The pupils of animal eyes
The eyes of animals pupils surprises by the diversity of their shape: sometimes circular (as in humans), sometimes oval, or straight (direction slots variable), or more complex (slot with multiple openings). The essential role of the pupils is to modulate the amount of light received by the retina.
See a few pages of the book "Ophthalmic Art", directed by Dr. Franck Rival, veterinary medical officer:
Shape and orientation of the pupil
Evolution has probably selected the most efficient forms of pupils for each of the species concerned, depending on the mode of life (day or night), the type of predation, and also some eye features such as colourful vision, the existence of a naturally multifocal lens... The characteristics of the animal pupils thus learn about the vision of animals.
The human species, primates and many aquatic and land animals have a circular pupil. The pupil of fish has usually a diameter fixed and is not reactive to light. The circular geometry of the pupillary border offers a variation of surface proportional to the square of the difference in diameter. Thus, between 2 mm and 7 mm, one can apply the amount of light captured from a fixed source in a same time varies by a factor of 25. This is not enough to adapt to the variations that exist between the dark night and day. Diurnal animals more often have a circular pupil: their retina is suitable for daytime, but weakly sensitive vision in night conditions. It is therefore point need to strongly reduce the surface of the pupil in case of strong illumination. Other forms of pupils (slots) allow a better control of the amount of light captured towards the retina. Living exclusively underwater species are not likely to be exposed to too strong light; Instead, the penetration of sunlight is reduced rapidly with depth, and then must capture the maximum amount of photons; size eyes and pupillary drive tends to increase in proportion with the size of the body of the animal. Swordfish is a predatory fish, swimming speed can reach 100 km/h, and that has big eyeballs and a large circular pupil allows the animal to its prey even in low light.
The Chameleon is a diurnal species, which has a relatively large eye for its size and a circular pupil, whose opening is close to that of the eyelids that surround the eyeball of the animal. The Visual system of the chameleons has peculiarities. Unlike the human visual system, that chameleons allows independent eye movements, be they slow or in jerks. The Chameleon's eye in particular has a unique feature: crystalline, negative power, form a system equivalent to a telescope of Galileo with the cornea. This can magnify (closer) image, which is useful to catch small prey such as insects. The accommodation, which is particularly powerful in this animal made by this pair of "lenses" opposite power, and a system of muscle striated (while she is smooth in humans for convenience) allows to develop very quickly, and at small distances (up to 60 Diotpries, about 1.5 cm). It is a remarkable Visual achievement, made possible through this autofocus very powerful. Chameleons Foveal retinal is also suited to this mode of predation since it is tilted inwards, which contributes to the magnification of the retinal image.
The eye of the Chameleon, "turret", allows the animal to its prey, which must be located within reach of achieving its language, that he plans on them in a Flash. The geometry of the anterior segment of the eye of the Chameleon can magnify the image of the observed target.
We find in reptiles of the pupils in vertical slot, as well as in some terrestrial mammals such as the Fox and the cat. This geometry allows for a wide field of vision more in the direction of the greater width of the pupil, a better vision in the perpendicular axis (the narrow axis of the pupil). It allows also further variations of surface, which is useful in diurnal and nocturnal animals. When the pupil is not round but shaped slot or elongated ellipse, the quality of vision is maximum in the opposite direction of the axis of the slot. Indeed, the smaller diameter allows the beams refracted in this axis to benefit from a greater depth of focus, and to be less sensitive to the effects of some aberrations. The vertical pupils allows the finest vision in close directions of the horizon. The vertical orientation of the pupils of the cat, some reptiles and other carnivorous predators facilitates hunting on flat ground.
Conversely, many ruminants have pupils in slot horizontally oriented. However, we should not lose sight of when these animals graze, the position looked down from their head restores a vertical orientation of their pupil towards the ground; This then allows them to better detect the arrival of a predator... The eyes of sharks are also equipped with oval pupils generally oriented according to a generally horizontal direction.
Some snakes like green python (Morelia viridis) also have vertical pupils, while their arboreal lifestyle does not seem to require a particular orientation for identification of prey.
However, in birds, which descended from reptiles, the pupils are basically round, and the iris is sometimes very pigmented: variations of the color of the iris appear to be used to increase the degree of sexual attraction with some penguins. The animal pupils of nocturnal animals considerably expand in the dark to capture better low light, and their retinas are also particularly sensitive, which makes only in daytime conditions, an excess of light could damage them. A pupil in slot is used to modulate the amount of light directed to the retina with more amplitude (up to a factor of 1000) than a circular pupil (factor 20 in humans). Finally, the elongated slot layout allows light rays to meet different portions of the lens, same devices. In some animals, the lens shows areas where the index of refraction and curvature vary, so as to induce a desirable multifocalite (longitudinal chromatic aberration compensation). These areas are arranged in concentric ways, and when the circular pupils contract, they hide the more peripheral portions of the lens. This does not occur in case of pupil is crack.
The gecko is mainly nocturnal reptile which presents an almost circular pupil the night. Day, this pupil adopts an overall rectilinear vertical form, equipped with side openings. Conformation in narrow slot allows to significantly reduce retinal illuminance, by a factor of 1000 compared to the conformation in full opening: the animal can hunt day without damaging his retina. In addition, the small circular openings in the secondary provide to the eye of this animal a system equivalent to the "Scheiner disk", which certainly allows him to appreciate distances accommodating to form a single image on the retina of the target ("system of" range finder").
When the watched object is developed on the retina, the 4 circular pupils allow the gecko to get a fine and contrasting vision.
The eye of the squid (or squid or squid) and cephalopods in general is an interesting topic of study. The squid evolving ocean, at depths where the ambient light is low, and its activity is mainly nocturnal. It is essential to capture the little ambient light, or the organic luminescent animals. The relationship between the size of the pupil (the eye in general) and the animal is certainly one of the most important of the animal world. Despite its resemblance to the eye of vertebrates, the eye of cephalopods (squid, giant squid) has features that are lacking in that of vertebrates. There is no blind spot, because the retina is "reversed": the photoreceptors are inside (the innermost layer) of the retina. The path of nervous nets that lead information to the animal's nervous system is simplified, unlike what happens in vertebrates, where Visual information is transported through the axons of the ganglion cells that intervening between the incident light and the photoreceptors. There are no blind spot in the Visual field. The photoreceptors are more numerous per unit area than in the human retina, which bodes well for a good visual acuity. These photoreceptors are only sticks, so colourful perception is no priori. The eye of the squid is equipped with a rigid, spherical lens (the cornea does not have an optical power important due to the low difference in index between water and the corneal tissue). The accommodation is made by the movement of the cristallinienne lens.