The pictures above illustrate three basic models of the compound eye. Adult insects have one pair of
compound eyes. As the name suggests, the compound eye is made up of a series of 'eyes' compounded
together - that is they have many lenses. Each lens is part of a prismatic unit called an ommatidium (plural
ommatidia). Each ommatidium appears on the surface as a single polygon or dome, called a facet. The
models above each show 60 such facets from 60 ommatidia arranged in 6 rows of ten. The facets may be
hexagonal (6-sided), squarish, circular or hemispherical. Hexagonal packing covers the surface of the eye
with the highest number of facets. However, eyes with hexagonal facets will have also have some
pentagonal (5-sided) or quadrilateral (4-sided) facets since hexagons cannot completely pack a spherical
surface without leaving gaps, whilst a combination of hexagons and pentagons can. If you were to use a
Zales promo code to get a large, expertly cut loose diamond, you would see similar geometric shapes cut
into the gemstone and it would give you a little bit of an idea of what an ommatidium would look like.
These geometries are important, because to some extent each ommatidium and its corresponding facet
behave as a single optical unit, and the more such units that fit into a given area the more resolution
(detail) the eye can see. The more ommatidia that add to the image, the more points or 'pixels' that go to
make up the final image. In vertebrates, like humans, the arrangement is quite different - a single 'facet'
and a single lens covers a retina of many sensory cells, where each sensory cell contributes one point or
'pixel' to the final image, so the retinal sensory cells are the optical units as far as resolution of the final
image is concerned. In insects, however, each facet encloses one ommatidium containing just 7 to 11
sensory cells. In the human retina, in its most sensitive region (known as the fovea) some 175 000
sensory cells per square millimetre are packed into an hexagonal array. In the insect, the compound eye
contains anything from about half a dozen ommatidia to 30 000 or more. For example, the wingless
silverfish have only a few ommatidia, or none at all, whilst the dragonfly has about 30 000 ommatidia in
each compound eye. Dragonflies catch prey on the wing and so they need better visual resolution, which
is why they have such large compound eyes and so many ommatidia. Often the density of the facets is
greatest in certain parts of the eye - those parts that are most often used for more accurate vision.
Similarly, in humans, the density of sensory cells in the retina declines away from the central fovea toward
the edges of the visual field, which is why the edge of your visual field is so fuzzy. For the same reason,
one can often sex flies by the size of their compound eyes - male flies have larger eyes that almost meet
in the middle of the face, since they need keener vision to help them spot females!
Insect eyes are one of the most prominent features of many insect heads and they vary tremendously in
colour, whether an insect is camouflaged or coloured to advertise itself as unpleasant to potential
predators or as attractive to potential mates, the colour and pattern of the eyes is very important!
Above: left, a single ommatidium from a compound eye. Right, the outer cells have been sectioned to
show the internal optic apparatus. A labelled version of this diagram is shown below:
Structure of a single ommatidium
This type of ommatidium is from a type of compound eye called an apposition eye and is characteristic
of diurnal (day-active) insects. Nocturnal insects have a modification to this plan, called the
superposition eye, which reduces spatial resolution but increases sensitivity to dim light. We will look at
these differences in more detail later.
Notice that light enters this ommatidium from above, through the corneal facet. The cornea and
crystalline cone together focus the light onto the rhabdom. This dual lens system forms the
light-focusing or dioptric apparatus of the ommatidium. Vertebrate, including human, eyes are similar in
this respect - the outer cornea and the lens together with the various liquids or gels in the eye focus the
There are usually seven or eight sensory cells (also called retinula cells, one of which is usually highly
modified) in each ommatidium, these surround the optic rod or rhabdom, which is a cylinder created by
a multitude of interdigitating finger-like processes (called microvilli) that extend from the sensory cells
and meet in the middle. This rhabdom is the actual light detector and contains high concentrations of
light-sensitive pigments called rhodopsins (rhodopsins require vitamin A for their manufacture). The iris
cells are also called pigment cells, since they are heavily pigmented to stop stray light entering the
ommatidium through the sides (such as light that has entered through neighbouring ommatidia).
