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Hoverfly eye with coating of dust

 
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rjlittlefield
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Location: Richland, Washington State, USA

PostPosted: Fri May 18, 2012 10:14 pm    Post subject: Hoverfly eye with coating of dust Reply with quote

I found this dead but fresh hoverfly being dragged across my driveway by a couple of ants. I thought it might make an interesting specimen if only I could get it cleaned up, so I took it away from the ants and brought it inside to my equipment. I thought I should take a look at what sort of dirt and debris I was dealing with, so rather than immediately pitching the fly into an ultrasonic cleaner, I put it under a stereo microscope. That turned out to be a fortunate decision.

To my great surprise, the specimen was far more interesting dusty than it would have been clean. The reason is that the dust revealed a fine array of hairs that I would have completely missed on a clean specimen.

Here is the beast as a single picture. Notice that it just looks like a dusty hoverfly with apparently naked eyes. You can see a few hairs near the bottom of the eye, but basically there's nothing interesting in this picture.



Here is the same stack processed into a crossed-eye stereo pair by Zerene Stacker PMax.

If you can see stereo, it will be immediately obvious that much of the dust does not lie on the surface of the eye touching the ommatidia. Instead, the dust is floating above that surface by a considerable distance.



Here's a closer view, a little bit left and below center of the full frame.



If you can't see stereo, then the above pairs are pretty boring at best. In that case, perhaps this rocking animation will make sense if you study it long enough.



Now, getting back to still pictures, here is the stacked composite alongside a single frame from the original stack.



In the single frame, it's possible to make out what is only hinted at by the stacked composites: in this area on the front of the eye, the entire surface is covered by a dense array of fine hairs, apparently one from each vertex between the ommatidia. These hairs trap most of the dust in a layer near the top of the hairs, well away from the ommatidia.

Elsewhere on the eye, the pattern is different. Over the large ommatidia on the sides and top of the eye, the hairs are shorter and more sparse, occurring at every 2nd or 3rd vertex instead of every one. At the back of the eye, I cannot see any hairs at all, even over the small ommatidia.

Now that I have this understanding, it's not too hard to generate a single image that shows some of these aspects of the distribution. Here is a single carefully selected frame, very heavily sharpened and levels adjusted to bring out the hairs.



What's most interesting to me in this exercise is that I never would have noticed most of these hairs by direct viewing of a clean eye under a stereo scope. The hairs are so thin that in direct view they get lost in the strong pattern of the ommatidia, just like they do in the first stacked result shown at the top of this thread. It was only the dust layer combined with the stereo view that gave me a clear idea there were hairs to be found, if only I looked hard enough.

All in all, a fascinating exercise. Thanks for listening! Very Happy

--Rik

Technical notes: Photographed at 5X using a Canon T1i camera with Nikon CFI BE 10X NA 0.25 objective on Canon 100 mm f/2.8L IS USM macro lens. Focus stacked with Stackshot at 15 microns, 110 frames. Stereo pairs at +-4.5% using Zerene Stacker synthetic stereo.
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Craig Gerard



Joined: 01 May 2010
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Location: Australia

PostPosted: Sat May 19, 2012 2:03 am    Post subject: Reply with quote

Excellent,

The majority of the 'dust' and debris is impaled on the pikes. What are those protruding spikes called?

That's a unique set of eye-lashes Smile


Craig
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ChrisRaper



Joined: 04 Oct 2011
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PostPosted: Sat May 19, 2012 2:29 am    Post subject: Reply with quote

Craig Gerard wrote:
What are those protruding spikes called?

Most keys just call them "hairs" but they can also be called "ommatrichia" Very Happy
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abpho



Joined: 17 Aug 2011
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PostPosted: Sat May 19, 2012 4:06 am    Post subject: Reply with quote

Very cool Rik. A great protective mechanism for the eye. I wonder if the fly can see it's own hairs. I also wonder how upset the ants must have been. Very Happy
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Craig Gerard



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PostPosted: Sat May 19, 2012 4:26 am    Post subject: Reply with quote

ChrisRaper wrote:
Craig Gerard wrote:
What are those protruding spikes called?

