17,000 Sensors: How a Moth Smells a Single Molecule

An Antenna Built for One Molecule

The antennae of a male silk moth carry roughly 17,000 trichoid sensilla — hair-like sensory structures — specifically tuned to bombykol, the pheromone released by females.

Seventeen thousand detectors, on an animal that cannot fly, does not feed, and has one thing to accomplish before it dies.

Single-Molecule Sensitivity

The number that should stop you: it has been estimated that a single molecule of pheromone may be sufficient to activate an olfactory neuron.

Not a concentration. Not a plume. One molecule arriving at one sensor is at the threshold of producing a response. It is difficult to build a detector more sensitive than that, because there is nothing smaller to detect.

Two Receptors, and One Is a Brake

Each sensillum houses a pair of olfactory receptor neurons, expressing two different receptors:

• BmOR1 — responds to bombykol, the attractant
• BmOR3 — responds to bombykal, a related compound

Here is the elegant part. Females emit bombykol as the attractant along with a small amount of bombykal, which negatively modulates the response to bombykol. The signal has a built-in brake, sent by the same animal doing the attracting.

A system with an inhibitor is far harder to spoof than a system with only an accelerator — and far more precise about which species is answering.

Getting the Molecule to the Receptor

A pheromone is oily and the sensillum interior is watery, so something has to ferry it across. That job belongs to pheromone binding proteins, concentrated in the sensillum lymph.

They do more than carry. BmorPBP changes shape depending on pH, and that conformational switch allows highly selective transport — meaning the carrier protein is itself part of deciding which molecules reach the receptor. Discrimination happens before detection.

Why It Is Feathered

The famous plumed antennae of a male moth are surface area. Seventeen thousand sensors need somewhere to sit, and they need to be presented to as much passing air as possible. The shape is the sampling strategy.

Related: bombykol · the silkworm brain