You might know those round-bodied, eight-legged creatures that make their homes in your basement as daddy longlegs or harvestmen. They’re related to spiders, scorpions, and other arthropods like horseshoe crabs but belong to an order known as Opiliones. The relationship between these various groups is constant ground for scientific debate given the differences in appearances and evolutionary adaptations. For example, spiders have eight eyes, horseshoe crabs have ten, and, daddy longlegs only have two. Or do they?
Researchers were surprised when looking at a daddy longlegs embryo under a microscope to discover what appeared to be two more sets of eyes. The discovery came about because researchers were using a process called fluorescent tagging, where a specific fluorescent protein is used to mark the location of other compounds, in this case opsins, a type of light-sensitive protein that exists in the eyes of many different species. As the embryos grew, researchers realized that the newly discovered eyes never fully developed. These four eyes appear to be vestigial remnants of eyes that one of their ancestors must have had and used, a fact bolstered by the fact that the oldest daddy longlegs fossils have 4 eyes. Interestingly, though, researchers were able to map the eye locations of contemporary daddy longlegs and compare those locations to some of their distant relatives like spiders and horseshoe crabs.
The result demonstrates that the vestigial eyes were about where these other arachnids have their extra pairs of eyes. This suggests that daddy longlegs and other arachnids have a direct, common ancestor, though it must be very ancient since the oldest daddy longlegs fossils are 400 million years old!
