Orthonevra nitida


Photograph by Al Denelsbeck.


Orthonevra nitida – syrphid fly or flower fly on a parsley flower. The fly is about 5 mm long. Mr. Denelsbeck is fascinated by the pattern on the eyes and asks, “I’ve only seen this tiny fly once … and couldn’t even make out the pattern of the eyes in the viewfinder. While I’ve identified it as Orthonevra nitida, no source has given the faintest idea how or why this eye pattern evolved, and I’m dying to know. Closely related species have different patterns, or virtually none at all.” Can any reader answer the question?


Dragonflies, horseflies and hornets have patterns on their eyes - see, for instance, http://www.dpreview.com/galleries/3[…]orsefly-eyes - or just go to Google Images and search for “horsefly eyes”.

(Sort of on topic…the fly’s eye design reminds me of Ben Kingsley’s facial tattoos in the upcoming “Ender’s Game” film.)

Striking color patterns are common in insect eyes, and in the compound eyes of some crustaceans. The most familiar examples in insects are seen in horse flies and deer flies, which often have species-specific iridescent patterns of color that also can differ between the sexes. Here is a link to an older paper that concludes that these kinds of patterns are for specialized ommatidia that respond to specific colors and/or direction of light, or may be used to see with enhanced contrast in some parts of the visual field. Interference filters. I do not know why this fly should have specialized eye areas arranged in a squiggly pattern, but the patterns seen on some other flies are also pretty striking.

Fascinating image!! I would need the creature to confirm what I am about to tell you, but here are my thoughts. Even with the fly in hand and under the microscope, it may be difficult to tell what the cause or reason for development, however.

To me, this is almost certainly camouflage. Sexual selection has been suggested and that too is a possibility. Biology is all about shelter (hiding), nutrition, and sex. I am going with camouflage because the color and pattern match the rest of the body very well. Here is how I believe the color is created. The visual pigments could reflect back some color, but not like this. The pseudopupils of many invertebrates such as moths, bees, and especially the mantis shrimp (a stomatopod) can appear to be multiple and completely black but those are ommatidia (individual units of a compound eye) that are completely absorbing the light as it goes into that particular ommatidium perfectly. That means that those unit(s) is/are looking directly at the camera. But these are black in appearance, not purple as this animal has.

I believe that all color on this creature is structural and not true pigments–like the feathers of birds. It is constructive and destructive interference of light–constructive for one color and destructive for all others. The structural color wavelengths come from the tracheoles. These are folded, tapering conduits for respiration–essentially stacked hollow tubes. They taper down to the smallest size that air/oxygen can travel through to nourish the eyes. Remember these insects don’t have lungs exactly. Once these tubes get that small, they fit nicely near and around the eye and even the individual ommatidia to nourish the cells. They are also small enough, fine enough and precise enough to reflect light of certain wavelengths and constructively amplify them. The evolution of certain colors to fit with the rest of the body coloration would be entirely random but patterns could easily form This pattern would assist in camouflage (or sexual selection) and hence make them safer from predators. The pattern formed gradually over many generations. Such change would probably take a few tens of thousands of generations to create this pattern–half a million years or maybe far less. The generation time is probably a year or two. That would create an advantage over those who did not have such coloration in this pattern. Sexual selection would occur in a similar manner. Decoration of the male is often female driven and is a potent force.

The previous comment about interference filters is on target too, although to me this does not quite seem like the same thing in this animal. Other explanations are possible making a good project for further study.

This is a very brief explanation of this process of the appearance of color but would fit. This mechanism is seen in the tapetal reflex of insects (and many vertebrates although the mechanism is different in vertebrates ) as well. The tapetal reflex is the mechanism of insect eyeshine, and explains why moths , for example, have eyeshine (often red). This is a fascinating and most interesting image. The photographer is to be congratulated. Ivan R. Schwab

Ivan Schwab is the author of Evolution’s Witness: How Eyes Evolved, to which I gave an effusive review here.

I’d like to thank everyone who responded - there’s a lot of interesting info here that I’m poring through, not quite explaining how this pattern occurred but fascinating nonetheless.

I had initially ruled out camouflage because the eyes seemed too different from the body, and too uniform to blend into anything, but this may have been skewed perspective from seeing them so close - at a slightly greater distance there may be a “dazzle” camouflage effect. I’ve only seen them the once, in circumstances where they stood out distinctly against their background. I’m also leaning away from sexual selection because both sexes are almost identical, but then again I’m unedumacated and may be missing something.

And I’ll definitely be picking up Dr. Schwab’s book when there’s a little more spare change in pocket - it looks enticing.

Thanks again!

My guess as to the purpose of the differentiation of eye facets is they better detect movement from different directions. Other species are supposed to have a similar pattern and it looks like the pattern of old castles and forts where overlapping fields of fire were found to be the best defense of the structure.

Something that’s an effective camouflage against the eyes of their predators (or some of them) might not be camouflage to human eyes.

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This page contains a single entry by Matt Young published on July 22, 2013 12:00 PM.

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