A team of researchers at the National Centre for Biological Sciences (NCBS) in Bangalore seems to have cracked the mystery as to how insects smell. Understanding the mechanisms underlying odour detection by insects may seem to be of pure academic interest, but the potential applications that may follow are tremendous.

“Its importance lies in the fact that insect behaviour, to a large extent, is governed by the sense of smell, ranging from finding a mate to searching for food or an appropriate place to lay eggs,” says NCBS scientist Gaiti Hasan who, along with her senior colleague Veronica Rodrigues, resolved the puzzle that has baffled scientists for decades.

Many deadly pests that ravage agricultural crops such as the ballworm or vectors that spread several infectious human diseases such as mosquitoes are insects. Significantly, odours play a more vital role in the lives of insects than in any other species of the animal kingdom.

“Understanding how the brain of an insect processes smell can lead to better odour-based insect repellents or effective strategies to control agricultural pests and insects that transmit diseases,” says Hasan.

What is significant about the work of the NCBS researchers, reported recently in the Journal of Neuroscience, is that it is for the first time that scientists have spotted a complementary smell detection mechanism. In other words, the NCBS scientists have found that the fruit fly — used as a model organism — employs two different routes to take olfactory receptor neurons to the brain for processing. Olfactory receptor neurons — which can be described as the workhorse of the nose — function as sensory cells, detecting a myriad of odours around them. They also serve as nerve cells and translate the chemical fingerprint of a smell into electrical signals that can be interpreted by the brain.

While it has a predominant G-protein pathway that is quite similar to those found in mammals, there is another parallel route that helps insects to detect odours. Knocking off the predominant sensory pathway — by breeding fruit flies that lack antenna — reduced the electrical response of the olfactory sensory neurons to odours by about 80 per cent. But their brain studies showed that electrical signals relating to smell still reached the brain, albeit feebly.

Insects like fruit flies are able to recognise and discriminate among a large number of distinct odours because they are endowed with more than 1,300 different olfactory receptor neurons, nearly four times more than in human beings.