In a new study, researchers reveal how the unique stalk-eyed fruit flies develop, reproduce, and interact with each other—and their work may shed light on eye-stalk evolution among arthropods. Shown here is a male Pelmatops tangliangi stalk-eyed fruit fly. (Photo by John Horstman/itchydogimages)

By Melissa Mayer

Melissa Mayer

If you peer into the right shrubs, the eyeballs that stare back might be some of the most intriguing known to entomology: eye stalks! These specialized eyes perched at the ends of antler-like stems are common among mollusks and crustaceans, but they show up in just eight fly families.

Among these, the stalk-eyed fruit flies, in the subtribe Pelmatopina, comprise eight species, including three in the genus Pelmatops, whose biology was poorly documented until now. In a paper published in July in the Annals of the Entomological Society of America, a team of researchers who have been studying these eye-popping insects in China describe how stalk-eyed fruit flies develop, reproduce, and interact with one another—and these observations could pave the way toward a better understanding of eye stalks as an adaptation.

Meet the Stalk-Eyed Fruit Flies

“What interests me most about this research is to explore the survival mysteries of this fantastic creature,” says Xiaolin Chen, Ph.D., associate professor at the Chinese Academy of Sciences and senior author on the study. “What is the function of the long eye stalk? And how did the long eye stalk originate?”

Between 2017 and 2020, Chen and her team observed two species of stalk-eyed fruit flies (Pelmatops ichneumoneus and Pelmatops tangliangi) in shrubs in the Sichuan and Yunnan provinces and reared specimens in the lab.

The specialized eyes of stalk-eyed fruit flies, perched at the ends of antler-like stems, are common among mollusks and crustaceans, but they show up in just eight fly families. The subtribe Pelmatopina comprises eight species collectively dubbed stalk-eyed fruit flies, including three in the genus Pelmatops. Shown here are a Pelmatops ichneumoneus female (A), P. ichneumoneus male, and P. tangliangi male (C). (Image originally published in Huangfu et al 2022, Annals of the Entomological Society of America)

It turns out these two populations may belong to one species. In the lab, male P. tangliangi and female P. ichneumoneus specimens placed together began to mate. That’s interesting because female P. tangliangi are so elusive that Chen and colleagues say they’ve never seen one. It’s possible that the populations’ differences in eye shape and stalk length could be simple variation, and more morphological and molecular data are necessary to figure that out.

The team reported other interesting mating behaviors, too. Male stalk-eyed fruit flies tend to hang out on shoots or stems where females are likely to look for good spots to lay eggs. The waiting males vibrate their wings, which may be a visual or auditory courtship signal for potential mates.

When multiple males were waiting on the same shoot, they sometimes engaged in combative behavior—like lining up their eye stalks, rearing up, flexing, and sticking out their forelegs. When two P. tangliangi males did this, the larger specimen was the victor, but the authors describe the combat as low-intensity and report no obvious injuries to either fly. Interestingly, interspecies combat between males of the two study species was less common.

These behaviors may help explain why eye stalks evolved in the first place. “[The paper] documents detailed combatting behavior,” says Yu Zeng, Ph.D., researcher at Chapman University and a coauthor on the study. “I think [this] directly supports the idea that these long eye stalks evolved as a consequence of sexual selection. Because they do use these eye stalks to show off.”

Yu Zeng, Ph.D.

Xiaolin Chen, Ph.D.

Filling in the Life Cycle

After mating, female stalk-eyed fruit flies lay eggs in shrub stems, and upon hatching the larvae burrow into the stem to feed. The team noted that the larvae rely on air holes in the stems, and air holes near the top of the stem indicate mature larvae ready to exit the stem and pupate underground.

One of the most fascinating aspects of life with eye stalks happens at the cusp of adulthood. The researchers observed new adults emerging from the pupal shell and noted that their eye stalks were always very short—just 0.3 millimeters long on average—and translucent. About 16 minutes later, the flies commenced rubbing their eye stalks with their forelegs, and those eye stalks began to elongate asymmetrically while the flies wiggled their bodies. The researchers reported they could see the tiny nerves within the translucent eye stalks and watch the stalks fill with body fluids as they elongated and straightened. Once the eye stalks straightened, they began to darken and harden. The entire process took just over 2 hours.

Researchers studying stalk-eyed fruit flies observed that, when new adults emerge from their pupal shell, their eye stalks were short and translucent (A). About 16 minutes later, the flies rub their eye stalks with their forelegs, and the eye stalks begin to elongate asymmetrically while the flies wiggle their bodies (B-G). The researchers reported they could see nerves within the translucent eye stalks and that the stalks fill with body fluids as they elongate and straighten (G). Once the eye stalks straighten, they begin to darken and harden (H, I). The entire process takes just over 2 hours. (Image originally published in Huangfu et al 2022, Annals of the Entomological Society of America)

“I think this paves the ground for a series of future publications on the evolution of these eye stalks, on taxonomy, on the evolution of eye stalks in general among arthropods,” says Zeng.

But the researcher’s connection to this insect goes deeper. Zeng grew up in one of the provinces where stalk-eyed fruit flies are found, and he’s blown away that these creatures were hiding in plain sight in local brambles.

“I never imagined there are these exotic insects in my hometown,” he says. “I’m in my 30s and, for kids about my age growing up in China, we read natural history books or documentaries, and most of them were imported. They would pay less attention to local domestic species. So, this is a good example of the underappreciated endemic biodiversity.”

Melissa Mayer is a freelance science writer based in Portland, Oregon. Email: melissa.j.mayer@gmail.com.