. . . or rather in its cephalothorax. Matthew Cobb sent me a link to this tw**t by Leilani Walker.
Jumping spiders are ambush predators, and need to follow and fix their vision on moving prey. How can they do this? It turns out that this group, contrary to what we expect for arachnids, can actually swivel their eyes about, something alluded to in the tw**t.
Here’s the video showing eye movement and its notes:
One of the most transparent jumping spiders I’ve ever seen. Note how visible the primary eyes are. Check out another video to look deep into a jumping spider’s spotted, moving eyes: https://www.youtube.com/watch?v=Dq5ky.
Watch carefully to see the movement: those black objects that are moving back and forth are the eye tubes (see below) scanning the environment, and happen to be visible through the transparent exoskeleton:
The explanation of this video, and of spider vision in general, is given in a piece by Gwen Pearson on Wired, “Spider vision made clear.” I’ll reproduce some of her text:
Spider eyes are different from insect eyes; they are not compound but simple. There is one lens for each eye, made of a thin layer of the cuticle. Below that is the retina, the actual light-detecting cells. Jumping spiders have a problem–how do they focus their eye? They don’t have an iris like we do, and their lens is solid.
The easiest way to deal with this is to angle your head, and you can see the spider carefully tilting his head to get a better look at the videographer pestering him. It’s those adorable head tilts that make photos of jumping spiders so very cute.
But for fine focus, more is needed. The evolutionary work-around for this (if you are a jumping spider) is to have eyes that are a bit of a tube:
The movement you’re seeing in the video is the front eye tubes and the muscles that adjust and point them. There’s a second lens at the end of the tube, and unlike the outer lens it’s flexible. Basically, jumping spiders have built themselves two little telescopes. By adjusting the angle and shape of the inner lens, the spiders can focus and zoom in on what they are looking at.
That’s remarkable. I know of no other group that has two lenses, one behind the other, in their eye. (I may of course be wrong.) Thanks to Gwen for the article and to Matthew Cobb, who brought it to my attention.
Finally, to save you trouble, I’ve embedded the second video as well, which uses special photography showing that apparently stationary eyes are actually swiveling about:
Notes on this one:
This jumping spider’s eyes have strange checkerboard spots on the inside, which you can see if you peer deep into its eyes. The eyes move side to side, as you may have seen in a different species here [first video above linked]. This is Habronattus aztecanus, mentioned also here.
sub
“I know of no other group that has two lenses, one behind the other, in their eye.”
How about our own eyes?
It is my understanding that most of the refraction is done by the cornea. What we call the lens is adjusted for finer control of the focus — rather like the spider’s arrangement.
The second video looks like a spider eye exam: “Is it better here, or here?”
I wonder what spider cataracts would be like.
I’m envisioning a scene out of Indiana Jones and the Something of Somewhere, where Indy is buried under a cataract of spiders, muttering “Maybe rats aren’t so bad.”
This is how they respond to the need of corrective vision.
Haha!
So spiders really are swivel-eyed maniacs?
There doesn’t seem to be much coordinated eye motion; the two eyes aren’t looking at the same thing. I guess at that small size scale, stereoscopic vision isn’t effective beyond a few centimeters.
I conjecture that they do use stereo vision for the final pounce.
Dr. Maddison also has some great public lectures:
https://www.youtube.com/watch?v=6sCQQmb7wvs&list=PL13407C34037C57B8
With a simple explanation of phylogenetics:
Sorry, I was trying to link to 8:18 in the second video
Of the third video
I have been able to see some of this while taking pictures of jumping spiders. I could see that the color of the two main eyes would change as it scanned me.
I thought I could get a little zebra jumping spider to display his front legs at my finger but he took one look & scuttled off to hide under a ledge. I actually felt bad for scaring him.
They seem to be able to see a fair distance too. About a month ago I came across a gorgeous jumping spider in our driveway. I called my wife over and we admired it for a few minutes as it nervously and continuously scanned us, shifting constantly. I then asked my wife to go in the house to get the camera. I didn’t want to leave it because I was sure it would disappear on me if I did.
I was paying close attention to the spider, and it to me, when my wife came back with the camera. She came through a door about 120 degrees away from the dirction the spider was facing (it was facing me, intently). The spider gave no sign of noticing her opening and closing the door. But suddenly as my wife approached to within about 7 meters the spider spun to face her, and then began rapidly spinning back and forth to face her, then me, then her. I was impressed that it had seen her from so far away and from such an angle! Probably noticed movement in one of its secondary eyes? Or maybe sound or vibration through the ground?
Before I could bring the camera to bear it took off on me.
A small reminder of the necessity in an “eye”, however constructed in detail, to have an opaque layer enclosing the sensing material so that there is directional response. Otherwise, if you had a completely transparent eye (apart from the light absorbed by the sensing element), then you wouldn’t be able tell whether any particular photon had come from *this* direction in stead of #that# direction.
I was going to say that this was a fundamental property of all optical systems, but then I thought about neutrino telescopes and thought I’d better stop now. There might be ways around it, but they’re not simple.
WordPress is misbehaving again.
Sinking to IE to clear the backlog of comments. Yeuchh.
I know people who hate spiders in general, but don’t mind jumping spiders. I think it is their sneaky, quirky qualities, and of course their ability to jump. I read on Wiki that some cultures call them “Octokittens”. That sounds appropriate…
Thanks for this in-depth look into their eyes.
Also their legs don’t stretch out in a scary way & even the larger ones are still pretty small.
They somehow manage to combine attentiveness and responsiveness without being threatening. And they kind of look like a cute cartoon of a spider to begin with- small, often fuzzy, no obvious mouth or fangs, and two large, reflective front facing eyes.
They _are_ some of our most “attractive” spiders. What’s not know is that, before a pounce, at least some of them put down a safety line.
Once, when I was in the bathroom, I saw a jumping spider on the window shelf. I slowly waggled my forefinger in front of him/her/it, and the spider jumped at it. He(generic) grabbed my forefinger briefly before realizing it wasn’t prey, and quickly let go. Then he climbed back up the thread he had put down, just before jumping.
Thanks so much for this article and the detailed references. Once again, my education rolls on! Cheers.
From a small child I was always fascinated by jumping spiders in the way that they ‘LOOK’ at you, for something that small to REALLY focus on something as large as me is about as close as we can get to the arachnids. In Italy I saw another species – I have no idea what it’s called, but it’s method of hunting an ant was to run in circles around the prey faster than the ant could run, extruding silk as it went so that the ant was eventually entangled and dispatched!
Wow, Jehovah must have been in a really “playful” mood when he created all these different kinds of eyes and pupils in animals- why didn’t he just come up with one design and give it to everybody? Huh?
On a related tack – why so many different varieties of butterfly fish? I’ve been holidaying in Rarotonga for a couple of weeks, and every swim in the lagoon produced dozens of fish varieties. Besides the omnipresent ‘standard’ threadfin butterfly fish http://www.pbase.com/hhf/image/55013177/original
every swim produced at least one other ‘new’ variant, always in pairs.
Since these all occupy (presumably) the same ecological niche, why so many different types? Why hasn’t natural selection narrowed them down to one?
cr
P.S. Maybe God has an inordinate fondness for butterfly fish…?
cr
Awww… Hard to notice the eye focusing science due to the Honorary Kitten™ theme.
Thanks for this, Prof CC (and the others) – fascinating.