Ancient daddy longlegs had an extra set of eyes
Hastocularis-argus
Image: National Museum of Natural History, France

Although daddy longlegs look a lot like spiders, they're actually more closely related to scorpions, mites and ticks. Unlike spiders with their famous eight peepers, they only have two eyes.

But that hasn't always been the case.

Scientists have used X-rays to scan an ancient fossil and found that 305 million years ago the daddy longlegs (or harvestman as it's really called) had four eyes. An image of one of the X-ray scans is shown above, taken by scientists at the American Museum of Natural History and the University of Manchester in the UK.

Their results have been published in Current Biology and also reveal that the four eyes were placed in an interesting way - in modern daddy longlegs, the eyes are either in the middle of the body or on the side of the body. But the ancient arachnid appears to have had both median and lateral eyes.

Further genetic analysis revealed that today's daddy longlegs have an unused "eyestalk-growing" gene in the location of their missing pair, suggesting they lost their third and fourth eye through evolution, National Geographic reports.

The results of this work may provide insight into how arachnid eyes have developed. And, it's also just really cool to imagine a four-eyed daddy longlegs running around 305 million years ago

The researchers were fortunate to find the fossil in the first place, as most arthropods' exoskeletons decompose over the years. This lucky specimen was surrounded in sediment that was compacted into rock with an iron carbonate mineral around it.