Seeing Beyond the Rainbow

A recent study, published in the journal Cellular and Molecular Life Sciences, proposed a biological mechanism that may explain how dragonflies see near-infrared light — far surpassing the capacity of human vision. This ability, termed “super red vision,” can also be found in other animals, such as butterflies and mantis shrimp. Looking across the rest of the animal kingdom, colour vision differs greatly between many species and ourselves.

While humans have trichromatic vision (meaning we can see red, green, and blue), the vision of other mammals, like dogs, is dichromatic (only seeing blues and yellows). As such, the concept of human colour vision, to a dog, is analogous to our conceptualisation of dragonfly sight — how can we see through the eyes of something which perceives a different set of colours than we do?

To understand this, it is important to cover the basic physics of vision. Visible light constitutes a very small portion of the electromagnetic (EM) spectrum, with a wavelength ranging from 380 to 750 nanometres (nm). As the wavelength of visible light increases, you progress through the colours of the rainbow, with violet and red corresponding to the shortest and longest wavelengths, respectively. Beyond the range of visible light, you reach near-infrared from 780 to 2500 nm — and at the opposite end of the EM spectrum, ultraviolet light from 10 to 400 nm.

Mammals can visualise wavelengths of light through various types of photoreceptor cone cells in their eyes. Each cone is defined by the presence of a different form of a light-sensitive protein called opsin. Opsin converts the light energy entering the eye into a chemical signal, which is then sent to the brain and processed into a coloured image. As such, the number of different opsin proteins, and by extension cone cells, determines the vision system of the animal. As before, it follows that dichromats have two types of cones, whereas we humans possess three.

It is in insects like the dragonfly where things become complicated. In addition to their 360-degree vision and photoreceptor cells that are slightly different from ours, opsin proteins in dragonflies are far greater — believed to be between fifteen and 33. As such, they can perceive not just visible light, but also polarised light, ultraviolet, and near-infrared. In comparison, our eyesight seems rather pathetic, and therefore, comprehending what the world might look like through theirs becomes all the more intriguing …

To see polarised light is to see with reduced glare. While our vision may be obscured by a reflective brightness covering the surface of a pond, for example, the dragonfly can make out the contents of the water. Inspecting a flower, dragonflies would see dots, lines, and traces across the petals — revealing trails of pollen and insectile footprints — whereas we would simply admire the block of visible colour. In terms of near-infrared (if you are a lover of nature documentaries or ghost hunting programmes), you will be familiar with night vision, where foliage is white, skies are black, and the haze of distant objects is reduced — unlike us, dragonflies do not require a special camera to see this.

Like any biological trait, the dragonfly’s impressive vision system is not without purpose — it is an adaptation that has been specifically evolved. For example, their capacity to see near-infrared enables them to distinguish males and females during flight, permitting the selection of a suitable mate. With their perception of polarised light, finding a body of water suitable for egg-laying is simple.

In sum, dragonfly vision is a testament to the diversity and specialisation of the animal kingdom — something you might not appreciate just by looking.

Illustration by Veronika Sullivan