How Many Eyes Do Spiders Have

Most spiders have eight eyes, but some species have six, four, two, or even no eyes. Even within a single species, the number of eyes may vary, but its always an even number. Key Takeaways About 99% of spiders have eight eyes. Some have six, four, or two. A few species have vestigial eyes or none at all.Spiders have two types of eyes. The large pair of primary eyes forms images. The secondary eyes help the spider track movement and gauge distance.The number and arrangement of spider eyes helps an arachnologist identify the spiders species. Why Spiders Have So Many Eyes A spider needs so many eyes because it cannot twist its cephalothorax (head) to see. Rather, the eyes are fixed in place. In order to hunt and evade predators, spiders need to be able to sense movement all around them. By having eyes spaced around its head, this spider gain an excellent range of vision. Mohd Faridz Azhar / EyeEm / Getty Images Types of Spider Eyes The two main types of eyes are the forward-facing primary eyes called ocelli and the secondary eyes. In other arthropods, the ocelli only detect light direction, but in spiders these eyes form true images. The principal eyes contain muscles that move the retina to focus and track an image. Most spiders have poor visual acuity, but ocelli in jumping spiders exceeds that of dragonflies (insects with the best vision) and approaches that of humans. Due to their placement, the ocelli are also known as antero-media eyes or AME. The secondary eyes are derived from compound eyes, but they dont have facets. They are usually smaller than the primary eyes. These eyes lack muscles and are completely immobile. Most secondary eyes are round, but some are oval or semilunar in shape. The eyes are identified based on placement. The antero-lateral eyes (ALE) are the top row of eyes on the side of the head. The postero-lateral eyes (PLE) are the second row of eyes on the side of the head. The postero-median eyes (PME) are in the middle of the head. Secondary eyes may face forward, or be on the sides, top, or back of the spiders head. The secondary eyes serve a variety of functions. In some cases, the lateral eyes expand the range of the primary eyes, giving the arachnid a wide angle image. The secondary eyes act as motion detectors and provide depth perception information, helping the spider locate the distance as well as direction of prey or threats. In nocturnal species, the eyes have a tapetum lucidum, which reflects light and helps the spider see in dim light. Spiders with a tapetum lucidum show eye-shine when illuminated at night. In some species, all eight eyes are located in front. I love nature / Getty Images Using Spider Eyes for Identification Arachnologists use spider eyes to help classify and identify spiders. Because 99% of spiders have eight eyes and the number of eyes can vary even within members of one species, the arrangement and shape of eyes is often more helpful than the number. Even then, the details of the spiders legs and spinnerets are more useful for identification. Eight Eyes: The day-active jumping spiders (Salticidae), flower spiders (Thomisidae), orb weavers (Araneidae), cobweb weavers (Theridiidae), and wolf spiders (Lycosidae) are common spiders with eight eyes.Six Eyes: Several spider families have species with six eyes. These include the recluse spiders (Sicariidae), the spitting spiders (Scytodidae), and some of the cellar spiders (Pholcidae).Four Eyes: Spiders belonging to the family Symphytognathidae and some spiders in the Nesticidae family have four eyes.Two Eyes: Only spiders belonging to the family Caponiidae have two eyes.Vestigial or No Eyes: Species that live exclusively in caves or underground may lose their sight. These spiders typically belong to families that have six or eight eyes in other habitats. Sources Barth, Friedrich G. (2013). A Spiders World: Senses and Behavior. Springer Science Business Media. ISBN 9783662048993.Deeleman-Reinhold, Christa L. (2001). Forest Spiders of South East Asia: With a Revision of the Sac and Ground Spiders. Brill Publishers. ISBN 978-9004119598.Foelix, Rainer F. (2011). Biology of Spiders (3rd ed.). Oxford University Press. ISBN 978-0-19-973482-5.Jakob, E.M, Long, S.M., Harland, D.P., Jackson, R.R., Ashley Carey, Searles, M.E., Porter, A.H., Canavesi, C., Rolland, J.P. (2018) Lateral eyes direct principal eyes as jumping spiders track objects. Current Biology; 28 (18): R1092 DOI: 10.1016/j.cub.2018.07.065Ruppert, E.E.; Fox, R.S.; Barnes, R.D. (2004). Invertebrate Zoology (7th ed.). Brooks / Cole. ISBN 978-0-03-025982-1.