Color world insects butterfly

The world is full of colours

But are colours really true?

We live in a colourful world – but it’s really only colourful, because we see it that way. If we had in our eyes only a single type of photoreceptive cell, we would only be able to see shades of one particular colour.

If we wanted to detect any difference in hue at all, we would have to have at least two kinds of receptors with maximum sensitivities in two different parts of the visual spectrum. Fact is that in the vertebrate eye two morphologically distinct receptor cell types occur: the rods and the cones. The rods are more sensitive to faint light intensities and have only one single peak sensitivity I in the blue part of the visual spectrum. The cones, on the other hand, which are functionally operational only at greater light intensities, occur in three varieties, each maximally sensitive to a different wavelength.

We are talking of tri-chromatic vision and the three receptor types correspond to blue, green, and red. Basically all colours and hues (and it is estimated that we can distinguish approximately 60,000 colour shades) can be created by a balanced stimulation of these three receptor types, but there has to be relatively bright light to excite them. When dim light conditions prevail, the rods take over and since their maximum sensitivity is in the blue, we tend to see the world in “bluish” tones at night when in reality galactic lights and moon light are slightly redder than sunlight, because of the selective absorption of shorter wavelengths by the dark moon rocks.

If this sounds rather technical, consider the psycho/linguistic aspect of colour vision: colours like red, green, blue, etc. are not physically defined, but subjective terms. Green traffic lights and the colour of a meadow are termed ‘aoi’ (blue) by Japanese (although a term for green ‘midori’ exists) and there are languages in which blue and green colours are described by the same term.

The colours insects see with their little eyes are likely to differ greatly from those that humans see, because while the human’s tri-chromatic vision is based on blue, green and red receptor types that of insects is based on ultraviolet (=UV, which we do not see), blue and green. Some butterflies may even possess two additional receptor types for purple and red as Prof Arikawa in Japan has shown and they would then interpret the colours of the world through a penta-chromatic system.

There is thus the potential for them to see even more colours and hues than we do and, what is even more amazing, as Dr Almut Kelber in Sweden has demonstrated, there are moths that can actually see colours at night under conditions of only moonlight present and light intensities below 0.3 lux. And why would that be useful? To see the colourful world of flowers, of course, because that does not cease to be colourful at night even, if we cannot appreciate its beauty at that time.

fly bee movement insects

For more reading about insects vision, you can click here for “Circus movements”

© Dr V.B. Meyer-Rochow and, 2016.
Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and with appropriate and specific direction to the original content.

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