The ommatidium is an energy transducer - light energy absorbed by rhodopsins in the rhabdom are
converted into electrical (strictly electrochemical) energy and the sensory cells send electrical signals
that encode the light stimulus to the optic lobes of the brain. There are a pair of optic lobes (or optic
ganglia) one innervating each compound eye.
Above: left, an insect (Aleochara bilineata) whose 450 or so ommatidia, each about 4 micrometres in
radius, pack into an hexagonal array in which each ommatidium has 6 (sometimes 5) nearest
neighbours. Right, a fly (Delia antiqua?) whose ommatidia pack into a square array in which each
ommatidium has 4 nearest neighbours.
How does the compound eye of an insect compare to the eye of a human?
First of all let's look at visual acuity. Visual acuity is the actual spatial resolution that the eye can see,
and can be measured, for example, by using a grating of alternating black and white vertical lines and
seeing how close together the striped must be when viewed at a set distance before the stripes merge
and can no longer be distinguished as a series of lines. Visual acuity is determined in part by the
maximum possible resolution of the eye, as determined by the spatial density of sensors - retinal cells
or ommatidia which determines the number of points that make up the final image. It is also determined
by the optical limits and imperfections of the lens system. Even if the optics are as perfect as possible,
light will still diffract (spread-out) by some degree as it passes through th lenses, such that a single
point of light coming from an object becomes an extended or blurred spot of light in the image. Insect
eyes are limited by this diffraction.
Ommatidial facets are very small, about 10 micrometres (or one hundredth of a millimetre) in diameter.
This allows many points to compose the final image. The type of eye we have considered so far is
typical of diurnal insects, such as flies (Diptera), wasps and bees (Hymenoptera), many beetles
(Coleoptera), dragonflies and damselflies (Odonata) and day-flying butterflies (Lepidoptera). This type
of eye is adapted for bright light and is called an apposition compound eye because the final image is
made up of discrete points, each point formed by a single ommatidium, placed side-by-side (apposed
to one another) to form an image which is a mosaic of points. This does not mean, however, that the
insect sees a disjointed image made up of points, nor that the insect sees multiple images, since the
brain integrates these images and so what the eye 'sees' and what the insect 'perceives' are two
different albeit related things.
In the human eye, the retina is made up of an hexagonal array of sensor cells, called rods and cones,
and the distance between adjacent sensors is only about 2 micrometres and so these sensors can
pack together to give a density (of about 175 000 per square millimetre) which is about 25 times
higher than the ommatidial density of the insect eye. This allows the human eye to detect greater
spatial detail or resolution in the object to give a more detailed image. The question is, why don't
insects have ommatidia that are only 2 micrometres in diameter? The answer is because diffraction
limits the performance of such small lenses. In fact the rhabdom acts like a wave-guide when it's less
than about 5-10 micrometres in diameter. A wave-guide is to light what a hollow tube is to air blown
through it - when you blow into a flute, the vibrations are confined in a narrow space and all but certain
frequencies of vibration cancel out and we get what we call standing waves which give the fundamental
and harmonic tones of the note played. In a light wave-guide a similar situation exists - light waves
vibrate only in certain frequencies that are confined (or guided along) the optical tube. As it happens,
when this occurs in a narrow rhabdom, it prevents the light from being focused into a point smaller
than about 5 micrometres in diameter. In short, such small optics are of no advantage as they are
unable to focus the light into small enough points. Lenses only work above a certain size.
Thus, the visual acuity of the compound eye is about one hundred times less than that of the human
eye due to design constraints. The only possible way to overcome this is to make the compound eye
larger. In fact an estimate can be calculated to show that the compound eye would need a diameter of
about 20 metres to see as much spatial detail as the human eye, which is about the size of a house!
