Papa Longlegs has hidden extra eyes from us

Guilherme Gainett, then a biologist at the University of Wisconsin-Madison, was looking through a microscope at the embryo of a daddy longlegs when he saw it – or rather, saw them. Daddy Longlegs, the group of beautifully long-legged arachnids also called harvestmen, are thought to have only two eyes. But on the animal’s body, illuminated with fluorescent markers, were four rudimentary eyes.

In a article published last week in the journal Current BiologyDr. Gainett, now at Boston Children’s Hospital, and his co-authors report that they believe they have discovered remains in the harvestman species Phalangium opilio of what may once have been fully functional eyes in the arachnids’ ancestors. Although these rudimentary eyes do not fully mature, they appear on the harvestmen’s bodies as they develop, formed by many of the same genes as the creatures’ true eyes.

Arachnids include spiders, scorpions, harvestmen and other arthropods, and it is difficult to predict the relationships among this vast group of organisms. To do this, researchers must draw on both genetic information from modern arachnids and fossils from those that are long gone. Interpretations of the relationships between arachnids can vary widely: Dr. Gainett and his graduate school advisor; Prashant Sharma, a biologist at the University of Wisconsin; and their colleagues stirred taxonomic controversy in 2022 when they published a family tree that placed horseshoe crabs, previously thought not to be arachnids, into the group.

For the current study, Dr. Gainett fluorescent tags to study the development of harvestman eyes. The tags are designed to latch onto opsins, light-sensitive proteins found in the eyes throughout the animal kingdom. Looking at the shapes seen on the tags, he matched the locations of the unexpected attacks on the harvestmen approximately to where extra eyes grow on spiders and horseshoe crabs. (Spiders typically have eight eyes, while horseshoe crabs have 10.) These findings suggest that the neural architecture that controls vision in the daddy longlegs may be quite ancient. The results could also help outline the family tree of arachnids.

The researchers believe this new finding helps explain some mysteries about daddy longlegs. Some of the oldest known fossils of harvestmen, in quartz deposits, appear to have two complete pairs of eyes; Modern organisms are only known to have one. The discovery of these vestigial eyes could help link the fossils, believed to be more than 400 million years old, more directly to their descendants, clarifying their place in the arachnid family tree.

The study, said Dr. Sharma, “suggests that they have changed very little over a very long time.”

The researchers also found that a variety of genes related to vision were involved in the formation of these rudimentary eyes, two of which are located just above the joints of the harvestman’s legs.

“Can they see with their shoulders?” Dr. Gainett asked. Probably not in the way most people would imagine: there are no lenses attached to these areas to focus the light. But perhaps these pieces of tissue are still sensitive to light, the researchers say. They could potentially help detect the difference between darkness and light, although behavioral studies are needed to explore that possibility.

The findings highlight the serendipity that sometimes comes from scientific discoveries: Dr. Gainett and Dr. Sharma had hoped to understand the process by which arachnids that do not need multiple pairs of eyes might give them up over evolutionary time.

“We started by saying, let’s look at how these animals lost their eyes,” said Dr. Sharma. “And they haven’t – they actually have more than we thought.”