"Daddy Longlegs: More Than Meets the Eye"

In a startling discovery, researchers have found that a certain species of daddy longlegs harbors four additional eyes that never fully evolve, hypothesizing that this arachnid lineage might be around 50 million years older than previously thought. This finding was reported in Current Biology on February 23.

Daddy longlegs, unlike spiders that can possess up to eight eyes, typically have two eyes at the maximum. This generalization among the approximately 6,500 species of daddy longlegs was challenged by a 2014 study that documented a fossil of a now-extinct species of daddy longlegs with four eyes.

Based on these findings, developmental biologist Guilherme Gainett, who was then at the University of Wisconsin–Madison, in collaboration with his team, carried out an investigation of the vision-related genes and proteins in embryos of the Phalangium opilio species. They discovered that along with the usual pair of front-facing eyes the species is born with, there were indications of another two pairs of eyes in the embryos, revealed through certain eye proteins. These additional sets included a pair of front-facing eyes and another pair on the species' head's side.

“There’s absence of any apparent lens or other external signs indicating their existence,” says Gainett, presently at the Boston Children’s Hospital. However, investigation of the molecular building blocks integral to vision unveiled the locations where the eyes of the arachnid's ancestors presumably existed.

The team also found that the Iporangaia pustulosa species possesses these vestigial eyes on the side of its head, which could suggest that all existing daddy longlegs might have these extra eyes. The researchers arrived at an estimate that the last common ancestor of the daddy longlegs group lived around 537 million years ago, which is about 50 million years earlier than previous estimates, by examining how widespread this trait was in the evolution history of daddy longlegs.

Gainett hypothesizes that these arachnids perhaps use these underdeveloped eyes for maintaining circadian rhythms or tracking changes in light intensity. He comments, “It opens a new dimension for studying the role of these evolutional remainders.”