(Click here to see this calculation). Dragonflies have among the largest eyes in the insect world, with
compound eyes several millimetres in diameter since they require quite sharp vision in order to catch
prey insects on the wing. Indeed, they can do this better than we could despite having less sharp
What about contrast?
Contrast is closely related to visual acuity in the sense of spatial resolution, but more exactly contrast
is the ability to distinguish similar shades of the same colour, say shades of grey, and is important in
defining the edges of objects. You can see the words on this page because the black type contrasts
strongly with the white page. However, this is much harder to read since the contrast is less. The dark
grey text in the line below has even less contrast with the black background:
For the same sort of reasons contrast falls as light levels fall - in dim light the contrast is less. To
overcome this, in dim light an optical system needs to collect more light. An astronomer's telescope
looking at dim galaxies far away would benefit by having a large diameter aperture (the aperture is the
opening at the end which directs light into the tube of the telescope) to gather in more of the dim light
coming from such far away objects. Alternatively, one can collect the light for longer periods of time -
an astronomer might leave their telescope trained on the same patch of sky for minutes or hours,
rotating the telescope to compensate for rotation of the Earth. Clearly, there is a limit to the length of
time that an animal's eye can gather light from the same object, since the animal world is dynamic and
if you don't see the predator quickly you are more likely to get eaten! Insect's are limited by the small
apertures of each ommatidium in the compound eye. Indeed the diurnal apposition type of eye can
only detect weak contrast in bright daylight, but can cope reasonably well in room-light, but these
insects stop flying if the light levels drop to below room-light, such as in Moonlight or starlight. It is
possible to calculate the number of photons entering each ommatidium each second (click here to see
the calculation). The insect eye collects light for about 0.1 second to form a given image, and it needs
to receive about one million photons (photons are particles or the smallest possible packets of light) in
this time period to maximise contrast and this is only achieved, in the apposition eye, in broad daylight.
The absolute minimum threshold for vision is about the same in insects and humans at about 1 photon
every 40 minutes, which is extremely sensitive! However, only very strong contrast could be detected
in such low light levels.
Humans are diurnal, and although they have a degree of night vision, human eyes are not particularly
good in twilight or Moonlight or starlight. Horses have adaptations that enable them to see better in
twilight than can humans, which is handy to spot predators working at odd hours!
Many insects are crepuscular (meaning that they are most active in twilight). Moths and beetles in
particular, but also some flies, some dragonflies and some butterflies fly at light levels comparable to
Moonlight. These insects may have apposition eyes with wider facets and they may collect light over a
longer time period (up to about 0.5 seconds?) before integrating the signal to produce the final image.
Moths and beetles, in particular, may have a different type of compound eye, called the superposition
eye. In this type of eye the iris cells only ensheath the top part of the ommatidium, around the facet
and cone. A translucent light-conducting rod connects the bottom of the crystalline cone to the
rhabdom which is now far beneath the cone. This is illustrated below:
Now each rhabdom not only receives light from its own facet and cone lens system, but it also
receives light from neighbouring ommatidia, since there is no screening pigment to prevent light
leaking between adjacent rhabdoms (the blue regions of the retinula cells in the diagram are
Above: the rhabdom light detector can receive light from neighbouring ommatidia in a superposition eye.
In this way, light from as many as 30 ommatidia may overlap and focus onto the same point. Clearly this
intensifies the image, improving sensitivity in dim light. However, the trade-off is that visual acuity is
reduced - 30 or so ommatidia are now working as one large ommatidium, so the final image will be made
from 30 times fewer points and spatial resolution will be reduced. The human eye makes a similar
trade-off - in dim light the eye relies upon sensors that combine their signals neurologically. Some
insects do this too. What we have described so far is known as optical superposition, since the light
itself is added together or superposed (literally light is placed on top of light). However, some insects
have optical apposition eyes that superimpose their signals neurologically, so-called neural
superposition. In neural superposition, it is the electrical signals from neighbouring ommatidia that are
added together by the nervous system, even though the light illuminates separate ommatidia by
The eyes of most insects are capable of adapting to light and dark. In diurnal insects with apposition
eyes, the pigment in the iris cells moves upward in the dark, exposing the rhabdom to light from
neighbouring ommatidia - effectively turning the eye from an optical apposition eye into an optical
superposition eye. Neural changes can further increase the sensitivity of dark-adapted insect vision.
