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The Organs of the
Senses of Perception Vision and Visual Organs
Introduction
Insects are provided with a
great variety of external sensory organs. These organs present
structural differences, in their cuticle part as well as in their
cellular part. The reception compartment, formed by cuticle,
the cell or group of sensory cells, and other associated cells is
called sensillia. The simplest type of sensillia is a
hair-like shape, full of nerves, mainly a group of hairs that have a
direct connection with a sensory cell. The eyes, or optical
sensillias, make up a completely different group or receptors that
structurally, have very little in common with the other sensory
organs. The external cuticle forms a transparent area (cornea)
that allows for the light to pass toward the reception cells.
These cells, composing the retina, are specialized nerve cells that
have a central axon forming the fibers of the optical nerve.
The primary function of the visual systems is to classify the
photons coming from different places in space, according to their
wavelength and polarization plane. The word "eye" is utilized,
in general terms, for those organs that are specifically sensitive
to rays of light that come toward it. They are capable of
transmitting the produced effect in its sensory cell to the central
nervous system. The eyes of the insects are always located on
their head, and the optic nerves are located in the protocerebral
region of the brain. The eyes, therefore, belong to the
preantennal region of the procephalon and are probably prostomial
organs in origin.
Among the animals, two types of eyes can be recognized; they are
simple and compound. The first are characterized for
possessing only one focusing structure (i.e. pupil, mirror or lens)
shared with all the photoreceptors that from the retina. The
compound eyes are formed by multiple focusing structures, each one
of them associated to a reduced number of photoreceptor cells,
making each unit an ommatidia.
In the insects, there are three types of visual organs
ontogenetically distinct: the dorsal ocelli, the stemmata (simple
eyes) and the compound eyes, the last two are of lateral position.
While the ocelli are a characteristic of adult insects, the stemmata
are found on the larvae of holometabolic insects. From the
point of functional vision, the dorsal ocelli are characterized by a
focal plane found behind the retina (this is, they are not focused),
which does not allow for an image to be formed, something that does
not occur in the stemmata. But the most important difference
falls in their ontogenetic origin. While the dorsal ocelli are
always associated to its own neuropile related with a media track of
the protocerebrum, the stemmata are linked to the same protocerebral
neuropile as the future compound eyes.
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