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Bug-Eyed

aging



J. W. Stewart
EYE
An insect with 100 eyes has left scientists, well, surprised

Parasitic insects known as Strepsiptera are so named for their twisted wings, from the Greek word strepsi, meaning twisted or turned, and ptera, wings. But wings are not their only warped feature, as scientists have found out. Indeed, these creatures, which prey on other insects such as paper wasps, have freaky peepers as well.

EYE
BUG'S-EYE VIEW. Eyelets in Strepsiptera each process a chunk of visual information, as opposed to the single points registered by the facets in compound insect eyes. Individual retinae invert the separate chunks as shown above. In reality, though, there is most likely less overlap between them.

Most bugs sport what are known as compound eyes, made up of hundreds--and sometimes thousands--of lenses that each sample an individual point in the visual field. Strepsiptera, however, have fewer and larger lenses, dubbed eyelets, clustered on either side of their head. Whereas the fruit fly Drosophila melanogaster may have some 700 facets per eye, one Strepsiptera called Xenos peckii has only 50 eyelets. And 15 fruit-fly lenses would cover the same area as one of X. peckii's lenses.

EYE

RECONSTRUCTION of part of the Strepsipteran visual system shows several individual eyelet lenses (yellow), retinae (red) and receptor cells and lamina (blue), as well as the tissue between eyelets (green).

Given such differences, researchers long suspected that eyelets might actually process whole chunks of a visual scene, not just separate points. But they had no proof. How do you view the world through Strepsipteran glasses?

Recently, though, the theory has gained solid support from a series of optical and neuroanatomical studies. Postdoctoral researchers Elke Buschbeck and Birgit Ehmer, along with Professor Ron Hoy of Cornell University measured numerous properties of X. peckii's eyelets and visual centers in the brain, all of which suggest that eyelets are in fact image-forming. Their work, published in the November 4th issue of Science, shows that Strepsiptera eyes may most closely resemble those of trilobites, marine invertebrates that swarmed in the earth's oceans 590 to 410 million years ago.

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FLUORESCENT STAIN highlights individual eyelets inside this scanning electron microscope of X. peckii's eyes.

Buschbeck and company first performed several optical measurements and found that, based on their plane of focus and spatial cutoff frequency, each eyelet should be able to resolve several thousand points in the bug's-eye view. Moreover, the light-gathering power per eyelet was at least 30 times greater than that of the individual facets in traditional compound eyes. Approximations of the eyelet's sensitivity also indicated that they captured enough light so that the 100-odd photoreceptors in their retinae might each resolve a single image point.

microsection
EYELET, shown in this microscope section from an X. peckii pupa, has a thick lens covering its retina.

The neuroanatomical findings were no less significant. The researchers discovered that the projections of each retina's receptor cells formed a nerve that terminated in the lamina, the region where a full image is assembled from the many points sampled by typical compound eyes. And the nerves from neighboring eyelets projected to adjacent areas in the lamina--all of which lends credence to the idea that the visual "chunks" perceived through eyelets are put together there, like puzzle pieces completing an entire scene. Also, the optic neuropils--or networks of nerve cells--comprised 75 percent of X. peckii's brain. Such a large and dense mesh of fibers wouldn't be necessary if each eyelet took in only a single point of the view.

From their findings, Buschbeck, Ehmer and Hoy created a compelling model of Strepsipteran sight. Each eyelet creates a partial--and inverted--image on its retina. This image piece is sent to the lamina so that it sits next to image pieces from nearby eyelets. And the image is reinverted at the lamina, thanks to the fact that the receptor fibers exiting the retinae cross over each other. The advantage of such a scheme may well be that eyelets have greater light-gathering and resolving powers than would be possible if Strepsiptera's tiny eyes were made from more traditional, point-processing facets.

The mystery is not completely solved. The scientists are unsure about the degree of overlap in the images caught by individual eyelets; although their model assumes there is none, it is not inconceivable. And they will continue to ponder why Strepsiptera has evolved with such a structurally and functionally different visual system from other arthropods. One hundred little eyes is no accident.


Images: BIRGIT EHMER and ELKE BUSCHBECK
Scientific American Special Universe

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