Nocturnal insects show a similar pattern, but with greater ranges in sensitivity, with the eye becoming
about 1000 times more sensitive to light in the dark. Thus, though insects may have the geometry of
apposition or superposition type eyes, most can change in functionality to some degree. Clearly,
however, the range of light-intensities which best suites each type of eye is restricted and best suited to
the life habits of the species. Humans similarly show dark adaptation, which occurs quickly over the first
ten minutes, then slows and takes some 30 minutes to complete. When you first switch off the light in a
room at night, you will find that at first you cannot see anything much, but after a few moments objects
will become clearer. Wait for half an hour or wake up in the middle of the night and you will see clearer
still. However, human eyes still work best in daylight and they are no where near as good in dim light as
those creatures that are most active in the dark. The graph below shows the increase in sensitivity of
the compound eye of the rove beetle Aleochara bilineata upon dark adaptation:
Above: Dark adaptation in the compound eye of Aleochara bilineata. This was measured using the
electroretinogram (ERG) technique, which uses electrodes to measure electrical activity in the insect
eye in response to pulses of light. The adaptation of this insect's eye is particularly rapid, being
complete after 10 to 15 minutes, but these insects are beetles and many beetles are known to fly in dim
light; fully diurnal insects may require nearer to 30 minutes to dark adapt (as does the onion fly, Delia
antiqua, for example), rather like humans. However, fast-flying diurnal insects also possess eyes that
dark-adapt very rapidly, and Aleochara bilineata will take to the wing in direct sunlight. The electrical
response of the eye (and underlying nervous tissue) measured here is generally proportional to the log
of stimulus light intensity. Since the light stimulus has remained at constant brightness here the
increase is due to an increase in sensitivity of the eye by about 100-fold. (Data courtesy of Skilbeck, C
and Anderson, M).
When you look at a conventional CRT (cathode-ray tube) television screen the image that you see
refreshes 25 or 30 times a second (depending where you live) but the image looks continuous. (You
may detect some flicker as the images change over as the screen refreshes through the corner of your
eye). Many electric lights also flicker on and off at 100 or 120 times a second, but this is too fast for you
to notice (unless the light is old and the rate of flicker becomes much slower). For any visual stimulus
that blinks faster than the flicker-fusion frequency of your visual system, the flickers fuse into a single
continuous image and the flickering cannot be perceived. The flicker-fusion frequency of human vision
is 15-20 times a second, which is why you can just make out TV screens flickering. The electric light
flickers much too fast for you to see it flickering. However, the flicker-fusion frequency for a honeybee is
about 300, so the bee will see the light flickering. Thus, although the spatial visual acuity of the
honeybee visual system is only 1/100 to 1/60 that of the human eye, its temporal resolution is much
greater! This helps account for the very fast reflexes of many insects. The dragonfly can intercept a
flying prey insect on the wing because its vision responds much faster than a humans. Fast-flying
diurnal insects have very high flicker-fusion frequencies. Slow fliers, like the stick insect, Carausius,
have flicker-fusion frequencies of about 40 per second.
Colour is what we perceive after our brains have processed visual information and represents the
wavelength of light coming from objects. (Click here to learn all about waves and wavelength). This is
an important point - what you see is what you perceive not simply what the eyes sense. Sensation is
the purely physical phenomenon whereby a sensor converts stimulus energy from the environment,
such as light, into encoded electrical signals in the nervous system. What you perceive is the result of
the nervous processing of these signals in the retina and brain as they are presented to the conscious.
