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Maddie Lykouretzos '23
Maddie Lykouretzos '23
Spiders’ eyes have evolved extensively and have endured complex changes to form the four pairs of eyes we know them to have today. Spiders owe a lot of these changes to the involvement of light-sensitive proteins called opsins.
The amount of opsin variation between different species of spiders remains critically understudied; however this does not mean that there are large amounts of variation. While opsin differences between species have yet to be fully understood, it is evident that spider eye placement and size plays a crucial role in helping spiders survive. With the exception of a few species that have only three pairs, spiders have four pairs of eyes which are split into four groups: Anterior Lateral, Posterior Lateral, Posterior Median, and Anterior median. For simplicity, the front pair of eyes, Anterior Median, are referred to as the principal eyes, while the eyes located on the sides of the head are the secondary eyes.
The eye placement in each species is reflective of their behavior, depending mainly on their hunting techniques. For example, members of the Araneidae family, which build webs to catch prey, have similar sized eyes spaced evenly around the head. Alternatively, members of the Salticidae family, commonly known as the jumping spider, have a large pair of principal eyes used to hunt and track prey. These principal eyes can actually move without the spider having to turn its head, allowing for more accurate tracking, which is seen in other species as well. The secondary eyes on a spider are more primitive and are responsible for simple tasks such as detecting movement and for providing a larger view of the surrounding area.
The differences between the secondary and principal eyes suggests different evolutionary paths. Unlike principal eyes, secondary eyes are unable to move inside the head and have inverted retinas. In turn, secondary eyes also have a tapetum which aids in night vision , allowing for better sensitivity to light and peripheral vision. This addition to the secondary eyes can easily be identified in animals such as cats where their eyes seem to shine or glow in the dark. The principal eyes, on the other hand, usually have color vision and are for more precise sight related tasks such as tracking and searching for prey.
Image from Molecule Evolution of Spider Vision shows the principal eye vs the secondary eye. Take note of the inverted retinas in the secondary eye and the added tapetum.
Spiders’ eyes have type II opsins, otherwise known as animal opsins which are part of the G protein coupled receptor family. These protein receptors are found in all eukaryotes and serve to detect molecules outside the cell in order to trigger a response from the body. In this case they respond to light. Type II opsins are classified into four groups: ciliary opsins (c-opsins), rhabdomeric opsins (r-opsins), Cniops (not seen in spiders), and Go/RGR opsins. Ciliary opsins attach to rods and cones in the eye, whereas r-opsins are attached to rhabdomeres, which are formed by light sensitive microvilli along the retina. C-opsins are subdivided into rod opsins and cone opsins. Their main function is to convert light into nerve impulses, and especially aid in UV sensitivity when it comes to spiders. Rod opsins include rhodopsin, which is used in night vision and is also found in spiders. There are four cone opsins, each of which correlate with the wavelengths that the animal can see - spiders have only two, allowing them to see green and ultraviolet. Not all opsins that a spider has are directly related to sight though: Melanopsins, a type of r-opsin, are involved in circadian rhythms which control the spider’s ‘internal clock’. They also help to accurately see in high-brightness situations. Spiders also have another non-visual opsin called arthropsin. The role of arthropsin remains largely unknown, but scientists believe that it plays a role in circadian rhythm, similar to melanopsin. Arthropsin may also be involved in spiders’ central nervous systems or possibly olfactory pathways. The fourth group of opsins, Go/RGR opsins, include Go opsins, RGR, peropsins, and neuropsins, all of which are found in spiders and work as visual pigment-like receptors to light.
While many aspects of spider eyes and the opsins behind them remain unknown, the available data shows the remarkable evolution that spider vision has gone through to help suit the unique lifestyle that varies from spider to spider.
Image from Molecule Evolution of Spider Vision shows the eye placement and size differences between species of spider. A few well-known species on the chart include the Salticidae (jumping spider), Lycosidae (Wolf spider), and the Araneidae (Orb-Weaver spider)
Bibliography
Morehouse, Nathan I et al. “Molecular Evolution of Spider Vision: New Opportunities, Familiar Players.” The Biological bulletin vol. 233,1 (2017): 21-38.
Terakita, Akihisa. “The opsins.” Genome biology vol. 6,3 (2005): 213.
Schumann, Isabell et al. “Immunolocalization of Arthropsin in the Onychophoran Euperipatoides rowelli (Peripatopsidae).” Frontiers in neuroanatomy vol. 10 80. 4 Aug. 2016,
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