Daddy longlegs look like they have two eyes. That doesn’t count the hidden ones

Two sets of vestigial eyes shed light on the arachnids’ evolutionary history

A macroscopic image of a daddy longlegs spider. Two eyes in the front of the head are clearly visible.

Despite its two-eyed appearance, Phalangium opilio has six peepers. The four extra eyes — leftovers of evolution — shed light on the evolutionary history of daddy longlegs.

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A species of daddy longlegs has been hiding four extra eyes. While the newfound peepers never fully develop, the vestigial organs suggest that this arachnid lineage is about 50 million years older than previously thought, researchers report February 23 in Current Biology.

Unlike spiders, which can have as many as eight eyes, the roughly 6,500 species of daddy longlegs have two eyes at most. But a 2014 study described a fossil of an extinct four-eyed daddy longlegs (SN: 9/19/14).

So developmental biologist Guilherme Gainett, then at the University of Wisconsin–Madison, and colleagues examined vision-related genes and proteins in Phalangium opilio embryos. In addition to the pair of front-facing eyes that the species is born with, certain eye proteins suggest that the embryos have two more pairs: another set of front-facing eyes and a set on the side of the head.

a fluorescent microscope image of a daddy longlegs embryo shows two main eyes in the front of the head (magenta) and two sets of vestigial eyes (green), one pair in the front of the head and one pair on the side.
Daddy longlegs have up to two functional eyes and at least one species has four hidden, underdeveloped ones. In this fluorescent microscope image of a Phalangium opilio embryo, the two working eyes appear magenta and four vestigial eyes look green. G. Gainett

“There’s no lens or any external indication that they are there,” says Gainett, now at Boston Children’s Hospital. But checking for molecular building blocks of vision showed where the arachnid’s ancestors once had eyes.

Another species (Iporangaia pustulosa) also has vestigial eyes on the side of the head, the team found, hinting that all living daddy longlegs might have them. By examining where the trait pops up throughout daddy longlegs’ evolutionary history, the team estimates that the group’s last common ancestor lived roughly 537 million years ago, predating a previous estimate by about 50 million years.

These arachnids may use the underdeveloped eyes to maintain circadian rhythms or detect changes in light intensity, Gainett says. “It just opens a door for investigating the function of these leftovers of evolution.”

McKenzie Prillaman was the Spring 2023 science writing intern at Science News. She holds a bachelor’s degree in neuroscience with a minor in bioethics from the University of Virginia and a master’s degree in science communication from the University of California, Santa Cruz.

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