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Index of Subjects Many thanks again Steve. I need time to thoroughly take this in and further investigate what I do not understand. The Insects are an amazing lot! Nancy Sent from my iPhone On 2013-07-28, at 1:29 PM, "Stephen R. Shaw" <srshaw@DAL.CA> wrote: > Hi Nancy, > The basic structure of compound eyes is pretty similar from the trilobites on forward, emphasizing the continuous evolutionary history, in which the eye is 'modular' with a repeated structure under each facet/lens. Some flies have >20,000 such modules (called ommatidia = 'little eyes'), the genetically famous vinegar fly Drosophila has ~750, and in one sort of parasitic bat fly (nycteribiid) there are ~3->0. You can think of it as a flexible way to make a wide range of eye sizes by prolonging or restricting developmental waves that model the structures out of initially undifferentiated cells. So these basic structures are responsible for movement detection, as you say, which is the current focus of some fancy work at a Howard Hughes outfit in Virginia, to nail down the mechanisms; but it's also responsible for colour vision, polarized light detection, some form vision, etc. There's some regional specialization, for example colour vision is sometimes different in the downwards-looking from the upwards-looking parts of the eye (dragonflies, drone bees), and polarized light detection is usually on the top, but not always. > > An advantage of this type of eye is that the lens of each facet in a bee or fly is very powerful, up with a 100X microscope objective lens. This gives a very short focus so another small insect can be seen/recognized in focus a very close range (~1 millimeter) whereas humans are stuck with a minimum focal distance of at least ~10 cm, even in young people. Another advantage is that some of the eyes are much 'faster' than ours -- they can still see objects even when making fast aerial flight turns that some execute, though this is a function of the nerve physiology not the optics. > > The big disadvantage, better known, is that the facets are tiny so that the resolution theoretically possible is very low, because of diffraction of light (the smaller the lens diameter, the worse the diffraction problem, which, crudely put, spreads out the light, defocussing images). In some crustaceans and day flying insects like butterflies, this has been partly overcome by the basic structure having become modified to form what is in effect a vertebrate-like, single lens eye (evolution again). That's where the retro-reflector square facets come in, as one solution to making a single image inside the eye (the powerful single facet lens can't then be used, so the front of facet is now flat, not dome-like). The other solution (some beetles, discovered in fireflies 100+ years ago) is to use a graded refractive index lens structure to achieve the same single image. This has only recently been perfected by humans in the form of parabolically graded-index, low dispersion glass fiber optics now used preferentially for fast communications instead of copper wire. > So insects and crustaceans developed at least two 'modern' optical tricks 100s of million years before we did, in overcoming the intrinsic diffraction limit imposed by the modular construction of compound eyes. (Maybe more than you wanted to know.) > Steve > ........................................... > Quoting nancy dowd <nancypdowd@gmail.com>: >> Hi Steve >> >> Thank you for this info. >> >> I take it the hexagonal facets in the lower eye are mainly useful to >> detect movement as in other insects? I read something about Mayflies >> having a dual function eye but it said nothing more so your >> explanation really helps. >> >> Nancy > .......................................... >> On Sat, Jul 27, 2013 at 7:43 PM, Stephen R. Shaw <srshaw@dal.ca> wrote: >>> Your pic is nice but not high enough magnification to see the detailed >>> structure of the eyes, even more alien-looking than you may think. >>> >>> At least in some male mayflies, the eye is conspicuously bi-lobed, with the >>> upper half optically quite different than the lower half and probably >>> specialized for spotting females (this has been studied in other insects, >>> mostly flies). >>> >>> The facets in the upper eye in some mayflies are square, not hexagonal as >>> they are in the lenses of the lower eye. Square facets in other arthropods >>> (crustaceans) have reflecting layers inside, lining their square optical >>> tubes, and behave like 2-Dimensional retroreflectors to focus light usefully >>> inside the eye into a single image. The mayfly's upper eye therefore >>> possibly may work similarly. The biological structures are somewhat >>> analogous to the retroreflectors left on the moon by astronauts. The moon >>> ones have been used to reflect back laser pulses aimed from earth, allowing >>> for instance the earth-moon distance to be monitored sensitively. >>> Steve >>> >>> Quoting nancy dowd <nancypdowd@gmail.com>: >>> >>>> I guess I never really looked closely enough at a mayfly to notice the >>>> prominent (almost alien looking) eyes: >>>> http://www.flickr.com/photos/92981528@N08/9367703219/ >>>> >>>> This one has its tail filaments snagged in a spider web. I freed it >>>> and it flew off. Any ID corrections are welcome. >>>> >>>> Nancy > >
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