Moving black spots on the eyes of a mantis

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rjlittlefield
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Moving black spots on the eyes of a mantis

Post by rjlittlefield »

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If you get close enough to see it, many insect eyes have a small black spot that follows your every move. This mantis makes a great example.

How does the mantis do that? It's an optical effect. Not an illusion, mind you -- there really is a small black spot that moves around on the surface of the eye. The catch is, the small black spot is not some physical structure that's moving, it's just the set of ommatidia (facets of the compound eye) that happen to be facing in the direction of the observer -- either your eyes or the camera.

This has one fascinating effect, though. If you look at the mantis eye through a stereo microscope, then each of your eyes sees the black spot in a different position. Your brain then proceeds to interpret the different positions as parallax caused by depth, and what you see looks for all the world like the mantis's compound eyes are big transparent balls, with the black spots painted on the inside of the eyes' back surfaces! Now that's an illusion -- and a very bizarre but convincing one!

--Rik

Technical: Canon 300D, Sigma 105 mm at 1:1, natural light, cropped. Captive mantis.

Mike B in OKlahoma
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Post by Mike B in OKlahoma »

Very interesting montage, thanks! (and I got an "error 500" when I tried to access it first time).
Mike Broderick
Oklahoma City, OK, USA

Constructive critiques of my pictures, and reposts in this forum for purposes of critique are welcome

"I must obey the inscrutable exhortations of my soul....My mandate includes weird bugs."
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rjlittlefield
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Post by rjlittlefield »

Thanks, Mike, for both comments.

BTW, I just noticed that the two frames in the lower right corner actually make a usable stereo pair (crossed-eye), if you concentrate on just the eye and ignore the legs and antennae. I'll try to get a better stereo pair tomorrow, but no promises -- it's a challenging problem.

--Rik

Erland R.N.
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Post by Erland R.N. »

Great photo sequence, displaying the effect very nice indeed. I mostly see it on dragonflies, and I've read that there infact should be I think six secondary spots around the main spot. They are probably hard to see though.
I would stay away from a stereo-microscope, to keep sane :-)
I must say that when I've held a dragonfly in my hands with very big eyes, and moved it very close to my own eyes, the surface seems to float too. Maybe the same effect.

Erland

Erland R.N.
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Post by Erland R.N. »

btw. the pose in the upper left picture is awesome.
Erland

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Cool Shots

Post by Raul G »

Cool shots
I love live specimens.
It is alive?.....isn't it?

rjlittlefield
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Re: Cool Shots

Post by rjlittlefield »

Caronte wrote:It is alive?.....isn't it?
Oh yes, very alive. That's why it's challenging to get a stereo pair, given that I don't have a camera that will shoot both pictures at the same instant.

--Rik

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Post by rjlittlefield »

Erland R.N. wrote:I've read that there infact should be I think six secondary spots around the main spot. They are probably hard to see though. ... I must say that when I've held a dragonfly in my hands with very big eyes, and moved it very close to my own eyes, the surface seems to float too. Maybe the same effect.
I think dragonfly eyes and mantis eyes are slightly different. I have only been able to find the one black spot in this mantis. (See larger image here.) Many butterflies, on the other hand, have numerous fuzzy dark spots that behave roughly the same way. The size, sharpness, and number of the spots may have to do with the resolution of the eyes, but this is just my speculation -- I have not read anything about it.

--Rik

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Post by rjlittlefield »

A follow-up posting, showing the spots as a stereo pair, can be found HERE.

--Rik

Irisoratoria
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Post by Irisoratoria »

Dear rjlittlefield.

The effect of the paying mantis coumpound eyes is called "pseudopupil" and it is an optical ilusion created because the omatidia aligned with the observer absorb the light while the non aligned omatidia reflect the light, creating a curious effect.

razashaikh
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Post by razashaikh »

Great piece of work there. Thanks for the brief explanation, didn't knew that!

AlxndrBrg
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Post by AlxndrBrg »

rjlittlefield wrote:
Erland R.N. wrote:I've read that there infact should be I think six secondary spots around the main spot. They are probably hard to see though. ... I must say that when I've held a dragonfly in my hands with very big eyes, and moved it very close to my own eyes, the surface seems to float too. Maybe the same effect.
I think dragonfly eyes and mantis eyes are slightly different. I have only been able to find the one black spot in this mantis. (See larger image here.) Many butterflies, on the other hand, have numerous fuzzy dark spots that behave roughly the same way. The size, sharpness, and number of the spots may have to do with the resolution of the eyes, but this is just my speculation -- I have not read anything about it.

