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Trilobites

Daddy Longlegs Have Been Hiding Extra Eyes From Us

The arachnids, which are not spiders, were thought to have only two
eyes, compared with many more on spiders.

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A close-up view of a brownish daddy longlegs spider on a foxtail.
Some of the oldest known fossils of harvestmen, or daddy longlegs,
seem to have two full pairs of eyes; modern organisms have only ever
been known to have one.Credit...Roman Chukanov/Alamy

By Veronique Greenwood

March 1, 2024Updated 1:08 p.m. ET

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 splendidly leggy arachnids also known as
harvestmen, have been thought to have just two eyes. But there on the
animal's body, illuminated with fluorescent markers, were what looked
like four more vestigial eyes.

In a paper published last week in the journal Current Biology, Dr.
Gainett, now at Boston Children's Hospital, and his co-authors report
that they believe they have discovered remnants in the harvestman
species Phalangium opilio of what may have once been fully functional
eyes in the arachnids' ancestors. Though these vestigial eyes don't
mature fully, they appear on the harvestmen's bodies as they develop,
shaped by many of the same genes as the creatures' true eyes.

Arachnids include spiders, scorpions, harvestmen and other
arthropods, and divining the relationships among this sprawling group
of organisms is tricky. To do so, researchers must draw on both
genetic information from modern arachnids and fossils of those that
have long since vanished. Interpretations of the relationships among
arachnids can vary widely: Dr. Gainett and his graduate school
adviser; Prashant Sharma, a biologist at the University of Wisconsin;
and their colleagues courted taxonomic controversy in 2022 when they
published a family tree that put horseshoe crabs, previously thought
not to be arachnids, in the group.

For the current study, Dr. Gainett used fluorescent tags to study the
development of harvestman eyes. The tags were designed to stick to
opsins, light-sensitive proteins that exist in eyes across the animal
kingdom. Looking at the shapes traced by the tags, he matched the
locations of the unexpected opsins on the harvestmen to approximately
where extra eyes grow on spiders and horseshoe crabs. (Spiders
typically have eight eyes, and horseshoe crabs have 10.) These
findings suggest that the neural architecture that handles the daddy
longlegs' vision may be quite old. The results could also help in
sketching out the family tree of arachnids.

The researchers believe that this new finding helps explain some
mysteries about daddy longlegs. Some of the oldest known fossils of
harvestmen, in deposits of quartz, seem to have two full pairs of
eyes; modern organisms have only ever been known to have one. The
discovery of these vestigial eyes may help link the fossils, which
are thought to be more than 400 million years old, more directly with
their descendants, clarifying their place on the arachnid family
tree.

The study, Dr. Sharma said, "suggests they have changed very little
for a very long time."

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