Daddy Longlegs Have Four Extra, Hidden Eyes, Researchers Say

The new discovery could help scientists unravel the mystery of how the arachnids evolved across some 537 million years

Close-up of insect with very long legs
While spiders can have up to eight eyes, daddy longlegs, which belong to a different order of arachnids called harvestmen, usually have just two eyes. Mathias Krumbholz via Wikimedia Commons under CC BY-SA 3.0 DEED

A species of daddy longlegs has the remnants of four extra eyes that may offer new insights into the creatures’ evolution, scientists reported last month in the journal Current Biology.

Contrary to popular belief, daddy longlegs are not spiders, but rather, they’re part of a group of arachnids known as harvestmen. While spiders can have up to eight eyes, the estimated 6,500 species of daddy longlegs usually have just two. But while looking through a microscope at an embryo of Phalangium opilio—a daddy longlegs species—scientists recently discovered four additional eyes that never fully develop.

The eyes are vestigial organs, or the remnants of body parts that no longer function—they are the “leftovers of evolution,” as study co-author Guilherme Gainett, who was a biologist at the University of Wisconsin-Madison when he conducted the research but now works at Boston Children’s Hospital, tells Science News’ McKenzie Prillaman. In humans, vestigial organs include wisdom teeth and the appendix.

The presence of vestigial eyes in daddy longlegs indicates the creatures are roughly 50 million years older than previously thought, according to the paper. Now, scientists suggest their family tree dates back to at least 537 million years ago, per Science News.

Image of daddy longlegs head under a microscope
The magenta areas show where the daddy longlegs species P. opilio's two functioning eyes form, while the green areas show where the vestigial eyes used to be located. Guilherme Gainett

By just looking at daddy longlegs, scientists could only see their two forward-facing peepers. Initially, they wondered why these harvestmen have just two eyes, while their spider relatives have more.

“We started out by saying, ‘let’s see how these animals lost their eyes,’” study co-author Prashant Sharma, a biologist at the University of Wisconsin-Madison, tells the New York Times’ Veronique Greenwood. The surprising answer was that “they haven’t—they actually have more than we thought.”

When the team began to search for vision-related proteins called opsins in the embryos under a microscope, they spotted the hidden organs. The evidence suggests P. opilio once had an eye on each side of its head, atop its frontmost pair of legs, as well as an additional pair of eyes that faced forward.

Though the four vestigial eyes do not fully develop today, the creatures might still get some use out of them. For instance, the organs might help daddy longlegs detect differences in light, which may influence their circadian rhythms.

The hidden pairs of eyes weren’t a total surprise to the researchers. In 2014, a 305-million-year-old fossilized daddy longlegs found in eastern France had four total eyes—two more than today’s living daddy longlegs. Together, these findings may one day help scientists piece together the mysterious evolutionary lineage of the arachnids.

“Terrestrial arthropods like harvestmen have a sparse fossil record, because their exoskeletons don’t preserve well,” said Sharma, who also worked on the 2014 paper describing the earlier fossil, in a statement at the time. “As a result, some fundamental questions in the evolutionary history of these organisms remain unresolved.”

The researchers supported their new findings with an analysis of genes linked to arachnid vision, which appeared to play a role in shaping daddy longlegs’ additional eyes. The team also discovered that another daddy longlegs species, Iporangaia pustulosa, has vestigial eyes as well, but only on the sides of its head.

It’s also not clear why some daddy longlegs’ eyes have evolved to stop developing—but that’s likely fodder for future studies.

“One of the goals of studying the genetics behind what we see in morphology is really understanding how evolution happens,” Gainett says in a statement. “What do you need to change in the underlying recipe of genes for the diversity of eyes to evolve?”

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