Most keys just call them "hairs" but they can also be called "ommatrichia" Very Happy

I needed to look that word up Cool

Quote:
Using both scanning electron microscopy (SEM) and transmission electron microscopy (TEM), the ommatrichia, or hair-like processes that are located between the facets of the compound eye, were examined on an adult male Megaselia scalaris (Loew) fly. Each ommatrichium was observed to be a tapering structure bearing a longitudinally grooved cuticle and are anchored tightly in flexible sockets. Ultrathin sectioning and TEM revealed a thick wall in the ommatrichia, and their function was proposed to be mechanoreception based on characteristics from both SEM and TEM observations.
Ref: http://www.ncbi.nlm.nih.gov/pubmed/15629651


Craig
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Macrero



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PostPosted: Sat May 19, 2012 7:06 am    Post subject: Reply with quote

Very interesting Rik ! Too bad that these details are lost when stacking ...

Regards !
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rjlittlefield
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PostPosted: Sun May 20, 2012 11:12 am    Post subject: Reply with quote

Thanks for the comments. A few thoughts...

Quote:
A great protective mechanism for the eye. I wonder if the fly can see its own hairs.

It does seem like good protection, both passive by catching dust and active by letting the fly feel when something is getting close enough to be a problem.

Judging from how little the hairs affect the view looking in, I suspect they don't have much effect on the view looking out. A slight loss of contrast, perhaps, and an occasional flare spot?

Quote:
Too bad that these details are lost when stacking ...

I think it is not so much "lost when stacking" as "lost in the big picture".

The stacked result faithfully reflects what I see with direct view through the stereo microscope. The stereo scope has a narrow aperture lens that gives relatively large depth of field, and the thin hairs get lost against the high contrast background of the ommatidia.

It is only when using a higher power, wider aperture objective that the hairs become well isolated as sharp features against a very blurred and thus low contrast background.

I wonder how this eye might be drawn by a scientific illustrator. The answer, I think, would depend on the intent of the illustration. If it were intended to show how the eye looks under a stereo scope, then the hairs would be omitted except for a few around the periphery. But if it were intended to emphasize the hairs as an anatomical feature, then the illustration would downgrade the ommatidia to be low contrast background, with the hairs drawn crisp and bold in front of them.

It would be great if the software were smart enough to understand the structure and offer the same sort of options. But I think that will not happen soon, since I'm still hard pressed to do it using a brain!

--Rik
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flyer2o12



Joined: 09 Jan 2012
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PostPosted: Sun May 20, 2012 12:51 pm    Post subject: Reply with quote

Does anyone know the reason for these hairs?
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ChrisRaper



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PostPosted: Sun May 20, 2012 1:48 pm    Post subject: Reply with quote

I would guess that they have some sensory purpose but they are probably not very important because within the same groups some flies have the hairs and others have none (or very few hairs) and they seem to have no great disadvantage. There are even some genera where some species have hairs and others don't.
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conkar



Joined: 18 Dec 2010
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PostPosted: Mon May 21, 2012 11:03 am    Post subject: Reply with quote

One of the most interesting things about photomacrophy is that you get the opportunity to see details that you would not otherwise see and you also then like to learn more about these findings.

I think insect eyes are interesting and I have also seen these fine hairs that you show here in your pictures.

Another detail that I have seen in fly eyes is that some have something that looks like eyelids. But to see them you have to raise the magnification a bit.

I have not photographed the eyes of a Hoverfly, so I don't know if they also have some kind of eyelids.

,,I updated my last picture post here with a 100% crop of a fly eye and at the upper left corner of that crop you can see the "eyelids".

Regards,
Conny
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rjlittlefield
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PostPosted: Mon May 21, 2012 1:55 pm    Post subject: Reply with quote

conkar wrote:
Another detail that I have seen in fly eyes is that some have something that looks like eyelids. But to see them you have to raise the magnification a bit.

I have not photographed the eyes of a Hoverfly, so I don't know if they also have some kind of eyelids.

,,I updated my last picture post here with a 100% crop of a fly eye and at the upper left corner of that crop you can see the "eyelids".

I have commented HERE about the image that I think you're referring to. It is not a physical structure, but rather the reflection of some environment, probably the edge of your diffuser.

At high magnification, it is easy to get misled by reflections from the very shiny ommatidia. Consider for example the thread HERE, "Eye of Fruit Fly". As I wrote in the discussion, "It's interesting to notice, in this picture, the ommatidia at lower left. They appear to be very complicated. But in fact, they are just smooth shiny bumps --- it is their environment that is very complicated! In each ommatidium, the dark disk is a reflection of the microscope objective, while the lighter ring is a reflection of the illuminated pingpong ball. The radial rays are simply reflections of the surrounding setae, of which there is one at almost every junction between ommatidia."

--Rik
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