Ancient Daddy Longlegs Had Extra Set of Eyes

Harvestmen, familiarly known as daddy longlegs, once had an extra set of eyes, according to an analysis of a 305-million-year-old fossil from France.

Though they share spiders’ eight-legged appearance and penchant for hanging out on bathroom ceilings, harvestmen are more closely related to scorpions, mites, and ticks. The arachnids are as ancient as they are ubiquitous, and can be found on every continent except Antarctica. (Related: “Early Daddy Longlegs Revealed in 3-D—Bugs Evolved Little.”)

A high-resolution x-ray scan of the 305-million-year-old harvestman fossil.
A high-resolution x-ray scan of the 305-million-year-old harvestman fossil (Hastocularis argus).
Photograph courtesy of the National Museum of Natural History, France.

And unlike spiders, which often have eight eyes or more, harvestmen have only a single pair of eyes.

But when scientists at the American Museum of Natural History in New York City and the University of Manchester in England scanned the fossil using sophisticated x-ray techniques, they found the arachnid had four eyes instead of two. (Related: “Biggest Fossil Spider Found.”)

Finding intact fossils of tiny arthropods like harvestmen is no mean feat, as the creatures’ delicate exoskeletons rarely stand the test of time.

Before the sediments surrounding the fossil “compacted into rock, an iron carbonate mineral called siderite grew around it to form a nodule, otherwise known as a concretion,” said study leader Russell Garwood, a paleontologist at the University of Manchester, via email.

“This stopped it being squashed with the rest of the rock. The fossil then rotted away, leaving a three-dimensional void in the rock.”

Eye-Openers

The research also revealed a difference in placement of the eyes. Modern-day harvestmen species have either median eyes (near the middle of the body) or lateral eyes (on the side of the body). (Related: “Photo in the News: Ancient Spider ‘Digitally Dissected.'”)

The harvestman fossil sported both median and lateral eyes.

A daddy longlegs on a fence in southern Ontario, Canada. Photograph by P. Raja, National Geographic YourShot

Genetic analysis further revealed that modern-day harvestmen possess an unused “eyestalk-growing” gene in the location of the missing peepers, suggesting that daddy longlegs gradually lost their extra eyes through evolution. (See “Spiders Evolved Spare Legs.”)

“Arachnid eyes in general are pretty variable, and different groups tend to lose or modify their eyes quite a bit,” said Garwood, whose study was published April 10 in the journal Current Biology.

It’s unknown why harvestmen lost their eyes, but it’s possible that spiders retained more eyes because they rely more on their eyesight for hunting prey. Harvestmen generally scavenge decaying plants and animals on the forest floor.

According to Garwood, the new findings may also help scientists better understand the genetics of eye development in arachnids at large, as well as shed light on other evolutionary mysteries.

“This work shows that by combining evolutionary-developmental biology, fossils, and evolutionary biology, we can explain otherwise enigmatic features in living groups.”

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Stefan Sirucek is a writer and journalist who reports from both sides of the Atlantic. He's written for the Huffington Post and Wall Street Journal. Follow him on Twitter at @sirstefan.