We can never know what an insect perceives, but we can ascertain how its sensors work and how the
nervous system manipulates and modifies this information. We can never know whether or not an
insect perceives colour in the way that humans do. What we can ascertain is whether or not they see
and respond to colour.
Nobel Laureate Karl von Frisch conducted classical experiments with honeybees, in 1914, and
demonstrated that honeybees do see colour. He trained bees to associate the colour blue with food. He
placed out a checkerboard series of paper squares, one blue and the others varying shades of grey.
This was to demonstrate that bees did not simply recognise the blue square by seeing it as a particular
shade of grey. He also covered the papers in a glass plate to rule out any odours associated with the
blue paper in particular, in case the bees could smell that the paper was different. He placed an
identical clean and empty dish on each square, but only the dish on the blue square contained food.
This ruled out any visual cues that the bees might use to find the food. The bees quickly learned that
the blue square contained food. The position of this square in the checkerboard was changed every 20
minutes, to prevent the bees remembering its position, but they still flew straight to the blue square, no
matter where it was. Even when no food was provided, the bees would fly to the blue square initially,
expecting to find food there (though they would soon learn that the food was gone). This demonstrated
that bees have true colour vision, and also that they were capable of learning. Furthermore, the bees
could not be trained to respond to a grey, black or white square - so the bees really see the colour blue
and do not see it as a shade of grey. The reason why so many insect-pollinated flowers are large and
brightly coloured, is to advertise their presence to insects. Flowers provide both pollen and often nectar
for food for the insect, in return for pollen dispersal and delivery to recipient flowers.
Colour vision is an advanced feature. Most mammals, including cats and dogs, see only in grey, black
and white - they cannot see colour. If your dog recognises your red car, then it does not recognise it as
red since the car would look grey to the dog, but it would be recognising other features about the car.
To a lion, a zebra is camouflaged, since its black and white stripes blend in with its grey surroundings.
Primates, including humans, are the exception among mammals. Your primate ancestors probably
evolved colour vision as an aid to finding fruit in trees, since fruit was a staple part of their diet. Many
birds and some fish also have colour vision, indeed their colour vision may exceed that of humans in
terms of the variety of colours they can see. For this reason, a zebra would never attract mates with a
bright show of colours, but a bird might (apart from which birds can fly away from predators that may
spot their bright colours whilst a zebra can't!). The colour vision of birds also explains why insects need
authentic camouflage colours to avoid being spotted by their avian predators, hence many insects are
coloured in shades of green, yellow or brown. Many insects also advertise their bad taste or toxic
make-up or stinging ability to would be predators by having contrasting stripes, as in bees and wasps.
The birds will see the stripes and their colours, whilst most mammals will see the contrasting stripes as
shades of grey.
The pigments in the retina, or in the insect eye, that detect light are called rhodopsins. These pigments
come in several types, but each has its own distinct colour and so each absorbs and responds best to
certain colours or wavelengths of light. How many such pigments do humans need to see the vast
number of hues that they can see? Perhaps surprisingly, the answer is only three! Humans have
sensors in the retina that respond best to blue light (440 nm, actually blue-violet?), or best to green
light (545 nm) or to red light (actually to yellow, orange and red light) a fourth type of sensor sees only
shades of grey, black and white (it is achromatic). Most real colours are a mixture of red, green and
blue. For example, a colour like this sky blue is about 3 parts red, 4 parts green and 5 parts blue.
