An eyespot (sometimes ocellus) is an eye-like marking. They are found in butterflies, reptiles, cats, birds and fish. Eyespots could be explained in at least three different ways. They may be a form of mimicry in which a spot on the body of an animal resembles an eye of a different animal, to deceive potential predator or prey species. They may be a form of self-mimicry, to draw a predator's attention away from the prey's most vulnerable body parts. Or they may serve to make the prey appear inedible or dangerous. Eyespot markings may play a role in intraspecies communication or courtship; the best-known example is probably the eyespots on a peacock's display feathers. The pattern-forming biological process (

morphogenesis Morphogenesis (from the Greek ''morphê'' shape and ''genesis'' creation, literally "the generation of form") is the biological process that causes a cell, tissue or organism to develop its shape. It is one of three fundamental aspects of deve ...

) of eyespots in a wide variety of animals is controlled by a small number of genes active in embryonic development, including the genes called Engrailed, Distal-less,

Hedgehog A hedgehog is a spiny mammal of the subfamily Erinaceinae, in the eulipotyphlan family Erinaceidae. There are seventeen species of hedgehog in five genera found throughout parts of Europe, Asia, and Africa, and in New Zealand by introductio ...

, Antennapedia, and the Notch signaling pathway. Artificial eyespots have been shown to reduce predation of cattle by lions.

Zoological distribution

In butterflies and moths

The eye-like markings in some butterflies and moths and certain other insects, as well as birds like the

sunbittern The sunbittern (''Eurypyga helias'') is a bittern-like bird of tropical regions of the Americas, and the sole member of the family Eurypygidae (sometimes spelled Eurypigidae) and genus ''Eurypyga''. It is found in Central and South America, and ...

, serve functions in addition to mimicry; indeed, it is unclear whether they actually mimic eyes. There is evidence that eyespots in butterflies are

antipredator adaptation Anti-predator adaptations are mechanisms developed through evolution that assist prey organisms in their constant struggle against predators. Throughout the animal kingdom, adaptations have evolved for every stage of this struggle, namely by avo ...

s, either in deimatic displays to intimidate predators, or to deflect attacks away from vital body parts. In species such as '' Hipparchia semele'', the conspicuous eyespots are hidden at rest to decrease detectability, and only exposed when they believe potential predators are nearby. Butterfly eyespots can mimic dead leaves for camouflage from predators, as seen in ''

Bicyclus anynana ''Bicyclus anynana'' (squinting bush brown) is a small brown butterfly in the family Nymphalidae, the most globally diverse family of butterflies. It is primarily found in eastern Africa from southern Sudan to Eswatini. Butterfly eyespots may play a role in mate recognition and sexual selection. Sexual selection drives the diversification of eyespots in different species of butterflies, as mates select for characteristics like size and brightness. Some species of caterpillar, such as those of hawkmoths (

Sphingidae The Sphingidae are a family of moths (Lepidoptera) called sphinx moths, also colloquially known as hawk moths, with many of their caterpillars known as “hornworms”; it includes about 1,450 species. It is best represented in the tropics, but ...

), have eyespots on their anterior abdominal segments. When alarmed, they retract the head and the thoracic segments into the body, leaving the apparently threatening large eyes at the front of the visible part of the body. Butterflies such as the blues ( Lycaenidae) have filamentous "tails" at the ends of their wings and nearby patterns of markings, which combine to create a "false head". This

automimicry In zoology, automimicry, Browerian mimicry, or intraspecific mimicry, is a form of mimicry in which the same species of animal is imitated. There are two different forms. In one form, first described by Lincoln Brower in 1967, weakly-defended m ...

misdirects predators such as birds and jumping spiders (

Salticidae 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 ...

). Spectacular examples occur in the hairstreak butterflies; they commonly perch upside down with the false head raised and shift their rear wings repeatedly, causing antenna-like movements of the "tails" on their wings. Studies of rear-wing damage support the hypothesis that this deflects attacks from the insect's head.

