Silk moths don’t have a nose, they smell with their antennae. But while the sense of smell of human men and women is developed in a relatively similar way, male and female moths live in completely different olfactory worlds, as researchers have discovered: The antennae and the olfactory hairs on them of male silk moths are highly specialized for sniffing out the pheromones of female conspecifics – Females, on the other hand, cannot detect their own smell at all.
“Since female silk moths cannot smell their own pheromone, it has long been suspected that their long olfactory hairs also have a specific function that is only found in females. A female’s only job is to find a suitable plant to lay her eggs after mating. It was therefore assumed that the long olfactory hairs of the females were specialized for the attractive scent of mulberry trees,” says Sonja Bisch-Knaden from the Max Planck Institute for Chemical Ecology in Jena.
The researchers tested this assumption by examining the reaction of silk moth olfactory hairs to various scents. To do this, they use natural scent mixtures such as those found in the leaves of the mulberry tree, caterpillar droppings or the body odor of butterflies.
It turned out that female silk moths actually smell the scent of mulberry leaves through their half-long olfactory hairs. But Sonja Bisch-Knaden’s team made another interesting discovery during their investigations: “What was surprising to us was that the nerve cells in the long olfactory hairs of the female silkmoth are not specialized for the perception of the host plant, as expected, but that one of the two nerve cells in long olfactory hairs is very sensitive to fragrances such as isovaleric acid and benzaldehyde.”
Isovaleric acid and benzaldehyde are scent components of silkworm feces. This appears to be an important source of information for some female silkworms. The researchers were able to show that the scents associated with caterpillar droppings had a stronger effect on mated females than on their virgin colleagues. While mated females were deterred by the smell, virgin females showed a specific reaction. The smell of feces probably helps the females to avoid mulberry trees that are already occupied by many caterpillars when laying their eggs.