Eye-popping Fruit Flies

Development into adulthood can cause a number of different biological changes in animals.

Many of us have experienced the distinct and sometimes rather uncomfortable effects of

puberty, however many other animals, such as primates, elephants, and fish, are known to go

through similar changes.

Unlike other animals, stalk-eyed fruit flies (Permatops Drosophila) go through a very unique

developmental change. At the cusp of adulthood, when new adult stalk-eyed fruit flies emerge

from their pupal shells, flies experience outward elongation of their eyes. At the start of

emergence from pupal shells, eye stalks are translucent and very short, remaining close to the

head. About 16 minutes later, flies begin to rub at their eye stalks with their forelegs, promoting

the elongation of the eyes stalks. Researchers observing this change reported how tiny nerves

were able to be seen through the translucent stalks. Stalks grew asymmetrically and fluid was

observed to be filling them as they elongated and straightened. Once eye stalks have fully

straightened, they begin to harden and turn dark. Generally this entire process would take a little

over 2 hours. Yet, there have been reports that in some individuals the process was concluded

in only 50 minutes.

Biologists have found eight different fly families in which eye stalks have evolved. However,

Permatops flies have received so little scientific attention that there are many questions still

needed to be answered about their basic biology. Only recently this year has the process of eye

stalk development been studied. Female stalk-eyed fruit flies were found to have shorter stalks

than males. Males were also observed engaging in combative behaviors. It is thought that

elongated eye stalks evolved as a consequence of sexual selection in which stalks are used to

show off. Males were seen lining up eye stalk to eye stalk, rearing up, flexing, and sticking out

their forelegs. When two males did this, it was found that the larger specimen was the victor.

Researchers did not observe any clashing, knocking, or locking of stalks while in combat. It is

also theorized that these eyes may have evolved to allow them to perceive danger while the rest

of the body is hidden away within greenery.

As stated earlier, there are still many more questions to be answered about stalk-eyed fruit flies.

Questions regarding the purpose of elongated eyes and other reasons why they might have

evolved in this species, the advantages and disadvantages of having these appendages, and

how might eyes function physiologically compared to common fruit fly eyes are all considered.

https://www.sciencenews.org/article/fruit-flies-eyes-head-eyestalks

https://entomologytoday.org/2022/08/12/new-insight-eye-popping-biology-pelmatops-stalk-eyed-

fruit-flies/

Gina Pasquale (9)