Ommatidium
Encyclopedia
The compound eyes of insect
s, mantis shrimp
and millipede
s are composed of units called ommatidia (singular: ommatidium). An ommatidium contains a cluster of photoreceptor cells surrounded by support cells and pigment cells. The outer part of the ommatidium is overlaid with a transparent cornea
. Each ommatidium is innervated by one axon
and thus provides the brain
with one picture element
. The brain forms an image from these independent picture elements. The number of ommatidia in the eye depends upon the type of insect and ranges from just a handful in the primitive Archaeognatha
and Thysanura
to around 30 thousand in larger Anisoptera dragonflies and in some Sphingidae
among moths
.
Ommatidia are typically hexagonal in cross section, and approximately ten times longer than wide. The diameter is largest at the surface, tapering toward the inner end. At the outer surface there is a cornea, below which is a pseudocone which acts to further focus the light. The cornea and pseudocone form the outer 10% of the length of the ommatidium.
The inner 90% of the ommatidium contains 6 to 9 in the case of some butterflies (depending on the species) long and thin photoreceptor cells often abbreviated "R cells" in literature and often numbered, e.g. R1 through R9. These "R cells" tightly pack the ommatidium. The portion of the R cells at the central axis of the ommatidium collectively form a light guide, a transparent tube, called the rhabdom.
In certain flies
, the rhabdom has separated into seven independent rhabdomeres. This has required the rewiring of the eye such that each ommatidium now has seven axons leading from it. The advantage to this arrangement is that it increases the number of picture elements by a factor of seven, without increasing the number of ommatidia.
Since an image from the compound eye is created from the independent picture elements produced by ommatidia, it is important for the ommatidia to react only to that part of the scene directly in front of it. To prevent light entering at an angle from being detected by the ommatidium it entered, or by any of the neighboring ommatidia, six pigment cells are present. The pigment cells line the outside of each ommatidium. Each pigment cell is situated at the apex of the hexagons and thus lines the outside of three ommatidia. Light entering at an angle passes through the thin cross-section of the photoreceptor cell, with only a tiny chance of exciting it, and is absorbed by the pigment cell, before it can enter a neighboring ommatidium. In many species, in low-light situations, the pigment is withdrawn, so that light entering the eye might be detected by any of several ommatidia. This enhances light detection but lowers resolution.
The size of the ommatidia varies according to species, but ranges from 5 to 50 micrometres. The rhabdoms within them may cross-section at least as small as 1.x micrometres, the category of "small" being assigned in some cross-species studies to those under 2 micrometers. Naively, microlens arrays can be seen as a biomimetic analogy of ommatidia.
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
s, mantis shrimp
Mantis shrimp
Mantis shrimp or stomatopods are marine crustaceans, the members of the order Stomatopoda. They are neither shrimp nor mantids, but receive their name purely from the physical resemblance to both the terrestrial praying mantis and the shrimp. They may reach in length, although exceptional cases of...
and millipede
Millipede
Millipedes are arthropods that have two pairs of legs per segment . Each segment that has two pairs of legs is a result of two single segments fused together as one...
s are composed of units called ommatidia (singular: ommatidium). An ommatidium contains a cluster of photoreceptor cells surrounded by support cells and pigment cells. The outer part of the ommatidium is overlaid with a transparent cornea
Cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...
. Each ommatidium is innervated by one axon
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....
and thus provides the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
with one picture element
Pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
. The brain forms an image from these independent picture elements. The number of ommatidia in the eye depends upon the type of insect and ranges from just a handful in the primitive Archaeognatha
Archaeognatha
Archaeognatha is an order of wingless insects, also known as jumping bristletails. They are among the least evolutionarily changed insects, appearing in the Devonian period along with the arachnids. The name Archaeognatha is derived from Greek Archaeos meaning "ancient" and gnatha meaning "jaw"...
and Thysanura
Thysanura
Thysanura is an order of insects, encompassing silverfish and firebrats, known for their three long caudal filaments.The families Machilidae and Meinertellidae of the jumping bristletails were once included with Thysanura....
to around 30 thousand in larger Anisoptera dragonflies and in some Sphingidae
Sphingidae
Sphingidae is a family of moths , commonly known as hawk moths, sphinx moths and hornworms, that includes about 1,200 species . It is best represented in the tropics but there are species in every region . They are moderate to large in size and are distinguished among moths for their rapid,...
among moths
Moths
Moths may refer to:* Gustav Moths , German rower* The Moths!, an English indie rock band* MOTHS, members of the Memorable Order of Tin Hats...
.
Ommatidia are typically hexagonal in cross section, and approximately ten times longer than wide. The diameter is largest at the surface, tapering toward the inner end. At the outer surface there is a cornea, below which is a pseudocone which acts to further focus the light. The cornea and pseudocone form the outer 10% of the length of the ommatidium.
The inner 90% of the ommatidium contains 6 to 9 in the case of some butterflies (depending on the species) long and thin photoreceptor cells often abbreviated "R cells" in literature and often numbered, e.g. R1 through R9. These "R cells" tightly pack the ommatidium. The portion of the R cells at the central axis of the ommatidium collectively form a light guide, a transparent tube, called the rhabdom.
In certain flies
Fly
True flies are insects of the order Diptera . They possess a pair of wings on the mesothorax and a pair of halteres, derived from the hind wings, on the metathorax...
, the rhabdom has separated into seven independent rhabdomeres. This has required the rewiring of the eye such that each ommatidium now has seven axons leading from it. The advantage to this arrangement is that it increases the number of picture elements by a factor of seven, without increasing the number of ommatidia.
Since an image from the compound eye is created from the independent picture elements produced by ommatidia, it is important for the ommatidia to react only to that part of the scene directly in front of it. To prevent light entering at an angle from being detected by the ommatidium it entered, or by any of the neighboring ommatidia, six pigment cells are present. The pigment cells line the outside of each ommatidium. Each pigment cell is situated at the apex of the hexagons and thus lines the outside of three ommatidia. Light entering at an angle passes through the thin cross-section of the photoreceptor cell, with only a tiny chance of exciting it, and is absorbed by the pigment cell, before it can enter a neighboring ommatidium. In many species, in low-light situations, the pigment is withdrawn, so that light entering the eye might be detected by any of several ommatidia. This enhances light detection but lowers resolution.
The size of the ommatidia varies according to species, but ranges from 5 to 50 micrometres. The rhabdoms within them may cross-section at least as small as 1.x micrometres, the category of "small" being assigned in some cross-species studies to those under 2 micrometers. Naively, microlens arrays can be seen as a biomimetic analogy of ommatidia.