In reptiles and mammals

Some reptiles, such as the

sand lizard The sand lizard (''Lacerta agilis'') is a lacertid lizard distributed across most of Europe from France and across the continent to Lake Baikal in Russia. It does not occur in European Turkey. Its distribution is often patchy. In the sand lizard' ...

of Europe, have eyespots; in the sand lizard's case, there is a row of spots along the back, and a row on each side. Many species of

cat The cat (''Felis catus'') is a domestic species of small carnivorous mammal. It is the only domesticated species in the family Felidae and is commonly referred to as the domestic cat or house cat to distinguish it from the wild members of ...

, including

Geoffroy's cat Geoffroy's cat (''Leopardus geoffroyi'') is a small wild cat native to the southern and central regions of South America. It is about the size of a domestic cat. It is listed as Least Concern on the IUCN Red List because it is widespread and abun ...

jungle cat The jungle cat (''Felis chaus''), also called reed cat, swamp cat and jungle lynx, is a medium-sized cat native to the Middle East, the Caucasus, South and Southeast Asia and southern China. It inhabits foremost wetlands like swamps, littoral ...

pampas cat The Pampas cat (''Leopardus colocola'') is a small wild cat native to South America. It is listed as Near Threatened on the IUCN Red List as habitat conversion and destruction may cause the population to decline in the future. It is named after ...

s, and

serval The serval (''Leptailurus serval'') is a wild cat native to Africa. It is widespread in sub-Saharan countries, except rainforest regions. Across its range, it occurs in protected areas, and hunting it is either prohibited or regulated in ran ...

s, have white markings, whether spots or bars, on the backs of their ears; it is possible that these signal "follow me" to the young of the species. There may be an evolutionary trade-off in this case between nocturnal camouflage and intraspecific signalling.

In birds

Male birds of some species, such as the peacock, have conspicuous eyespots in their plumage, used to signal their quality to sexually selecting females. The number of eyespots in a peacock's train predicts his mating success; when a peacock's train is experimentally pruned, females lose interest. Several species of pygmy owl bear false eyes on the back of the head, misleading predators into reacting as though they were the subject of an aggressive stare.

In fish

Some fish have eyespots. The

foureye butterflyfish The foureye butterflyfish (''Chaetodon capistratus'') is a butterflyfish (family Chaetodontidae). It is alternatively called the four-eyed butterflyfish. This species is found in the Western Atlantic from Massachusetts, USA and Bermuda to the Wes ...

gets its name from a large and conspicuous eyespot on each side of the body near the tail. A black vertical bar on the head runs through the true eye, making it hard to see. This may deceive predators in two ways: into attacking the tail rather than the more vulnerable head, and about the fish's likely direction of travel. The foureye butterflyfish eyespot is thus an example of self-mimicry. For the same reason, many juvenile fish display eyespots that disappear during their adult phase. Some species of fish, like the spotted mandarin fish and spotted ray, maintain their eyespots throughout their adult lives. These eyespots can take a form very similar to those seen in most butterflies, with a focus surrounded by concentric rings of other pigmentation. Deilephila elpenor 11.JPG, Larva of elephant hawkmoth (''Deilephila elpenor'') displaying eyespots when alarmed File:Notodontidae - Cerura vinula.jpg, Larva of puss moth (''Cerura vinula'') in a defensive posture, with black eyespots visible just above its head File:Gray Hairstreak (One more time...) (6222138633).jpg,

Automimicry In zoology, automimicry, Browerian mimicry, or intraspecific mimicry, is a form of mimicry in which the same species of animal is imitated. There are two different forms. In one form, first described by Lincoln Brower in 1967, weakly-defended m ...

gray hairstreak The gray hairstreak (''Strymon melinus'') is also called the bean lycaenid or cotton square borer. It is a member of the Lycaenidae family, known as the gossamer-winged butterflies and the second-largest family of butterflies. It is one of the mos ...

