Eyespot (mimicry)

Gladeye Bushbrown (Mycalesis patnia) with a large eyespot on its under wings

An eyespot (sometimes ocellus) is an eye-like marking. They are found on butterflies, reptiles, birds and fish. In members of the Felidae family (such as the Leopard Cat and Leopard), the white circular markings on the backs of the ears are termed ocelli, and they are functionally similar to eyespots in other animals.

Eyespots may be a form of automimicry in which a spot on the body of an animal resembles an eye of a different animal to deceive potential predator or prey species; to draw a predator's attention away from the most vulnerable body parts; or to appear as an inedible or even dangerous animal. In larger animals, eyespots may play a role in intraspecies communication or courtship – the most well-known example is probably the eyespots on a peacock's display feathers. The white spots on the back of Felid ears serve a social function, by communicating the animal's mental state (excited, calm, aggressive, etc.) to conspecifics in the gloom of dense forest or in tall grass.

The eye-like markings in some butterflies and moths and certain other insects, as well as the Sunbittern (a bird) do not seem to serve a mimicry function. In some other cases, the evolutionary function of such spots is also not understood. There is, however, recent evidence to suggest that eyespots in butterflies serve as an anti-predatory device, by either scaring off potential predators or deflecting attacks away from vital body parts. Butterfly eyespots may also play a role in mate recognition and sexual selection, playing a similar role to the eyespots found on larger organisms mentioned above. Pattern formation of concentric spots is probably less complex than the conspicuous eyespots might suggest; it cannot be ruled out that eyespots are simply a morphogenetic "spandrel" which in any one particular case may or may not have been adapted for a particular function.

Evolution of the eyespot at the molecular Level

Rear view of a serval, displaying ocelli on the ears

Butterfly eyespots are formed as a result of a morphogenetic signaling center, or organizer called the focus. This signaling center induces its neighboring cells to produce specific pigments which pattern the eyespot. Foci morphology and evolution appears to be the result of an altered regulatory circuit composed of the same patterning mechanisms conserved in insect wing patterning. This altered regulatory circuit is able to pattern both the anterior and posterior eyespots independent of the anterior/posterior wing compartmentalization restrictions.

Breaking the highly conserved compartmentalization restrictions put forth by insect wing patterning, by redeployment of early developmental signaling sources like the canonical hedgehog (Hh) pathway, Distalless (Dll), and engrailed (en), this altered regulatory circuit is able to pattern both the anterior and posterior eyespots independent of the A/P compartmentalization restrictions through increased localized levels of hh signaling. Increased hh signaling raises expression of its receptor ptc and transcription factor. Normally, in the fruit-fly Drosophila, engrailed(en) acts in the posterior compartment to restrict ptc and ci expression to the anterior compartment by repressing transcription of ci, thereby preventing expression of ptc. However, This rogue expression of ptc and ci seen in the posterior compartment of some butterfly wing disks, is an indication that during the evolution of eyespots, there was a divergence in the repression or expression of these genes from that of drosophila. From an evo-devo perspective, understanding the redeployment and plasticity of already existing regulatory mechanisms in butterfly eyespot loci development may give more insight into a fundamental mechanism for the evolution of novelty structures.

Morphogenetic models

Eyespot patterning is thought to occur by morphogenetic activity, however the exact model and modes of action have yet to be elucidated. A number of experiments have been done to understand the mechanism of eyespot patterning all of which indicate morphogenetic activity.

Effects of cautery on the butterfly wing eyespot foci are indicative of a long range signaling mechanism or morphogen gradient, both in its spacial and temporal aspects. Cautery studies have shown that different results in eyespot size and color can result depeding on the time in development at with the cauterizing damage is administered. While these findings support morphogenetic activity, they cannot simply be explained by a source/diffusion model. Observations of cautery have given rise to two different explanations of how eyespots are patterned: 1) A source/threshold model, in which the focus is the morphogen source and, 2) the sink model, in which the focus is generating a gradient by removal of the morphogen at the foci.

Further investigation of genes involved in wing development and morphogenetic activity has led to the discovery that hh has a primary role in the morphogenetic signaling center of the foci. In a manner that is similar to Drosophila, hedgehog (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; patched (ptc) the hh receptor, and cubitus interruptus (ci) the hh transcription factor are seen throughout the mid to late fifth instar as well, which further implies a role for hedgehog 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 foci cell fate determination relies on high concentrations of hh in surrounding cells.

Eyespots in fish

See also: Underwater camouflage and mimicry

The foureye butterflyfish has false eyes on its back end, confusing predators about which is the front end of the fish

Some fish species also display eyespots. The foureye butterflyfish gets its name from a large dark spot on the rear portion of each side of the body. This spot is surrounded by a brilliant white ring, resembling an eyespot. A black vertical bar on the head runs through the true eye, making it hard to see. This can result in a predator thinking the fish is bigger than it is, and confusing the back end with the front end. The butterflyfish's first instinct when threatened is to flee, putting the false eyespot closer to the predator than the head. Most predators aim for the eyes, and this false eyespot tricks the predator into believing that the fish will flee tail first.

See also

• Animal coloration
• Mimicry
• Ocellus