The eyes of spiders vary significantly in their structure, arrangement, and function. They usually have eight, each being a simple eye with a single

lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements'' ...

rather than multiple units as in the

compound eye A compound eye is a visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidia, which are tiny independent photoreception units that consist of a cornea, lens, and photoreceptor cells which dis ...

s of insects. The specific arrangement and structure of the eyes is one of the features used in the identification and classification of different species, genera, and families. Most

haplogyne The Haplogynae or haplogynes are one of the two main groups into which araneomorph spiders have traditionally been divided, the other being the Entelegynae. Morphological phylogenetic studies suggested that the Haplogynae formed a clade; more re ...

s have six eyes, although some have eight (

Plectreuridae Plectreuridae, also called plectreurid spiders, is a small spider family confined to the Southwestern United States, Mexico, and the Caribbean. Only two living genera are known—the nominate genus ''Plectreurys'' and ''Kibramoa''. In the past, ...

), four (e.g., ''

Tetrablemma ''Tetrablemma'' is a widespread genus of armored spiders first described by Octavius Pickard-Cambridge in 1873. It only has four eyes; an unusual trait for spiders, found only here and in some species of ''Caponiidae'', though the two are not cl ...

'') or even two (most

Caponiidae Caponiidae is a family of ecribellate haplogyne spiders that are unusual in a number of ways. They differ from other spiders in lacking book lungs and having the posterior median spinnerets anteriorly displaced to form a transverse row with the ...

). In some cave species, there are no eyes at all (e.g.

Stalita taenaria ''Stalita taenaria'' is an araneomorph spider species in the family Dysderidae. The species is classified as a member of troglofauna, more precisely a troglobiont species, meaning such spiders are obligate cave-dwellers adapted to living in dark ...

). Sometimes one pair of eyes is better developed than the rest. Several families of hunting spiders, such as

jumping spider Jumping spiders are a group of spiders that constitute the family Salticidae. As of 2019, this family contained over 600 described genera and over 6,000 described species, making it the largest family of spiders at 13% of all species. Jumping spi ...

s and

wolf spider Wolf spiders are members of the family Lycosidae (). They are robust and agile hunters with excellent eyesight. They live mostly in solitude, hunt alone, and do not spin webs. Some are opportunistic hunters, pouncing upon prey as they find it or ...

s, have fair to excellent vision. The main pair of eyes in jumping spiders even sees in colour.

Structure and anatomy

Spiders' eyes are

simple eyes A simple eye (sometimes called a pigment pit) refers to a form of eye or an optical arrangement composed of a single lens and without an elaborate retina such as occurs in most vertebrates. In this sense "simple eye" is distinct from a multi-le ...

, or ''ocelli'' (singular ''ocellus''), meaning their eyes have a single cuticular lens above a simple

retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...

. The retina is concave and composed of visual and pigment cells, which lie beneath a cellular

vitreous body The vitreous body (''vitreous'' meaning "glass-like"; , ) is the clear gel that fills the space between the lens and the retina of the eyeball (the vitreous chamber) in humans and other vertebrates. It is often referred to as the vitreous humo ...

Categorisation

Most spiders have eight eyes, which tend to be arranged into two rows of four eyes on the head region. The eyes can be categorised by their location and are divided into the anterior median eyes (AME), anterior lateral eyes (ALE), posterior median eyes (PME), and posterior lateral eyes (PLE). The exact arrangement of the eyes varies significantly by family, and to a lesser degree genus, so is often used as a diagnostic feature to identify or categorise spiders, especially in the field. They can also be categorised by structure into the ''principal eyes'' (or ''main eyes''), which are always the AME, and the ''secondary eyes'', which are always the PME, ALE, and PLE. In six-eyed species, it is always the principal eyes which are absent. The secondary eyes normally have a light-reflecting layer, the tapetum, that makes the eyes appear pale. The tapetum differs considerably between spider families but can be separated into three main types: ''PT'' (primitive type), ''CT'' (canoe type), and ''GT'' (grate type). Some spiders, such as jumping spiders, have no tapetum in their secondary eyes.

Principal eyes

In most species, the principal eyes are more visually acute than the secondary eyes, at the cost of their sensitivity. They lack a tapetum entirely, and the retina is everted, meaning the ''rhabdomeres'' (light-sensitive parts of the visual cells) point towards the incoming light. Some species can move the retina using between one and six muscles, greatly increasing the

field of view The field of view (FoV) is the extent of the observable world that is seen at any given moment. In the case of optical instruments or sensors it is a solid angle through which a detector is sensitive to electromagnetic radiation. Humans a ...

. No actual focusing takes place; it is unnecessary, since the small lenses and short focal length of most spiders' principal eyes lead to a wide

depth of field The depth of field (DOF) is the distance between the nearest and the furthest objects that are in acceptably sharp focus in an image captured with a camera. Factors affecting depth of field For cameras that can only focus on one object di ...

. When a spider has colour vision, it is typically the principal eyes which are responsible.