(''Strymon melinus'') has false head at rear File:Zauneidechse 1469 (detail).jpg,

Sand lizard The sand lizard (''Lacerta agilis'') is a lacertid lizard distributed across most of Europe from France and across the continent to Lake Baikal in Russia. It does not occur in European Turkey. Its distribution is often patchy. In the sand lizard' ...

(''Lacerta agilis'') with rows of eyespots File:Glaucidium californicum Verdi Sierra Pines 2 (detail).jpg, Pygmy owl (''Glaucidium californicum'') with eyespots behind head File:Serval from back.jpg,

Serval The serval (''Leptailurus serval'') is a wild cat native to Africa. It is widespread in sub-Saharan countries, except rainforest regions. Across its range, it occurs in protected areas, and hunting it is either prohibited or regulated in ran ...

(''Leptailurus serval'') from back, ocelli clearly visible on the ears File:Chaetodon capistratus 14zz.jpg,

Foureye butterflyfish The foureye butterflyfish (''Chaetodon capistratus'') is a butterflyfish (family Chaetodontidae). It is alternatively called the four-eyed butterflyfish. This species is found in the Western Atlantic from Massachusetts, USA and Bermuda to the Wes ...

(''Chaetodon capistratus'') has false eyes near tail and disruptive eye mask concealing eye

Morphogenesis

Butterfly eyespots are formed during embryogenesis as a result of a morphogenetic signalling centre or organizer, called the focus. This induces neighbouring cells to produce specific pigments which pattern the eyespot. Early experiments on eyespot morphogenesis used

cautery Cauterization (or cauterisation, or cautery) is a medical practice or technique of burning a part of a body to remove or close off a part of it. It destroys some tissue in an attempt to mitigate bleeding and damage, remove an undesired growth, or ...

on the butterfly wing eyespot foci to demonstrate that a long range signaling mechanism or morphogen gradient controlled the formation of eyespots in both space and time. The findings cannot be explained by a simple source/

diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemica ...

model, but could be explained by either a source/threshold model, in which the focus creates the morphogen, or by the sink model, in which the focus generates a gradient by removing a morphogen which was created elsewhere. Several genes involved in eyespot formation have been identified that can fit into these models, but only two of them have been functionally tested. These genes are the transcription factor Distalless (Dll) and the ligand (a signalling substance that binds a

cell surface receptor Cell surface receptors (membrane receptors, transmembrane receptors) are receptors that are embedded in the plasma membrane of cells. They act in cell signaling by receiving (binding to) extracellular molecules. They are specialized integral m ...

) Hedgehog (Hh). Butterfly eyespot morphology appears to be the result of the evolution of an altered version of the regulatory circuit which patterns the wings of other insects. This rogue regulatory circuit is able to pattern both the anterior and posterior eyespots independent of the usual anterior/posterior wing compartmentalization restrictions seen in the fruit fly '' Drosophila''. The altered regulatory circuit redeploys early developmental signaling sources, like the canonical

hedgehog A hedgehog is a spiny mammal of the subfamily Erinaceinae, in the eulipotyphlan family Erinaceidae. There are seventeen species of hedgehog in five genera found throughout parts of Europe, Asia, and Africa, and in New Zealand by introductio ...

(Hh) pathway, Distal-less (Dll), and engrailed (En), breaking the anterior/posterior compartmentalization restrictions through increased localized levels of Hh signaling. In turn, this raises expression of its receptor

Patched Patched (Ptc) is a conserved 12-pass transmembrane protein receptor that plays an obligate negative regulatory role in the Hedgehog signaling pathway in insects and vertebrates. Patched is an essential gene in embryogenesis for proper segme ...

(Ptc) and transcription factor. Normally, in '' Drosophila'', engrailed acts in the posterior compartment to restrict Ptc and '' Cubitus interruptus'' (Ci) expression to the anterior compartment by repressing transcription of Ci, thereby preventing Ptc expression. From the perspective of evolutionary developmental biology, understanding the redeployment and plasticity of existing regulatory mechanisms in butterfly eyespot locus development has given more insight into a fundamental mechanism for the evolution of novel structures.

