WHAT??
do insects really see?
Since the compound eyes of insects are comprised of a mosaic of small facets, the popular media
likes to represent insect mosaic vision like this.
However, insects actually see like this.
Let us examine the structure of the insect compound eye and learn how the compound eyes
visualize the landscape for insects.
The main visual structure of insects is the compound eye.
The compound eye appears to be comprised of many hundreds or thousands of tiny facets.
Each facet is actually the corneal lens for an individual photoreceptor unit called an
ommatidium.
Let us look at the detailed structure of an ommatidium.
An ommatidium is divided into light-gathering and light-detecting components.
The corneal lens is transparent cuticle that is secreted by two modified epidermal cells
called corneagen cells.
Corneagen cells secrete the corneal lens when new cuticle is formed at the time of the molt.
The corneagen cells secrete the corneal lens then later differentiate into the primary
pigment cells.
Below the corneal lens is the crystalline cone.
This structure is either secreted or formed by four cells called Semper's cells.
Light enters the corneal lens and is focused on the light detecting apparatus by the crystalline cone.
The light detecting apparatus of each ommatidium contains 7 to 8 light-sensitive photoreceptor
cells called retinula cells.
Retinula cells are actually photoreceptor neurons that detect wavelengths of light.
The light-sensitive, dendritic region of the retinula cell is the rhabdomere.
Together, all of the individual rhabdomeres are referred to as the rhabdom.
Rhabomeres consist of parallel microvilli containing light-detecting visual pigment
molecules embedded into their plasma membranes.
Light entering the cornea and cone of an ommatidium is focused on, and detected by, visual pigments
embedded in the rhabdomere membranes of the retinula cells.
The retinula cells are surrounded by 12-18 secondary pigment cells so that each ommatidium
can be functionally isolated from its neighbors.
Ommatidia sit on a basement membrane in the retina, and axons from the retinula cells
combine below the membrane to form the optic nerve that leads to the protocerebrum of the
insect brain.
Let us see how insect compound eyes function to detect light and perceive visual information
. Insect species that are active during the
daytime, when light is abundant, have photopic ommatidia in their compound eyes.
The photopic ommatidium was earlier called an apposition ommatidium because the base
of the crystalline cone is in direct contact, or, as it were -- in apposition to the rhabdom.
In photopic ommatidia, incoming light is focused on the base of the crystalline cone and directly
onto the rhabdom, the light-sensitive, sensory region of the retinula cells.
Light rays that enter the corneal lens at angles are absorbed by screening pigments
located in the cytoplasm of the pigment cells that surround photopic ommatidia.
The importance of the screening pigments is seen best from the perspective of the retina.
The screening pigments prevent light entering at angles through one ommatidial lens from
activating the rhabdom in adjacent ommatidia of the retina.
Light entering the corneal lens of an ommatidium stays within that ommatidium.
In photopic eyes, each ommatidium serves as an individual visual unit for viewing only
that portion of the visual scene that is directly in line with the ommatidium’s position within
the compound eye.
Next, we will compare the structure and function of the photopic eye of day-active insect species
to the eye of night-active insect species.
Ommatidia in the compound eyes of nocturnal insects are referred to as scotopic ommatidia.
The structure of scotopic ommatidia is similar to that of photopic ommatidia except that
scotopic ommatidia, appear to have an open space, referred to as the clear zone, between
the crystalline cone and the retinula cells.
The clear zone actually contains transparent, crystalline tracts that arise from the retinula
cells and extend to the crystalline cone.
. The crystalline tracts act as “light guides.”
In scotopic ommatidia, incoming light is focused on the base of the crystalline cone and conducted
to the rhabdom by the crystalline tracts.
Unlike photopic ommatidia, scotopic ommatidia are sensitive to the changes in light intensity
that occur during the day and night cycle, and they detect light differently during the
light and dark phases of the daily circadian cycle.
During the daytime, when light is abundant, scotopic ommatidia detect light similar to
photopic ommatidia.
The shielding pigments in the secondary pigment cells absorb light rays that enter the corneal
lens at an angle, and each ommatidium detects light independently of neighboring ommatidia,
just as in photopic ommatidia.
However, as night approaches, and the intensity of light decreases, the shielding pigments
of scotopic ommatidia contract upward toward the distal end of the secondary pigment cells.
Distal contraction of the shielding pigments in scotopic ommitidia opens the basal portion of the pigment cells
so that light entering the dioptric apparatus of one ommatidium can pass through that ommatidium
and strike the rhabdoms in adjacent ommatidia.
In this manner, scotopic ommatidia interact cooperatively
to form multiple, superimposed images on the
rhabdoms of neighboring ommatidia.
This interaction compounds the sensitivity of the scotopic ommatidia to light.
Because scotopic ommatidia form multiple overlaid images on adjacent ommatidia, the scotopic
ommatidium is sometimes referred to as a superposition ommatidium.
The superimposed images are probably poorly defined, but detailed vision is less critical
to nocturnal insects -- than is the enhanced ability to detect dim light.
Hence, the night-phase superposition compound eye has low visual acuity, but high light
sensitivity.
Let us observe how the shielding pigments react in the scotopic ommatidia of the retina
during several light and dark cycles, to illustrate the interactions between adjacent ommatidia.
Next we will see how the mosaic eye perceives the visual scene.
Let us now enter into the compound eye and see the world as viewed by an insect.
Here is how the world might appear to an insect from within a single ommatidium.
Each ommatidium views the visual scene according to its position in the compound eye and the
width of its lens.
The view from each ommatidium is then integrated by the nervous system to provide a view of
the entire visual scene.
Here is the mosaic view as observed by an insect with this type of eye.
The lens and cones of insect ommatidia are not capable of changing their focus so insects
are probably near-sighted and distant objects appear blurry.
Note that the observed scene is most distinctive when broken patterns caused by the edges of
structures occur within the field of view of a single ommatidium, or across adjacent
ommatidia.
Notice how conspicuous the pattern is when it has sharp contrasts of light and dark within
the field of view of an ommatidium.
Conversely, the scene is least distinctive when a nearly similar pattern occurs within
the field of a single ommatidium or between adjacent ommatidia.
Likewise, movement within the field of a single ommatidium is poorly observed, but movement
across the field of several ommatidia provides a high degree of sensory input.
Here is how something close might appear as it moves past.
Note that the most information is provided as it crosses the fields of adjacent ommatidia.
Finally, let us juxtapose a compound eye from a species such as a dragonfly that has many
more ommatidia per unit area.
Note that by increasing the number of ommatidia per unit area a greater number of individual
ommatidia detect broken patterns within their field of view as compared to the single larger
ommatidium.
Since each ommatidium acts as a visual unit, this has the effect of increasing the detail,
or resolution, that the insect can detect in its field of view.
Note how many ommatidia the flying insect crosses in this situation.
This increases the sensitivity of the dragonfly’s eye to the presence of its prey.
Therefore, insect vision is developed to be most sensitive to changing patterns and movement,
rather than a highly-resolved, detailed view of the visual scene.
Such as is found with the vertebrate eye.