Secondary eyes

The secondary eyes have an inverted retina, meaning the rhabdomeres face away from the incoming light. They also typically have a tapetum made up of crystals, which are likely guanine. Because incoming light passes through the rhabdomeres both before and after being reflected by the tapetum, the sensitivity of the eyes is essentially doubled. The exact structure of the tapetum varies significantly between families and genera, so is sometimes used as a diagnostic feature. Despite this variation, it can be divided into three main groups: # The primitive-type tapetum fills the entire eye cup with holes only for the nerves; it is found mainly in the

Mesothelae The Mesothelae are a suborder of spiders (order Araneae) that includes a single extant family, Liphistiidae, and a number of extinct families. This suborder is thought to form the sister group to all other living spiders, and to retain ancestral ...

, and

Mygalomorph The Mygalomorphae, or mygalomorphs, are an infraorder of spiders, and comprise one of three major groups of living spiders with over 3000 species, found on all continents except Antarctica. Many members are known as trapdoor spiders due to the ...

(the more 'primitive') spiders. # The canoe-type tapetum is formed by two walls divided by a median gap, where the nerves exit; it is found in many families, including

Theridiidae Theridiidae, also known as the tangle-web spiders, cobweb spiders and comb-footed spiders, is a large family of Araneomorphae, araneomorph spiders first described by Carl Jakob Sundevall in 1833. This diverse, globally distributed family includes ...

Clubionidae The sac spiders of the family Clubionidae have a very confusing taxonomic history. Once, this family was a large catch-all taxon for a disparate collection of spiders, similar only in that they had eight eyes arranged in two rows and conical anter ...

, and

Amaurobiidae ''Amaurobiidae'' is a family of three-clawed cribellate or ecribellate spiders found in crevices and hollows or under stones where they build retreats, and are often collected in pitfall traps. Unlidded burrows are sometimes quite obvious in cru ...

. # The grate-type tapetum is the most efficient; the long vitreous body and spherical lens combine to produce a sharp, in focus image. It forms rows of crystals which lie beneath the visual cells, and the nerves exit through the gaps in the 'grate'. This type is found primarily in hunting spiders such as

s and

fishing spider ''Dolomedes'' is a genus of large spiders of the family Pisauridae. They are also known as fishing spiders, raft spiders, dock spiders or wharf spiders. Almost all ''Dolomedes'' species are semiaquatic, with the exception of the tree-dwell ...

Function

Most spiders' eyes can detect little more than brightness and motion, so vision plays only a minor role in behaviour. However some species, such as jumping spiders,

s, and deinopids, have more developed eyes which they use in hunting and courtship. In species which can perceive colour, it is typically the principal eyes with colour perception.

Detection of polarised light

All spiders possess photosensitive microvilli which have the capacity to be sensitive to the polarisation of light, as they preferably absorb light oscillating parallel to their long axis. These microvilli are arranged orthogonally in the eyes of several families, including

s, and

nursery web spider Nursery web spiders (Pisauridae) is a family of araneomorph spiders first described by Eugène Simon in 1890. They resemble wolf spiders (Lycosidae) except for several key differences. Wolf spiders have two very prominent eyes in addition to the o ...

s. This is significant, because this orthogonal arrangement is also present in the 'dorsal rim area' used by some insects to detect the polarisation of light. Additionally, many spider families' secondary eyes possess polarising tapeta which may enhance the perception of polarised light. Despite this fact only one species, ''

Drassodes cupreus ''Drassodes cupreus'' is a species of spider in the genus ''Drassodes'', family Gnaphosidae. A ground-living nocturnal hunter, it spends the day in a silken retreat. Description Females are 10–18 mm, males 10–13 mm. The carapace (t ...

'', has been observed to detect polarised light with their secondary eyes. Agelenids and lycosids have been shown to detect polarised light with the principal eyes. The

tarantula Tarantulas comprise a group of large and often hairy spiders of the family Theraphosidae. , 1,040 species have been identified, with 156 genera. The term "tarantula" is usually used to describe members of the family Theraphosidae, although ...

species

Aphonopelma hentzi The Texas brown tarantula, also known as Oklahoma brown tarantula or Missouri tarantula (''Aphonopelma hentzi''), is one of the most common species of tarantula living in the Southern United States today. Texas brown tarantulas can grow to leg ...

has also been shown to make use of polarised light in navigation.

Evolution

The evolution of spiders' eyes has received little attention, and as such not much is known on the subject. The principal and secondary eyes likely evolved separately, with the principal eyes being homogeneous to the ocelli of insects but the secondary eyes being derived from compound eyes. Since the secondary eyes lack the remnants of the borders between the

ommatidia The compound eyes of arthropods like insects, crustaceans and millipedes are composed of units called ommatidia (singular: ommatidium). An ommatidium contains a cluster of photoreceptor cells surrounded by support cells and pigment cells. T ...

typically present in eyes evolved through fusion of the ommatidia, it is more likely that they developed through separation.

References

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