The Tribune - Spectrum


Sunday, September 28, 2003

Scent of a silkworm
Nutan Shukla

A male silkworm’s antennae are covered with smell receptors
A male silkworm’s antennae are covered with smell receptors

THE ability of certain insects to detect the courtship scents of members of their species is almost supernatural. The unmated female silkworm moth, also known as silk moth, can attract males from a distance of more than 7 km by releasing a pheromone called bombykol ester in the atmosphere (a specific chemical used as a signal by silk moth females to attract males), provided the wind is in the right direction. In the breeding season the female flutters her wings, scattering the scent produced by special glands on her abdomen. It drifts through the air in unimaginably small quantities until its tiny particles come in contact with the feathery antennae of distant males.

Females secrete their sex pheromones in very minute quantities. The amount of quantity can well be understood by the fact that in order to obtain 12 milligrams of bombykol ester the German biologist A.F.J. Butenandt and his colleagues had to get extracts from 2,50,000 silver moths. Each female produces about one-hundredth of a micron (one micron is equal to 1 millionth of a gram) of the scent and the power of this minute amount under natural conditions is amazing. According to experts, theoretically, a single female is capable of producing enough of her magical fragrance to excite more than a million males. However, in practice a female can only signal her presence over a limited area.

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Male antennaes are ever alert to incoming scent, checking and analysing it until precisely the right one is received. Their detection ability is so amazing that the receptor cells on the antennae are, however, capable of receiving, analysing and identifying as little as a single molecule of the enchanting substance, but it takes just 300 molecules to stimulate sexual activity in a male silk moth and to induce him to fly in search of a female, claims German biologist Schneider, who succeeded in determining experimentally the sensitivity of a male moth’s antennae. This level of sensitivity to scent is almost inconceivable to humans.

The male’s feathery antennae are covered with smell receptors, each of which consists of a hollow hair containing two nerve endings immersed in fluid. Pheromone molecules enter the hair through pores on its surface, then travel down a small tube and touch the receptor cell. There are about 60,000 to 65,000 sensory hairs on the insect’s antennae, of which three-fourths are sensitive to bombykol. As the male is aroused by the enchanting substance, he sets off in search of a mate, flying in a zigzag pattern. Meanwhile, he also keeps on comparing the concentrations of the fragrance, which each antennae picks up. By flying in the direction of increasing concentration, he finds his way to the waiting female.

Till the late 19th century, scientists were not aware about how insects find each other. One day his excited son informed a French naturalist, Jean Henri Fabre, who was working on a giant female peacock moth in his laboratory, that his room was full of almost bird-size moths. When the scientist reached his room, he found there were numerous male silkworm moths round the cage that contained a female and all were flapping their wings struggling to enter it. This compelled the naturalist to wonder how the female managed to attract her numerous suitors. He came to the conclusion that it was the scent though he could not support his claim with proofs.

For almost 80 years, nobody believed Fabre. Few years back, chemists not only succeeded in identifying the pheromone, but also confirmed that in the breeding season female silkworm moths release this chemical from their glands and at the same time flap their wings to make it waft into the air and get carried off to other places.