ITHACA, N.Y. -- An unusual type of eye -- resembling a tiny raspberry andpossibly following a design principle that vanished with the extinction oftrilobites hundreds of millions of years ago -- lives today in a parasiticinsect, Cornell University biologists report in the Nov. 5 issue of thejournal "Science".
The compound eyes of most insects have many hundreds of lens facets, eachsampling only one small point in the insect's visual field, but thecomposite lens eyes of strepsipteran insects have no more than 50 facets.
Fewer facets does not mean poorer vision, the Cornell biologists believe.The strepsipteran lenses are larger, and each has about 100 receptors,forming an individual retina behind each lens. According to theinvestigators, this kind of eye is well equipped to sample not points but"chunks" of the visual field, greatly improving the visual capabilities ofthese strange insects.
"No other insect that we know of has eyes quite like this," said Ron Hoy,professor of neurobiology and behavior at Cornell and co-author, withCornell postdoctoral associates Elke Buschbeck and Birgit Ehmer, of the"Science" report. "The only place one may see a comparable eye structureis in the fossils of some kinds of trilobites," he says, referring to theextinct arthropods that lived in shallow seas during the Paleozoic era.
Strepsipteran insects, such as the "Xenos peckii" species studied by theCornell biologists, don't dwell in water, but they do have short, secretivelives. The seldom-seen parasites are hidden in the bodies of common paperwasps (See fact sheet, "Looking at an Odd-ball Insect," attached). Female"X. peckii" never leave their wasp hosts, and when males do, they are on aspecific, hurried mission: In the approximately two hours before they die,the males have to find another wasp that is parasitized by a female "X.peckii", mate with the female and depart. "Sex pheromones from femalesprobably help males to locate the general neighborhood of a wasp with afemale parasite," Ehmer
says, "but the male presumably relies on his vision once he is close to thewasp." She said that the importance to the insect of the visual systemalso is apparent from the volume of optic lobes dedicated to processingvisual information, which Ehmer estimates to be 75 percent of the insect'sbrain.
Compared with a fruit fly ("Drosophila melanogaster) "with more than 700facets in each compound eye, "X. peckii" has only about 50 facets per eye.Each of the "X. peckii "facets, referred to as "eyelets" by the biologists,is about 65 microns (65 millionths of a meter) in diameter and covers aboutthe same area as 15 of the smaller fruit fly lenses. The fruit fly eye hasonly eight photoreceptors per facet, with each facet contributing to onesample point.
"The need for a tiny insect eye to gather lots of light and bring imagesinto sharp focus on the receptors may explain the advantage of the eyesthat deliver images in chunks instead of points," Buschbeck says. "Thiscomposite lens arrangement allows the insect to have many morephotoreceptors in a given area than would be possible with a compound eye.If you only have so much space on your head for eyes and you want to gatherthe most light, you want a composite lens eye," she says.
"The larger lenses of the strepsipteran insects are similar to a large lensof a camera," Buschbeck explained. "Large insect lenses admit more light,support more photoreceptors and permit higher resolution. Anotherimportant condition for vision is that the image is focused on the retina(just as the camera lens image should be focused on the film) and we haveconfirmed this with measurements made through the insect's lens."
However, an insect viewing the world in fewer but larger chunks of thevisual field would seem to have an inverted, mirror-image problem. Likeany simple lens, each eyelet inverts or reverses its individual portion ofthe overall image. Without some means of correction, the parasitic insecttrying to read the first three words of this sentence and seeing each wordwith one of its eyelets might instead read "tuohtiW emos snaem."
The correction comes about, the Cornell biologists believe, because ofchiasmata, which are X-shaped nerve crossings named for the Greek letterchi. The biologists found that behind each of the eyelets is a nerve thatconnects that eyelet to the brain. The nerve exhibits a chiasma, rotatingthe nerve 180f around its own axis and re-inverting each portion of theimage.
"We're certainly not claiming that this insect descended from thetrilobites or anything like that," Hoy commented. "What strepsipteraninsects and trilobites have in common is that they are arthropods -- but soare lobsters and crabs and all the other insects -- and that the structuresof their eyes appear analogous -- at least from what one can tell from thebody surface of ancient fossils."
Research resulting in the "Science" article, "Chunk Versus Point Sampling:Visual imaging in a Small Insect," was supported by grants from the U.S.National Institutes of Health and from the Deutsche Forschungsgemeinschaft.
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