Communication with light

Insect Communication Using Light

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Insects see the world around them very differently from humans because their eyes work differently than ours do.

A human eye is a precise optical instrument made up of a lens and pupil which adjust to control the focusing of images on the fovea so that they can be transferred to the brain for processing via the optic nerve. Insect eyes are very different in that each eye is made of hundreds of optical instruments known as ommatidia for this reason insect's eyes are referred to as compound eyes. Each individual ommatidium is similar to the human eye.

Both the human eye and an ommatidium have a way to focus the images that come in on the light rays. The human eye uses the cornea (in combination with the lens and pupil) to do this while the insect ommatidium uses the cornea and crystalline cone. These structures adjust so as to focus the image accurately onto a sensory material so that they can then be transferred via an optic nerve. The sensory material for insects is known as the rhabdom while in humans it is known as the fovea.

In insects the ommatidium structure varies slightly depending on whether the insect is diurnal (day active) or nocturnal (night active) since the ommatidium will have to be sensitive to vastly different light intensities. The image below is an example of an ommatidium found in apposition eye used by diurnal insects. Nocturnal insects use what are called superposition eyes which compromise their spatial resolution in order to be more sensitive to lower light intensities.

The main functional difference between these types of eyes is that a human eye will focus a single sharp image on around 175,000 sensory cells in the fovea and an insect's compound eye will have each ommatidium focus an image on just 7 to 11 sensory cells. This might make more sense if each sensory cell is considered to be like a single pixel in a digital camera, the higher megapixel a camera the higher resolution photos it will take. Due to size and structural constraints the most accurate compound eye will only be around 100 times less then that of a human eye that is unless the insect is large enough to have compound eyes 20 meters in diameter! For the exact calculations showing why this is true click on the off site link here. However what the insect's brain interprets via the optic nerve is not many disjointed images, but rather a single combined image.

The number of ommatidium in an insect's compound eye ranges from few to none in insect orders that do not need much visual stimuli such as the wingless silverfish (Archeognatha) to a whopping 30,000 for highly aerial insects like dragonflies (Odonata). In addition male insects, especially flies, have more ommatidia than females, presumably to spot their female counterparts more easily.

close up of a compound eye showing corneas
of approximately 450 ommatidia
http://cronodon.com/BioTech/Insect_Vision.html

basic structure of an insect ommatidium
http://cronodon.com/BioTech/Insect_Vision.html

basic structure of the human eye
http://www.scientificpsychic.com/workbook/chapter2.htm

Another important aspect of how insects use of light to communicate with their surroundings is what their visible spectrum of light is. In 1914 Biologist Karl Von Frisch did an experiment to prove that insects (in his experiment honeybees) could indeed differentiate colors. He did this by training honeybees to associate the color blue with food. To learn more about the details of his experiment click on the off site link here. This fact was already suspected prior to Frisch's 1914 experiment because so many insect pollinated flowers have bright colors but it is still a surprising one because so many other animals include cats and dogs do not see in color.

Insects as well as humans are able to see in color due to special pigments in their retinas called rhodopsins. In humans there are only 3 rhodopsins which each respond to a different wavelength of light. One for light rays with a 440 nanometer wavelength (blues), one for light rays with a 545 nanometer wavelength (greens), and one to light rays with a 700 nanometer wavelength (reds). We also have an additional sensor which responds to shades of gray. Insects color vision operates similarly except with different rhodopsins. They have a rhodopsin which allows them to see light rays with an even shorter wavelength (around 365 nanometers) and are often missing the rhodopsin which allows us to see light rays with an even longer wavelength (red light).

The short wavelength light rays are known as ultraviolet and are invisible to the human eye. Remarkably insects use their ultraviolet vision to view the patterns of ultraviolet light emitted by the sun in order to navigate, even in very low light situations.

A physical property to note is that light is an electromagnetic wave which allows it to travel through the vacuum of space. Without this property insects would not be able to see the ultraviolet light emitted by the sun and consequently would not be able to navigate.

Entomologists who study insects frequently take advantage of insect's ultraviolet navigation habits to trick them. A great way to do this is to hang a white bed sheet near a lamp emitting ultraviolet rays at night. Wait a few minutes (especially in tropical regions) and soon the light sheet comes ALIVE!

Insects at a light trap
http://www.exploratorium.edu/biodiversity/place/b-lighttrap.html

Want to know more? Check out how insects use Pheromones!!!