By Victoria Gill
Science reporter, BBC News
The silkworm moth is a "model organism" for studying insect pheromones
X-rays have revealed the structure of a protein that shows how insects smell.
The protein, found in the antennae of silkworm moths, is involved in their detection of pheromones - chemical signals that affect insects' behaviour.
Discovering exactly how insects "smell" these chemicals could help to develop new methods of pest control, and insect repellents for humans.
Scientists at the UK's Rothamsted Research and the University of London carried out the study.
The structure was the 100th to be solved using the UK's Diamond Light Source synchrotron.
The synchrotron generates intense beams of electrons that can be used to probe structures down to the molecular level.
Studying this odour-binding protein has revealed where exactly on its structure the pheromone molecule binds.
"We know that [the moth's] odorant binding proteins pick up pheromones at pores on the outside of their antennae and carry them through a watery layer to nerve endings," said Dr Jing-Jiang Zhou, the senior researcher at Rothamsted who led this study.
But how exactly the proteins bind these pheromone molecules is still "a bit of a mystery", according to Professor Linda Field, head of the insect molecular biology group at Rothamsted.
"We know they have to bind [together] before they trigger a reaction at the receptor, but looking at how they interact is difficult."
The team wanted to know if the pheromone molecules were simply transported to a receptor by the protein, or if they formed a protein-pheromone complex.
See the light
The protein changes its structure when it is bound to a pheromone
The x-ray images revealed that the structure of the protein changed when it was bound to a molecule of the moth pheromone, bombykol.
From this, the researchers surmised that this protein-pheromone complex triggered the olfactory (or smell) receptor and, in turn, the mechanism that allows the moth to process the odour.
Plants release these chemical odours to attract insects.
By studying this mechanism the scientists hope to develop more effective methods of pest control that are less toxic to other organisms - by directly targeting the pests' sense of smell.
"In the long term we want to find how this [binding] works and to block it," Professor Field told BBC News. "My research team is working on mosquitoes and aphids to develop systems to protect both crops and humans."
Professor Field explained that insects were "incredibly sensitive" to pheromones and host odours.
"A male moth is able to locate a female from miles away and a mosquito can find a human in a field full of cows," she said.
Rob Lind, senior technical specialist from crop protection company Syngenta, explained that targeting these pheromones was a safe and useful method of controlling specific pests.
"It is already used to control codling moth in apples," he said. "We use pheromone disruption - odours that confuse the males so they can't find the females. The females then lay infertile eggs so the crop is protected.
"It's like pest contraception."
Dr Lind explained that pheromones and odour-binding molecules are could be thought of as locks and keys.
"These are very specific keys and each species has its own particular one.
"In the future, if scientists can find out how they keys work, they could design new ones that fit better."
Dr Zhou concluded: "It's not just the farming community which stands to benefit from this work.
"These new insights will be fed into the development and refinement of biosensors where detection sensitivity is paramount - in areas like blood tests."