A Burst Of Colour
The Christmas festivities are well under way, and we all know how colourful this season can be. Red, green, yellow, you name it, chances are there is a decoration in your house of that colour. Thinking about the sheer variety of colour I am surrounded by at the moment led me to dedicate my next post to a smaller, and often forgotten realm – the world of the invertebrates.
Christmas to insects isn’t the same, for many insects can only see the higher part of the visible light spectrum and part of the ultraviolet light spectrum. An exhibit in the Natural History museum showed what some flowers could look like through an insect’s eyes, and I was curious about what difference in anatomy caused this interspecies variation in vision.
It’s all about the receptors
Many insects that can detect ultraviolet light have eyes which contain ultraviolet light receptors which aren’t present in human eyes. Human eyes contain three types of photoreceptors, we are trichromatic; red, blue, and green . When light enters the eye, its wavelength determines whether it is absorbed by these receptors, and the combination of signals produced by the photoreceptors due to the light absorbed is what determines which colour the brain perceives (as a quick side note; discussing colour perception is very theoretical, as, the sensors stimulated and colour of light absorbed can be scientifically proven, however, how the brain of the organism interprets those signals is impossible to state with 100% accuracy. Predictions can be made, but perception of colour is in the eye of the ‘bee-holder’ only after all .).
Insects are trichromatic also, however they do not possess the receptor which absorbs red light. Instead, insects possess the ultraviolet light receptor, leading to many invertebrates being unable to sense red light but instead being able to sense light towards and in the ultraviolet spectrum. Each of the insect’s prismatic lens containing units (or ommatidia ) contains eight light detecting cells; four respond to yellow-green light, two respond to blue light, and the other two respond to ultraviolet light . This fact has forced flowers to develop petals which are attractive to insects not only in the visible light spectrum, but also in the ultraviolet light spectrum.
One hundred and eighty
When looking at many petals in the visible light spectrum, they may seem drab and a bit boring to you and me, however, when placed under an ultraviolet lamp, or photographed with an ultraviolet camera, these petals reveal a hidden world. Patterns, like dart boards, suddenly appear and offer a small glimpse into the world of the invertebrate. Vibrant colours illuminating the pollen rich areas of the flower act as a target for the insects flying overhead . They have adapted so their petals are not only attractive to the human eye in the visible spectrum, but they exploit the ultraviolet sensing ability of the insects to become highly practical in their marketing strategy. No beating around the bush, if the insect wants nectar, it knows exactly where to find it. With a precise flutter of the wings, the insect hits the bullseye, receiving a sugary reward.
The photoreceptors present in the eye is what makes all the difference when perception of colour is involved. Without the relevant receptor, an entire spectrum of colours is inaccessible and a world is hidden to us. Think about how different Christmas could be with just a simple receptor missing.
 HORSE ARMOR. (2016) Insect vision. [Online] Available from: http://www.horsearmor.net/pages/Insect-Vision.html [Accessed: 23rd December 2016]
 RIDDLE, S. (2016) How Bees See And Why It Matters. [Online] Available from: http://www.beeculture.com/bees-see-matters/ [Accessed: 23rd December 2016]
 [Online] Available from: http://cronodon.com/BioTech/Insect_Vision.html [Accessed: 23rd December 2016]
 STARR, B. (2013) Hidden Patterns: How A Bee Sees The World Of Flowers. [Online] Available from: https://www.visualnews.com/2013/04/08/hidden-patterns-how-a-bee-sees-the-world-of-flowers/ [Accessed: 23rd December 2016]