actually help when you’re learning to ID spiders. This morning, I picked out a few examples — from both recent and older photos — to illustrate some of the variations that might be seen by anyone who takes the time to examine a few backyard spiders.
At the top of this post, you’ll see a Jumping Spider (Family Salticidae). Members of this family do not build webs for capturing prey. Instead, they prowl around looking for prey to capture — which is exactly what the spider in the above photo has managed to do. As you can see, this spider has a very large pair of eyes at the center front. Eyes located at the center front are called “anterior median eyes”. When you see a big set of eyes like that, it usually means that you’re looking at a spider that is a hunter — like the jumping spiders — that must have very good eyesight so that they can locate and capture prey. By the way, this spider does have eight eyes — but four of them aren’t too visible. If you look hard, you’ll see the rear-most pair of eyes back on the side of the head region. Those are called “posterior lateral” eyes. There is actually another pair of eyes in between the front and rear pairs of eyes — unfortunately not too visible in this photo, but they are the “posterior median” eyes.
The next spider has a completely different eye arrangement. This spider is a Misumena vatia – often called a Flower Crab Spider. It belongs to the Crab Spider Family (Thomisidae). These spiders don’t move around a lot. Instead, they find a suitable location and then let their prey come to them. They often sit in the center of an attractive flower, looking upwards, legs raised skyward, waiting for an unwary bee, hover fly, or other victim to come looking for pollen or nectar. I have often seen one of these spiders sit motionless for hours as they wait for their prey. I’ve also found the odd dead bee inside of a flower blossom, and when I’ve investigated, found a spider hanging onto it down inside the bloom. As you can see, this spider’s eyes are all contained within a very small area facing front and center. No doubt, they suit this spider’s hunting style, which is to concentrate full attention on an object which comes directly into the spider’s grasp. By the way, here’s a photo of the whole spider just to give a better idea of how it looks when it’s waiting for prey to come within its grasp. I might add that, despite this fairly sedentary hunting technique, these spiders seem to be incredibly successful — which probably says something for patience and perseverance.
The next spider is some species of Orbweaver (Family Araneidae). I think it’s probably a Marbled Orbweaver (Araneus marmoreus), but I always find the various species of Araneus a bit difficult to sort out. In any case, this spider spins a large, circular web which can be 20 inches or more in diameter. It then sits in a safe spot, either on the web or nearby. I usually find them sheltered under the bent tip of a Timothy grass seedhead, just to top and side of their orb. They are said to have poor eyesight — which isn’t too surprising considering the minute size of their eyes. These spiders depend on feeling vibrations from struggling insects in their webs rather than on seeing their prey. When they feel vibrations and tugging on strands of their webs, they crawl to that spot to kill and wrap their prey.
I’ve included these last two spiders as examples of hunting spiders with interesting eye arrangements. I’m quite sure that the above spider is a Dark Fishing Spider (Dolomedes tenebrosus), and is a member of the Nursery Web Spiders (Family Pisauridae). As North American spiders go, this is one of the larger species. As can be seen, the 4 anterior eyes are fairly small and lined up in a row at the front. The center rear pair (posterior median) of eyes are quite large, with a small pair of rear side (posterior lateral) eyes a little off to the sides. This spider hunts in or near water and has good vision which makes it able to capture prey such as large insects, and aquatic creatures such as tadpoles and small minnows.
The last spider of the group is this handsome unidentified Wolf Spider which I photographed last weekend (April 2). To see a full view of it, click here. Although it’s difficult to see all of the eyes, what I would point out is that Wolf Spiders (Family Lycosidae) have a pair of fairly good-sized posterior median eyes (center rear eyes), and also a somewhat conspicuous pair of lateral posterior eyes (the rear set on the sides of the cephalothorax). Both Pisauridae and Lycosidae spiders have a reflective layer of cells at the back of the eye (tapetum) which give them good night vision.
Well, hopefully my explanations weren’t too confusing and I didn’t make any particularly grave errors (feel free to post a comment if you spot an error). There’s a lot more to the topic of spider eyes and vision, but I’ll have to leave some of that for another day. Got to get out hiking as it’s a very nice day here in eastern Ontario!
Btw, for those who are interested in such things – I’m sometimes asked about favourite field guides, etc.. For spiders, a good, basic guide that would get someone started with spiders is “Spiders of the North Woods” by Larry Weber (Kollath-Stensaas Publishing, 2003). Sorry I haven’t put up a link as I’m in such a hurry to get out of here. One of these days, maybe I’ll get around to putting up a fancy page of favourite books… but don’t hold your breath. (-:
Tags: spider eyes, spider vision, Jumping Spider, Salticidae, Crab Spider Family, Thomisidae, Misumena vatia, Orbweaver, Araneidae, Araneus marmoreus, Dark Fishing Spider, Dolomedes tenebrosus, Pisauridae, Wolf Spider, Lycosidae