The compound eye is an eye comprising of many individual lens systems, called ommatidia. Compound eye is present in many members of phylum Arthropoda such as insects and crustaceans. Compound eye comprise thousands of ommatidia. Ommatidia are small self-regulating photoreception units that consists of a cornea, lens and photoreceptor cells which can discriminate brightness and color. (Marshall etal, 2003.p.459)
Most of the insects have a pair of large prominent compound eyes, composed of units called ommatidia. A single compound eye may have 30,000 ommatidia, such as in large dragonflies.

Fig 1.1.Compound eye of a dragonfly.
As compared to the single- aperture eye, compound eyes have poor image resolution. The benefit of compound eye is that they possess a very large view angle, have ability to detect fast movements and also detect the polarization of light in some cases. (Volkel,R.,Eisner, M. etal june 2003.p. 461-472)
There are two types of compound eyes one of them is apposition eyes, that form multiple inverted images, and the other is superposition eyes , that form a single erect image.(Gaten, Edward 1998)
1. Apposition Eyes:
These can be divided into two groups. The typical apposition eye has a lens focusing light from one direction on the rhabdom, while light from other directions is absorbed by the dark wall of the ommatidium. Mantis shrimp is an advanced example of this type of eye.

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Fig. 1.2 Apposition eye
Neural superposition eye:
In the other kind of apposition eye, each lens forms an image, and the images are combined in the brain. This type is found in strepsiptera. This eye is also known as the neural superposition eye.

Fig 1.3 Neural superposition eye
2. Superposition eyes:
It is divided into three types
a. The refracting
b. The reflecting
c. The parabolic
a. The refracting superposition eye : It has a gap between the lens and the rhabdom, having no side wall. Each of the lens takes light at an angle to its axis and then reflects it to the same angle on other side. The resultant image is formed at the half of radius of eye, the region where the tips of rhabdom lie. This type is found mostly in nocturnal insects.

Fig 1.4 Refraction superposition eye
b. The parabolic superposition eye: This type is found in mayflies, the parabolic surfaces. In this type the inside parabolic surfaces of each facet focus light from a reflector to a sensor array.

Fig 1.5 Parabolic superposition eye.
c. Reflecting Superposition eyes: Long bodied decapod crustaceans such as prawns, shrimps and lobsters possess this type of eye they use corner mirrors instead of lenses this type is not found in insects.
Good fliers like flies , prey catching insects like dragonflies possess specialized zones of ommatidia that is organized into fovea area that produces acute vision. The acute zone has larger facets and flattened eyes. Thus due to flattening more ommatidia receive light from a single spot and thus produce higher resolution.

Structure of compound eye:
Compound eyes cosists of a few to 28,000 receptors, called ommatidia, that fused into the multifaceted eye.

Fig 1.6 Structure of ommatidium
Structure of ommatidium :The outer surface of each ommatidium is a lens and is one facet of the eye. Below the lens is the crystalline cone. The lens and the crystalline cone are light gathering structures. Certain cells of an ommatidium called retinula cells, have a special light gathering area called rhabdom The rhabdom converts light energy into nerve impulses.

Fig 1.6 The structure of compound eye
Pigment cells support the crystalline cone, and sometimes the rhabdom, and prevents the light that strikes one rhabdom from dazzling into an nearby ommatidium. (Miller and Harley , page 255 seventh edition )
Benefits of compound eye :
Many insects form an image of form but the construct of image establishment holds no special importance for most species. The compound eye is much better suited for detecting movements. The movement of a point of visible emission less than 0.ace point can be easily identify when light successively smash nearby ommatidia. This is the main reason that how insects choose moving prey and bees become attracted to flowers. Compound oculus can detect wavelength of light falling in UV spectrum , They can also perceive polarized light that help them in navigation and orientation.( miller and Harley p 255)
Ocelli of insects
It has been found that the Ocelli are very simple photo-receptors or light detecting organs. A single lens and many sensory cells made up ocelli. Ocelli can not produce comple image. They can only detect movements in the surroundings. Insects posses ocelli- a character of Arthropods.

Fig 1.7 The eye of fly.
How ocelli differ from compound eyes:
Ocelli have only a single structure inpite of having multiple units. They are called simple eyes. Adult insects possess ocelli along with compound eyes. Ocelli are found at their back or face. Dragon fly and may flies also have ocelli, these appear as tiny swellings.
Functions of ocelli:
? Ocelli are helpful in detection of small changes in light intensity.
? Ocelli can affect changes in behavior because it has been found that they are associated with some largest neurons of the nervous system.
? Ocelli control circadian rhythms in some types of insects by performing activity of light detection that falls on their body and then nervous system conveys message to different regions of the body.
? In an experiment performed on Cockroaches it has been found that the stimulation of light caused by ocelli has the ability to alter the features of the optomotor performance .
Antennae of insects:
Antennae are the organs which are not used for touch but are used for the sense of smell. Many individual joints form paired antennae. Mobility of them is obvious. Filiform attenae are the most basic form having many segments with unequal sizes. This type found in many groups like in book lice, scorpion flies, Beetles, Dragonfly, Grasshopper etc. but differences among structures persist among them.

Fig 1.8 Insect Antennae
Types of Anttenae:
Filliform antennae:
Among all the types the most simple type is filliform antennae. Different variations in the crucial one structure give rise to other types.

Fig 1.8 Filliform anttenae
1. Setaceous antennae: There are many junctions. The antenna narrowing gradually from the base to the tip e.g. Bristletails, Cockroaches and stoneflies etc.

Fig 1.9 Setaceous antennae
2. Pectinate: They gave the look of a comb. The segments are longer on one side.
Examples: Wasps and beetles

Fig 1.10 Pectinate
3. Geniculate: It is found in ants and beetles there found a bent along antennae

Fig 1.11 Geniculate
4. Plumose:
These look like feathers. Each segment carries a number of fine thread like branches for example found in flies.

Fig 1.12 Plumose
5. Clavate :
A club appearance is found tip of antennae becomes wider for example in beetles, utterflies etc.

Fig 1.13 Calavate
6. Lamellate :
A fan appearance is given because segments toward the end are plate like and flattened. Example, beetles.

Fig 1.14 Lamellate

7. Serrate:
A saw appearance is found for example in beetles because the segments are angled on one side.

Fig 1.15 Serrate
8. Moniliform:
Appeared like a string of beads eg. Beetles.

Fig 1.16 Moniliform
Structure of antennae :
The scape, the pedicel and finally the flagellum which often comprises many units called flagellomeres. A collection of sensory organs called johnston’s organ are present in stem region. Torulus is the socket region that holds scape. A membrane closes this socket that holds scape. The antennae hinges on the firmly leaped projections called antennifer from edge of trolus. Scape connected muscles help in as a whole movement of antennae. Flagellomeres serves as indicative features and number vary among different types.