Assuming that you are not colour blind, then it stimulated the blue sensors in your retina most,
stimulated your green sensors quite a bit and your red sensors least of all. Your retina and brain then
blended the three colours to give the correct shade of blue. The whole spectrum of hues that humans
can see is generated by blending these three primary colours: red, green and blue. For this reason
humans have trichromatic colour vision (trichromatic literally means 'three-colour'). People who are
colour-blind, however, are usually dichromatic (they can only see two colours) although some people
may be totally colour blind and able to see only shades of grey, like our lion. Most birds and goldfish
and a few humans are tetrachromatic (they can see four primary colours) and some birds may be
pentachromatic (they can see five primary colours) and so are capable of seeing more hues than your
So, what about insects? The graph below shows an example of an insect visual spectrum, for the rove
beetle Aleochara bilineata. It tells us how sensitive the eye is to each wavelength of light:
Above: the visual spectrum of Aleochara bilineata (unpublished data courtesy of Skilbeck, C and
Anderson, M). This shows the electrical response of the compound eye (measured with electrodes) in
response to pulses of light of definite wavelength. Measuring the electrical response of an animal's eye to
light is a technique called the electroretinogram (ERG). The greater the electrical response measured
from the ERG, the greater the sensitivity of the eye to the particular wavelength used. In this case we can
see peaks in sensitivity to light at around 365 nm (ultraviolet) and 545 nm (blue-green). The dotted
vertical line indicates the cut-off for human vision - humans cannot see wavelengths to the left of this line,
which are ultraviolet (UV). Humans also cannot see beyond about 750 nm, which is infrared (IR). Notice
that the longer the wavelength, the redder the light and the shorter the wavelength, the bluer the light.
This insect cannot see red light at all well, which is typical of many insects. It can, however, see ultraviolet
light clearly - so it can see some colours that humans can see, but it cannot see red very well.
The peak in the ultraviolet spectrum helps insects to navigate. Sunlight forms a pattern of polarised
ultraviolet light in the sky - a pattern that humans cannot see. This pattern indicates the position of the
Sun in the sky, even if it is cloudy, allowing insects to navigate by using the Sun as a compass, along with
their own internal biological clocks. The insect knows what time of day it is and thus it knows where the
Sun is in the sky and can use this to sense compass bearing, so it knows whether it is flying North, East,
South or West.
How does an electroretinogram work?
By measuring the voltage generated across the insect eye in response to a pulse of light, the following
type of trace can be seen on an oscilloscope:
Above: two examples of an electroretinogram. In each case the square pulse indicates when the flash of
light was administered (with time along the bottom horizontal axis) and the trace above each stimulus
shows the response of the insect eye. The response of the eye consists of a rapid change in voltage
(called the b wave) followed by a slower, longer-lasting change in voltage (called the c wave). Probably
the b wave is from the eye itself, and the c wave from the underlying nervous tissue in the optic ganglion
of the brain. One of the traces is for a dark-adapted insect eye, the other for a room-light adapted
insect eye, can you tell which is which? The top trace is for the dark-adapted eye, which is why the
response is larger - the eye has become more sensitive to light (the response is recorded in milliVolts,
mV, or thousandths of a Volt and is due to electricity generated by the eye as it encodes light energy
into electrical energy). Thus, a bigger response indicates higher sensitivity. The above traces were
obtained with white light beam pulses of 0.187 seconds duration. The light was passed through heat
filters to remove the heat (so the insect does not respond to the heat as well). Now, by using narrow
band-pass filters, it is possible to produce a beam of light of a specified colour. For example, a red filter
might let light through at 600 nm (and a bit either side) whilst a blue filter may let through light only at
around 450 nm. Thus, one can measure the response of the eye to different colours of light. The
responses have been standardised by testing the eye only in the dark-adapted state. The graph below
shows the spectrum obtained for female onion flies (Delia antiqua):
Again we have a peak in the ultraviolet - a very strong peak. Onion flies fly more than Aleochara and so
probably need to use the Solar compass more often and perhaps more precisely (?). Being a rove
beetle, Aleochara spends a lot of its time on or under the ground in burrows that it digs, folding its wings
under its elytra for protection. Aleochara will only fly in direct sunlight. The onion fly also has good
sensitivity in the green part of the spectrum, but it has greater sensitivity to blue light. Again this may
indicate a strong flier and as this insect feeds on onion plants, a high sensitivity to the blue-green leaves
of onions perhaps helps it to spot them more easily (though odours will also be very important).
