Male moths and pheromone detection - finding females

Male polyphemous moth showing its large antennae

While butterflies are known for their daytime activity and slender antennae, moths are known for the polar opposite.  With broad, feathery antennae and nocternal behaviours, they stand apart from their Lepidopteran relatives.  However, another difference lies in antennae differences between males and females.  Butterflies show no significant difference, while in moths, the males' antennae are much larger than the females'.  Why?

It is all about pheromone detection.  

Females release specific chemicals called sex pheromones from abdominal glands whose sole purpose is to aid in attracting males.  The chemicals may be specific to the species or may be a unique combination of several agents.  The pheromones are released into the air and are dispersed through diffusion and air currents.  The tiny filaments (sensilla) composing the male moths' antennae have receptor sites for that/those specific chemicals.  When stimulated, the brain receives a signal telling the moth which direction to fly in.  Equal concentrations bilaterally indicate movement in the proper direction and increasing concentrations confirm direction and indicate proximity.

Consider that moths are active at night (nocturnal) while butterflies are active during the day (diurnal).  Butterflies rely on vision and tend to fly towards others of their own species.  Subtle differences between males and females give cues as to which is which.  For moths, vision is limited as they fly at night.  This is why pheremones are important.  They can't see prospective mates.  But they can "smell" them.

The word "smell" is appropriate and is not so different from how we detect certain odours.  In the polyphemous moth, females release a particular chemical (trans-6, cis-11-hexadecadienyl acetate), and males detect it.  In the male of the same species, receptors called PBPs (pheromone binding proteins) bind to this specific chemical and cause receptors to react to the combination.  The combined pair breaks down quickly to allow new receptors to take place.

The amazing thing here is that a moth can detect a single molecule.  As concentrations increase, the male continues to alter or maintain course until the female is reached.  It is at this point that mating behaviour begins.

Adult polyphemous moths have no mouth parts.  They cannot feed and have no way to gain energy.  They live only 4 to 10 days at maximum.  Mating has to be done within that window.  The success of the species is attributed to the ability of the male to detect a female, even if it is only one molecule at a time.

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