--Rik
There are different kind of of compound eyes, that while they have a similar exterior their inner workings are quite different. The original type is apposition compound eyes (which this mantis has), where each facet of the has a dedicated rod of light receptor cells beneath it. This is common in day active insects, bee's, mantises, dragonflies etc. Many nocturnal insects such as moths and beetles have superposition compound eyes where the optical element of the ommatidia is decoupled from it's sensory cells, so that the light from one ommatidia can reach several light receptor rods (rhabdomes). So it's basically a layer of lenses (i.e. the surface of the compound eye) then a gap (not sure if this is an air gap or some kind of transparent liquid) and below it a retina. This utilizes the light more effectively, but I can't imagine how their signal processing circuits actually manage to form a coherent picture from it - maybe it's quite low resolution and that is why it's not used in diurnal animals.

I'm a bit rusty when it comes to these things, been more than a few years since I studied sensory biology - but you can read this article from my old professor, he's brilliant, I'm sure it's very understandable even without being a specialist.
https://www.researchgate.net/publicatio ... iny_brains

Edit: I forgot to say - apposition eyes have one main pseudopupil, while superposition eyes have a central one with many smaller surrounding it. If you see a drawing of the difference between these eyes it is quite obvious why

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Post by rjlittlefield »

Alex, thank you for reviving this thread and providing the link to Eric Warrant's paper. I've printed a copy of that, and will be reading it when time permits.

As it happens, I've recently purchased a couple of books on eyes, and I think I can provide an extract that bears on some of the issues that you've raised.
AlxndrBrg wrote:superposition eyes...but I can't imagine how their signal processing circuits actually manage to form a coherent picture from it
It turns out that the coherent picture is formed by strange optics, not signal processing.

Quoting from pages 191-193 of Animal Eyes by Michael F. Land and Dan-Eric Nilsson (2nd Edition, ISBN 0199581142),
The real surprise is optical. All superposition eyes produce a single deep-lying erect image in the vicinity of the retina. Not only does this distinguish them from apposition eyes, which have multiple inverted images, but also from camera-type eyes where the image is inverted. Clearly we are dealing here with something quite out of the ordinary.
...
The credit for the discovery and elucidation of this remarkable piece of optics is due to Sigmund Exner, who worked on the problem throughout the 1880s and published his complete findings in 1891. Exner showed that the only way an erect image could be formed was for the optical elements to behave in a rather strange way, as shown in fig. 8.3a. Basically what each has to do is not to form an image from a parallel beam as in a conventional lens, but to redirect light back across the element's axis, to form another parallel beam on the same side of the axis (Fig. 8.3b). Exner realized that although a single lens wouldn't do the job, a two-lens telescope would, and he went on to demonstrate (as well as he could with the technology of the time) that such structures were indeed present in the superposition eyes of insects.
So, with a superposition eye the retina actually has a "simple" job, quite analogous to our own in terms of the connectivity of the neurons that are required.

The neural circuits in most apposition eyes also have it "simple", since each ommatidium only produces a single summed output that represents a small neighborhood within a single erect image.

However, in some apposition eyes such as those of true flies (Diptera), the output of each ommatidium is not a single value but instead a mini-image consisting of seven samples from the inverted image formed by the lens. Dealing with an array of inverted images would seem to be a nightmare, but the eyes have a very simple and clever way of dealing with it: the nerve bundle coming out of each ommatidium simply does a 180-degree twist before feeding into the next layer down. That physical twist neatly undoes the optical inversion caused by the lens, so that again, what most of the retina deals with is just a single erect image. There is some discussion of this at https://en.wikipedia.org/wiki/Ommatidium.

As for the pseudo-pupils, the situation is not as simple as apposition versus superposition. Dragonfly eyes are apposition also, but they often have prominent secondary pseudopupils (here, for example). For a long time, I too thought that the secondary pseudopupils represented separated areas of the eye that were all seeing the same part of the environment. But it turns out that the darkness of the secondary pseudopupils is actually a magnified view of dark pigments around the photosensors, rather than the photosensors themselves. So, the main pseudopupil indeed shows which ommatidia can see the camera lens, but the secondary pseudopupils do not. No matter how complicated the pattern of secondary pseudopupils, no additional processing is required at the neural level. Some further discussion and links at http://www.photomacrography.net/forum/v ... 218#246218 .

Cool stuff!

--Rik

houstontx
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Post by houstontx »

This sequence is great. I interpret it as:
top left - wake up
next two - work
next four - lunch
next three - work
last two - sleep

Pretty much sums up my day! And thus the cycle continues...

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