So, these insects may at least be dichromatic (with UV and green sensors), possibly trichromatic (with
UV, blue and green). However, this data is not enough to prove that they have colour vision. Although
the eye has the necessary sensors, we do not know whether the brain interprets colours as brightness or
as colour. Behavioural experiments, similar to those done on bees, might be able to answer that. Some
moths have peaks in the UV, blue and green and also in the red or infrared and so may be
[Technical note: it is important to calibrate the apparatus to ensure that the 'intensity' of light at all colours is the same,
otherwise, for example, if the blue light was brighter then this would bias the result. One way to do this is to keep the
energy of the light beam constant at all wavelengths, this yields what is called the spectral efficiency of the insect.
However, eyes do not respond to colours on the basis of the energy in the light beam, rather they respond to the number
of photons present in the beam, or more accurately the photon flux density. Keeping photon flux density across the
wavelengths measures what is called the spectral sensitivity. Note that since the energy of a photon, E = Planck's
constant (h) x frequency of light, the red light at 650 nm contains almost double the photon flux density as blue light at 350
nm. However, since the eye responds to the log of stimulus intensity, and when the eye is not very sensitive to red light,
as in this case, the spectral sensitivity curve differs little from the spectral efficiency curve, the peaks are simply shifted to
the right slightly. However, spectral sensitivity is the preferred method these days as it is considered more accurate and
representative (although one has to consider the quantum efficiency of the eye, so the spectral sensitivity does not
measure the response to a number of absorbed quanta, only to the number of incident quanta).]
Did you know?
Many insect-pollinated flowers contain ultraviolet pigments that only their pollinating insects (and perhaps
birds) can see. Many flowers are more strikingly coloured in the UV than in the visible spectrum.
Furthermore, markings, visible only in the UV, act as taxi markers to guide the landed insects to the
pollen and nectar food rewards.
So, we have seen that the insect compound eye is designed very differently to the vertebrate eye. The
insect eye has much poorer spatial resolution, due to its design constraints, but some have much higher
temporal resolution. Like mammals, insects can adapt to see in low light levels at night, and like primates
and birds, at least some of them can see colour. Most insects can also see ultraviolet light (whether as a
colour or as shades of grey) which helps them to navigate using the Sun. It has been said that insect
evolution hit a brick wall with the compound eye - unable to achieve better spatial resolution, but some
insects may have other types of eye that should have been able to evolve to become more like
vertebrate eyes. So, why doesn't any insect have visual acuity as high as a human? The answer
probably lies in neural processing. Even if an insect had an eye as spatially acute as a humans, where
would it fit the large brain required to process such detailed images? In the end, insect eyes are highly
adapted to the insect way of life. Indeed, insects rival the vertebrates as dominant terrestrial life-forms on
Earth, so they clearly are highly evolved!
Things to do!
Here is a simple experiment on insect vision that you can perform at home. All you need is a wooden
cone, 6 to 8 inches in length, to act as a mould (such as the handle of a paintbrush of appropriate size),
black paper, glue and tissue paper. Make a model of the insect eye by wrapping pieces of the black
paper around the mould to make cones and glue the overlapping edges together and cut half an inch
from the tip of each cone. Make about 20 or so such paper cones and then pack (and glue) them
together, with the wide ends directed outwards and the narrow cut ends forming part of the surface of a
sphere. A bit of tissue paper held against the narrow tips serves as the retina. Take the model into a
dark room with the wide ends directed toward a remote lamp, such that only one to three of the cones
are illuminated, and the rest will remain dark so long as the tissue paper is firmly pressed against their
narrow openings. This is an apposition eye, in which each cone or model ommatidium is optically isolated
and an accurate image is produced. Next, move the tissue paper away from the cones and the image will
become less distinct as light is scattered to neighbouring ommatidia, this is the superposition eye. (Idea
taken from: Simple Experiments with Insects, by H. Kalmus, Heinemann press).
Coming soon: more on insect vision ...