Distal-less

The Distal-less gene is present in almost all eyespot organizers, making it an ideal candidate to carry out major functions of eyespot formation. During the wing imaginal disc development Dll, has two expression domains separated by a temporal component. First Dll is expressed in a group of cells in the center of what will become the focus and eventually the eyespot. This expression starts during the middle of the fifth instar

larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. ...

and lasts until the pupal stage. The second domain starts around 20 hours after pupation around the original central cluster of cells, in an area in which a black ring of the eyespot will be formed. Functional experiments using transgenic ''

Bicyclus anynana ''Bicyclus anynana'' (squinting bush brown) is a small brown butterfly in the family Nymphalidae, the most globally diverse family of butterflies. It is primarily found in eastern Africa from southern Sudan to Eswatini.Drosophila'' fruit flies, Hh is expressed in all cells in the posterior compartment of the developing butterfly wing during the mid fifth instar of butterfly wing development. However, in butterflies, Hh expression is significantly higher in those cells that flank the potential foci. Higher transcription levels of Hh, along with other known associates of the Hh pathway, namely patched (Ptc) the Hh receptor, and cubitus interruptus (Ci), the Hh transcription factor is seen throughout the mid to late fifth instar as well, which further implies a role for Hh signaling in eyespot development and patterning. Furthermore, cells that are flanked by the cells expressing the highest level of Hh signaling are fated to become the foci, indicating that focus cell fate determination relies on high concentrations of Hh in surrounding cells. However, this observation has not been totally confirmed as a rule for multiple butterfly species. Studies tried to extrapolate the result of Hh pathway involvement by looking for the expression of Ci in ''Bicyclus anynana''. Here they observed that both seem to be expressed in eyespots, suggesting a relation with the Hh signaling pathway. However, other studies did not find evidence of Hh expression in ''B. anynana''.

Notch

The Notch (N) gene expression precedes an upregulation of Dll in the cells that will become the center of the focus. This makes N the earliest developmental signal, so far studied, that is related with the establishment of the eyespots. Loss of N completely disrupts Dll expression, and eventually eyespot formation, in several butterfly species. A variety of other wing patterns are determined by N and Dll patterns of expression in early development of the wing imaginal disc, suggesting that a single mechanism patterns multiple coloration structures of the wing.

Evolution

Butterfly eyespots are formed by an interplay of at least 3 genes, namely Distal-less (Dll), spalt (sal), and Antennapedia (Antp), hence their evolution has been shaped by differential expression of these genes in different butterfly taxa, as shown in ''

Bicyclus anynana ''Bicyclus anynana'' (squinting bush brown) is a small brown butterfly in the family Nymphalidae, the most globally diverse family of butterflies. It is primarily found in eastern Africa from southern Sudan to Eswatini. Eyespots painted on the rumps of cows have been shown to reduce cattle predation in Africa. The study authors, Cameron Radford and colleagues, note that in the

Sundarbans Sundarbans (pronounced ) is a mangrove area in the delta formed by the confluence of the Padma, Brahmaputra and Meghna Rivers in the Bay of Bengal. It spans the area from the Baleswar River in Bangladesh's division of Khulna to the Hooghly R ...

, forest users wear face masks with eye markings on the backs of their heads in the hope of reducing tiger attacks. In the study on 2061 cattle in 14 herds over 4 years, 683 were given eye markings, 543 were painted with crosses, and 835 were unpainted. None of the eyed cattle were predated, but 4 cross-marked and 15 unmarked cattle were killed, one by a leopard, the rest by lions. Both the eyespots and the cross markings provided statistically significant protection. The cattle were always in mixed groups of marked and unmarked animals; it is not known whether marking all animals in a herd would provide effective protection.

References

Animal anatomy Antipredator adaptations Mimicry {{DEFAULTSORT:Eyespot (pattern)