INSECT EYES
by
Jan Parmentier
In an earlier article in Micscape I discussed
the eyes of mollusks. In this article I will try
to give some insight into the ways an insect
sees the world. Insects can have a number of
eyes and moreover, eyes of different types;
single eyes and compound ones. In contrast
with our eyes, insect eyes are immovable and
unable to focus.
Insects are short sighted.
Butterflies are probably the most far sighted;
Compound eyes in the head of a Crane-fly
they can see perhaps a few meters, while
bumblebees only have a range of a half meter. But many insects are helped a lot by
their sometimes unbelievable sensitivity for scents.
It is of course impossible for humans to perceive what an insect sees. But we think
that a compound eye, consisting of 2 to 30 000 lenses will project a sort of mosaic
picture. In most cases the compound eyes will see only form and movements, but
the dragonfly, which has to catch his prey in flight, undoubtedly must have a
detailed view of his near surroundings.
With its two enormous compound eyes, each made of 30 000 lenses and the
additional single eyes, the dragonfly is very well adapted to its predatory and
active life.
Insect eyes can change considerably between the larval and the adult stage.
When you look into the details of the compound insect eye, there are many
differences when you compare them with our eyes. But amazingly there is a very
close resemblance between the genes that control the development of eyes in a
house fly and in man, making it a little bit easier to understand the many times that
eyes developed independently in different classes of animals.
Other sets of
control genes cause the differences between animal groups. (A gene is a stretch
of DNA in a cell nucleus with the information, necessary to make enzymes, the
proteins that regulate all cell functions. Control genes regulate other genes, by
switching them on and off).
We find compound eyes even in
trilobites, crustaceans already
present 500 million years ago.
The lens parts of their eyes are
often marvelously preserved, due
to the fact that the material of
these eyes consists of calcite
crystals.
Even more amazing is that these
lenses are biconvex, composed of
The eye of a trilobite (5 mm. wide)
a
doublet,
a
construction
developed
by
Huygens
and
Descartes, to correct for spherical aberration. Evolution constructed these eyes
half a billion years before man thought of it!
When we look into more details of a compound
insect eye, we have to learn the terms that
describe such an eye. One single, complete eye,
situated in a compound eye, is called an
ommatidium. On the outside we find the lens,
directly followed by a crystalline cone. The
cone is connected to long cells, called the
rhabdome (in fact the photoreceptor surface),
surrounded by several retina cells, which
secrete the rhabdome.
Crystalline cone,
rhabdome and retina cells are surrounded by
pigment cells that have the function of optically
isolating each ommatidium from its neighbours.
The retina cells are connected to nerve cells,
leading to the optic ganglion.
Section through the eye of a honey-bee
In insect species that are active at twilight or
at night we find that the rhabdome is not
connected to the crystal cone. Consequently
the ommatidia are not completely isolated from each other, leading to a better
light yield but also resulting in a less sharp image. Both types of eyes are
combined in mayflies. In some night owls the light yield is improved by a reflecting
layer. We see their eyes lit up in the dark when we direct a light beam at moths.
Seeing is a photochemical process. Photons
are caught on the rhabdome by a compound
that we call retinal. When it absorbs light
energy, retinal changes its form from bent to
straight. Retinal is connected to a membrane
bound protein, opsin. This complex is called
rhodopsin. When retinal changes its form, it
separates from the rhodopsin and the opsin
triggers a nerve cell. The nerve cells conduct
the signal to the brain, telling the brain that
the rhabdome has seen a photon. (The free
retinal will later be reduced to vitamin A, and
in a second reaction, from this vitamin retinal
is formed again, which recombines with the
free opsin. That is why we need vitamin A for
a good night vision).
Seeing, by a human or by a bumblebee, is a
process of incredible subtlety.
As
microscopists we can at least see an important
part of the apparatus involved in this process!
Close-up of the honey-bee's eye
References:
1. S. Sutton-Vane, The Story of Eyes. Phoenix House 1960, London.
2. W.H.Freeman, Brian Bracegirdle, An Atlas of Invertebrate Structure. Heinemann Educational
Books, 1971, London.
3. Kurt Fiedler, Johannes Lieder. Mikroskopische Anatomie der Wirbellosen. Fischer Verlag, 1994,
Stuttgart.
4. P. Suppon, Chemistry and Light, Royal Society of Chemistry,1994, Cambridge.
5. Riccardo Levi-Setti, Trilobites. 1993, University of Chicago Press.
The pictures of honeybee's eyes were taken from a slide made by Johannes Lieder.
The picture of the crane-fly's head was taken from a NBS slide.
Thank you to Laurie for suggesting we visit the link to this article. 