Discoveries

April 15, 2024

Four Eyes

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Most spiders have more than two eyes – but not daddy longlegs.

These basement-dwelling arachnids – which are technically not spiders – are known to have only two forward-facing peepers.

However, this wasn’t always the case, the Smithsonian Magazine reported.

Daddy longlegs belong to an arachnid group known as “harvestmen,” which is comprised of more than 6,000 known species. A majority of them have only two eyes.

Recently, researchers studying embryos of Phalangium opilio – a daddy longlegs species – discovered four additional eyes that never fully develop.

They explained that these eyes were vestigial organs, which are remnants of body parts that no longer function and are considered “leftovers of evolution.” In humans, these organs include wisdom teeth and the appendix.

The findings showed that the P. opilio had an eye on each side of its head, including a pair that face forward. Although they don’t develop, the harvestmen could still gain some benefit from them, such as detecting differences in light, the researchers added.

They noted that their study and other research on fossilized daddy longlegs showed that the arachnid species are around 50 million years older than previously thought.

Their lineage tree now dates back more than 537 million years. with the new findings providing scientists additional insight into their mysterious evolution.

This story originally appeared in the April 15, 2024 newsletter, The Wedge Between Two Giants.