Technical | PPC93 December 2018

Scientist and zoology lecturer, Dr Roger Santer, is looking through the eyes of a tsetse fly to ascertain how they see colour. The research will inform the effective trapping of these blood-feeding insects and help to control the spread of a deadly tropical disease in Africa. The research approach Dr Santer describes is directly transferable to the control of other insect pest species.

SPEED VIEW:

• Light detection by the human eye is different to that of many animals
• Understanding how animals see colour can help create more efficient coloured traps for disease vectors
• Tsetse flies can transmit the parasitic micro-organisms that cause sleeping sickness in humans
• Research showed phthalogen blue-dyed cotton is an attractive fabric, but the effect could not be recreated with more useful polyester fabrics
• We can use the fly’s eye view of attractive fabrics to make more effective and robust colour traps.

Colours are as much created by nervous systems as they are physical properties of the world. This means that many animals perceive colour differently to humans, making their perception and use of colour a fascinating area for scientific research. But unravelling the mysteries of how animals see colour is of much more than just academic interest - it can be useful in the fight against disease by helping us to create more efficient coloured traps for disease vectors such as tsetse flies.

For humans, the visible wavelengths of light span from a little under 400nm to about 700nm, and these wavelengths correspond to a spectrum of colour sensations ranging from violet to red. To visualise this spectrum, think of a rainbow.

But to understand how those colour sensations actually come about, we also need to think about the light detecting machinery in the human eye. In the retina are two principal kinds of photoreceptor: the rods and cones. The rods are much more sensitive than the cones and can operate under dim light, but they do not contribute to colour vision – this is why our night vision is essentially greyscale. The cones, meanwhile, only operate under much brighter light conditions and it is their responses that provide the basis for colour sensations.

In humans, there are three kinds of cone cell, each sensitive to light in a different region of the visible spectrum. These are often called the blue, green, and red cones (though abnormalities in the green or red cones are reasonably common, resulting in red-green colour blindness). Rather than detecting the exact spectrum of wavelengths entering the eye, our nervous systems generate colour sensations simply by comparing the relative responses of our three types of cone cell. We can easily demonstrate this with a little experiment.

First, focus your eyes on the centre of the coloured circle below for about 30 seconds. Then focus your eyes on the centre of the blank white square to the right. Hopefully, you saw a ‘phantom’ - a differently coloured circle that slowly faded with time.

What happens here is that as you stare at the first circle, a different one of your cone cell types is strongly excited in the part of your retina looking at each coloured segment of the circle - the blue cones are excited by the blue segment, the green cones by the green segment, and the red cones by the red segment. Consequently, that cone type gets ‘tired out’ by the constant stimulation, and is briefly unable to respond to light as a result (we call this adaptation).

This means that when you move your eyes to look at the blank white square, within the area of your retina that viewed each original coloured segment there are two cone types that have not been excited and are raring to go, and one that is still tired out and cannot be excited. As a result, you perceive a colour that is not really there because of the difference in excitation across those three cone types.

For example, viewing the blue segment tires out your blue cones in that part of your retina, so when you divert your attention to the white area, only the green and red cones are able to become excited. When only those two cone types are excited, our brains interpret that as yellow.

So, colour sensations clearly depend on the light detecting photoreceptors in our eyes, but what you might not realise is that the light detecting machinery in the human eye is different to that possessed by many other animals. If those animals have a different complement of photoreceptor types, they must also have different perceptions of colour.

For example, lots of primates have three cone types like we do, but most other mammals only have two because they only have a single type of photoreceptor sensitive to longer wavelengths of light - they do not have separate green and red cone types. So, despite the common saying, red rags really don’t stand out strongly to a bull’s eye view!

The eyes of insects differ further still from our own. For example, bees have three photoreceptor types, but these are sensitive to UV, blue, and green light; and the humble housefly has five main kinds of photoreceptor spanning UV, blue, and green regions of the spectrum. But why should we care about the colour vision of flies?

Well, the tsetse flies of sub-Saharan Africa are blood-feeding flies that can inflict a nasty bite. These bites also transmit the parasitic micro-organisms that cause sleeping sickness in humans, and a similar disease called nagana in cattle. Sleeping sickness is fatal if not properly treated, and because there are no vaccines or prophylaxes to prevent infection, controlling tsetse is an important part of controlling the disease.

For the species of tsetse that transmit the majority of sleeping sickness, colours are the main attractant used to lure them towards control devices. Tsetse control devices are constructed from coloured fabrics. These include traps and simple insecticide-impregnated targets. In each case, the coloured fabric attracts tsetse to the device, where they are either caught or dosed with insecticide when they land upon it.

Decades of entomological field research identified phthalogen blue-dyed cotton as an extremely attractive fabric to tsetse flies, and therefore the best one from which to make traps and targets.

However, modern polyester fabrics are lighter, more robust in the field, and carry insecticide more effectively, so they have superseded cotton as the material of choice for tsetse control devices.

Unfortunately, phthalogen blue dye cannot be applied to polyester, so alternative blue dyes have to be used. But here’s the catch: field trials comparing the attractiveness of phthalogen blue-dyed cotton and various blue polyesters have shown that the former attracts up to twice as many tsetse as the latter.

The key to understanding this problem is to realise that what appears blue to the human eye is irrelevant to the tsetse because it perceives colour using a completely different complement of photoreceptors. Luckily, fly photoreceptors have been so extensively studied that it is possible to calculate how they would respond to the spectrum of light reflected from a fabric sample - essentially, we can use mathematics to take the fly’s eye view of the fabrics used to attract them. Applying these methods to extensive measurements of tsetse attraction to differently coloured fabrics from previous field studies has allowed us to model how each of a tsetse’s five main types of photoreceptor likely contributes to attraction.

Essentially, we have created models that explain the visually guided behaviour of tsetse using the visual information that is actually available to the fly’s nervous system. This analysis also suggests important differences in the way that some blue polyesters and phthalogen blue cottons appear to tsetse. Building on this work, we’re now going a step further. Because our models tell us which photoreceptor signals increase attraction, and which decrease it, we are now developing polyester fabrics that excite the fly photoreceptors even more effectively. In this way, we hope to create highly attractive polyester fabrics that can increase the attraction of tsetse to control devices and thus improve the effectiveness of tsetse control.

This work has real potential to improve the health and wellbeing of poor, rural communities in sub-Saharan Africa by providing more efficient devices for the control of tsetse flies and prevention of sleeping sickness. Sometimes intriguing scientific questions can lead to unexpected but important applications!

About Roger

Roger Santer is a lecturer in zoology at Aberystwyth University, joining in 2010. Following his PhD in invertebrate neuroethology at Newcastle University, he held research positions at Newcastle University (2003-2006), and the University of Nebraska-Lincoln (2006-2008), and a lectureship in biology at the University of Limerick (2008-2010). Roger’s interest is in animal behaviour and the neural mechanisms that underlie it. His current research is on visually-guided behaviour, mainly conducted in insects and arachnids using a range of electrophysiological, behavioural, and computational techniques.

This article was originally published in Aberystwyth University magazine, PROM
aber.ac.uk

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