Crab spiders can (sort of) turn their eyes!

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MarkSturtevant
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Crab spiders can (sort of) turn their eyes!

Post by MarkSturtevant »

A friend recently gave me a lovely flower crab spider (Misumenoides sp.). I set about doing some quick focus stacks of it, "as I do", when I noticed something strange about the eyes. Here is the final stacked picture. Note the red frontal eyes, and the larger dark area around them. The dark areas delimit the bundle of pigmented retina cells deep in the eyes, which are visible here through the translucent cuticle. The individual pictures used to make this stack clearly show that the retinas can move from side to side!
ImageFlower crab spider by Mark Sturtevant, on Flickr

Click on the picture below, which will take you to the Flickr page. Play the short movie and you can see the movement for yourself. I chose to animate this as a movie since posting a gif animation is maybe a bit beyond me. Its simply two frames from the stack, played repeatedly several times. Anyone can feel free to steal the frames and post a looped gif animation here if you like.
ImageFlower crab spiders can "turn" their eyes! by Mark Sturtevant, on Flickr

This sort of thing is well known in jumping spiders, which can pivot their bundle of retina cells around in order to scan their surroundings. This is shown quite vividly here in this translucent jumping spider: https://www.youtube.com/watch?v=Yvz7NVAnKY4
Maybe it is known that crab spiders can do this too? In any case, it was certainly new to me!
Mark Sturtevant
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Re: Crab spiders can (sort of) turn their eyes!

Post by rjlittlefield »

MarkSturtevant wrote:
Fri Sep 17, 2021 4:25 pm
[scanning of the large central eyes is] shown quite vividly here in this translucent jumping spider: https://www.youtube.com/watch?v=Yvz7NVAnKY4
Maybe it is known that crab spiders can do this too? In any case, it was certainly new to me!
Thanks for this link!

I was not aware that internal movements of the eye tubes could be seen so easily from the outside in any kind of spider. I had always thought that clearly seeing the movements in jumping spiders required special optics to watch the retina through the cornea, for example as described in reference #24 in today's https://en.wikipedia.org/wiki/Jumping_spider :
Google search on youtube jumping spider eye movement found me a couple of other videos showing the same effect, but not nearly as well as the link that you listed.

--Rik

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Re: Crab spiders can (sort of) turn their eyes!

Post by MarkSturtevant »

It was unexpected. And since I wouldn't think that crab spiders have especially well developed vision, maybe the ability to move retinas is pretty widespread among spiders.
Mark Sturtevant
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Re: Crab spiders can (sort of) turn their eyes!

Post by rjlittlefield »

MarkSturtevant wrote:
Sat Sep 18, 2021 4:28 pm
maybe the ability to move retinas is pretty widespread among spiders
It seems so.

https://www.journals.uchicago.edu/doi/f ... 086/693977 ("Molecular Evolution of Spider Vision: New Opportunities, Familiar Players", Nathan I. Morehouse, Elke K. Buschbeck, Daniel B. Zurek, Mireille Steck, and Megan L. Porter) says in the caption of Figure 1,
Figure 1. Basic properties of the spider visual system. (A) Spiders have four eye pairs, named after their relative positions on the head, including anterior-posterior and lateral-medial positioning. These names are retained even in the face of dramatic differences in size and exact positioning between families, as indicated by examples of real spiders from the families (B) Eresidae (Stegodyphus dumicola Pocock, 1898) and (C) Salticidae (Habronattus altanus (Gertsch, 1934)). The anterior median eyes are called “principal” eyes and differ in their internal anatomy and developmental origin from all other eye pairs, called “secondary” eyes. These differences (illustrated in D) include whether the rhabdoms of the photoreceptors are everted (principal) or inverted (secondary), whether the eyes have a reflective tapetum (secondary) or lack one (principal), and whether the retinas can be moved by dedicated muscles (principal; not illustrated) or lack any musculature (secondary).
and in the text...
Finally, the principal eye retinas are also unusual in that they can be moved behind their fixed lenses by dedicated muscles attached to the retinal envelope (Land, 1969a). The sophistication of this musculature and the resulting retinal movements differ across taxa. Some spider groups, such as the ctenids, lycosids, and thomisids, have only four retinal muscles and exhibit a modest repertoire of retinal movements composed of microsaccadic “twitches” of 2–4° and larger displacements of up to 15° (e.g., C. salei; Kaps and Schmid, 1996). The microsaccades are thought to help avoid adaptation of the principal eyes to nonmoving stimuli, whereas the larger gaze movements often preempt and lead body turns (e.g., C. salei; Kaps and Schmid, 1996) or track prey without betraying their position by moving their bodies. However, salticids have elaborated both their principal eye musculature and associated retinal movements. These animals have six retinal muscles that enable not only vertical and horizontal displacements in a range of up to 50° but also torsional movements of the retina (Land, 1969a). The result is a sophisticated repertoire of microsaccadic, tracking, and scanning movements that allow these retinas to explore features of complex scenes in ways we are only beginning to understand (Land, 1969a; Canavesi et al., 2011). The secondary eyes universally lack eye musculature or retinal movement.
Interesting stuff!

--Rik

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Re: Crab spiders can (sort of) turn their eyes!

Post by Bob-O-Rama »

Some spiders have 3 axis IBIS? :shock: Kinda makes the iPhone seem somewhat less impressive.

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