. . . 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.