Extreme preservation gives fly’s eye view

The eyes had it, even 45 million years ago.

Two flies stuck in Baltic amber still have enough soft tissue to confirm predictions that their kind had already evolved a fancy, open array of photoreceptors, according to a paper published online December 16 in Proceedings of the Royal Society B.

Preserved, ancient eyes have turned up before, but “usually you don’t get the internal parts,” says Andrew Parker of the Natural History Museum in London. The bulging red eyes on these flies have what Parker says could be the oldest retinas yet examined.

And possibly best of all, “this time we could take it apart,” Parker says. An intriguing fly eye he wrote about in 1998 had a fascinating outer surface, but the keepers of the specimen were not open to any cutting to see the inside.

The new specimens belonged to a family of long-legged, or dolichopodid, flies, whose members are buzzing around today.

Parker, Gengo Tanaka, now at Gunma Museum of Natural History in Japan, and their colleagues were able to study details only a few micrometers wide of the inner structure of the flies’ eyes. Like their modern cousins, the flies had compound eyes made up of myriad lens and photoreceptor units.

Amber has preserved the fly eyes so well that researchers could see the arrangement of the rhabdomeres, the pigment-rich photoreceptors in cells that catch light coming through each lens.

Like modern long-legged flies, these preserved flies had little clusters of individual cells bearing rhabdomeres, dotting the tissue like asterisks.

That arrangement differs from photoreceptors in many other kinds of insects today, which have rhabdomeres fused into one structure that zings the light captured by the cells down a single waveguide pathway. Separating the cells improves the absolute sensitivity of the eye to light.

Biologists studying family trees have calculated that open rhabdomeres evolved independently in at least four insect groups. The preserved flies now show that the open array structure had already reached much of its modern form by 45 million years ago, Parker says.

“The most important aspect of this paper is the description of a fossilized visual system with unprecedented detail, down to the cellular level,” says Todd Oakley of the University of California, Santa Barbara, who studies the evolution of vision.

The eyes of the ancient long-legged flies, it turns out, also had an anti-reflection grating covering the outer surfaces, Parker says. It’s a version of the grating Parker has written about before, one that has since inspired an efficiency-boosting covering for solar panels.

Parker and his colleagues push their speculation into even more detail — into the molecular realm. Geneticists have now identified a single Drosophila gene, nicknamed spam for spacemaker, that encodes a protein that keeps rhabdomeres from fusing. So Tanaka and his colleagues contend that the Baltic amber specimens suggest spam itself has been around for 45 million years.

That’s going a bit far considering how much we still have to learn about spam, says Daniel Osorio of the University of Sussex in Brighton, England. What he would like to hear speculation about, though, is what the open rhabdomeres suggest about the lifestyle of the two flies in amber.