Tobacco hawkmoths have come a long way since the days when they were a pest. Richard White remembers that their taste for tobacco leaves was once notorious, but scientists soon realised the insect's were easy to work with,because of their size. Hawkmoths soon began making regular appearances in the literature, especially neurobiology, and White has spent much of the last twenty years working on the insect's visual system. He explains that the insects don't seem to integrate visual information from their compound eye in the same way as mammals. Photoreceptors in distinct regions of the insect's eye structure direct specific behaviours, although little is known about the neural systems that link the receptors to the behaviours they control. `We know some aspects of the inputs and the outputs, but it's difficult to get in between' says White. So mapping the distribution of photoreceptor cells in the eye of the tobacco hawkmoth seemed like a good place to start unravelling the complexities of insect vision(p. 3337).
Unlike butterflies, which forage in the full light of day, hawkmoths forage after dark, viewing the pale flowers that they feed on with three visual pigments tuned to blue, green and ultraviolet wavelengths. So to get a high-resolution glimpse of the photopigments' distributions throughout the moth's eye, White turned to immunocytochemistry, to map the position of each photoreceptor with antibodies.
First, his team had to generate antibodies that were sensitive enough to detect the difference between the closely related rhopsin molecules. Huihong Xu set about the tricky task of cloning and expressing the three proteins, but cloning isn't always straightforward, so the team changed tack, raising antibodies against peptides derived from the three proteins. This time they had more success. With the antibodies in hand, White and his team began probing the moth's retina. But again the team ran into a technical hitch. The moth's eye is intrinsically fluorescent, ruling out the use of fluorescence labels, so the team resorted to mapping the photopigments' distributions with coloured stains and microscopy. Armed with the tools of the technique, the team began mapping the distributions of all three photopigments, in the 27,000 ommatidia that make up the insect's eye.
At first the team focused on the structures of the individual ommatidia,identifying three photoreceptor cell types within each eye structure's retinula. Then they looked at the distribution of photopigments across the whole eye. There was a clear pattern. Although the receptors that were tuned to green were uniformly distributed across the eye, the ultraviolet and blue receptors seemed to be more localised, with the blue receptors located exclusively in the ventral half of the insect's eye. Which is exactly the same place that the image of a flower would fall on the foraging insect's photoreceptors. White explains that hawkmoths are strongly attracted to feed at blue objects, and blue light is just one of the many wavelengths reflected by white flowers, so having the receptors that trigger feeding located at the same point as the image of their favourite dining opportunity makes perfect sense for a moth foraging in its dim nocturnal world.