By Scott Solomon, Rice University
One of the most common ways in which insects communicate with one another is through pheromone communication. Like hormones, pheromones are chemicals that are made in specialized cells within a gland and have an effect elsewhere. But while hormones have an effect somewhere in the body of the organism itself, pheromones are produced by one individual and have an effect on another individual.
Experiments by Jean-Henri Fabre
The French naturalist Jean-Henri Fabre, who had a flair for clever experiments and provocative writing, discovered the power of pheromones by accident. After capturing a female moth and keeping it alive in a small enclosure in his home in southern France, he was shocked to later find his house filled with male moths that had flown in through an open window.
Noting that they had arrived at night when it was dark outside, Fabre suspected that the male moths were attracted by a chemical cue given off by the female. “Are there,” he wrote, “in point of fact, effluvia similar to what we call odor… absolutely imperceptible to ourselves and yet capable of impressing a sense of smell better-endowed than ours?”
To test this idea, he put female moths in different types of enclosures, some of which were tightly sealed and some of which had small openings. No males arrived at the enclosures of females kept in tightly sealed containers, but females kept in enclosures with even the slightest opening were swarmed by males.
In another experiment, Fabre kept a female moth in a wire mesh container overnight, then transferred her to a glass container the next day. The male moths swarmed around the open container where the female moth used to be but ignored the container where she could be seen but not smelled. Fabre concluded that there must indeed be a chemical that attracts the male moths to the female.
This article comes directly from content in the video series Why Insects Matter: Earth’s Most Essential Species. Watch it now, on Wondrium.
Pheromones Get Potential Mates Excited
Fabre’s idea that moths use chemicals to find mates remained unproven until 1959 when a German biochemist isolated a substance called bombykol from female silkworm moths. Male moths that were presented with a pure solution of bombykol began rapidly flapping their wings, performing what’s known as a flutter dance—an indication of their interest in mating.
So Fabre was right—moths do use chemicals to find mates. The term pheromone was coined in 1959, the same year that bombykol was isolated. It was the beginning of a golden age in the study of how chemicals influence insect behavior.
If You Smell Dead, You’re Dead
In 1958, an American naturalist named Edward Osborne Wilson had reported a discovery of another way that chemicals control insect behavior. Wilson was fascinated by the social behaviors of ants and wondered whether these behaviors might be controlled by chemicals. One behavior that Wilson was curious about was the way that dead worker ants are removed from the colony and deposited in a refuse pile.
Removing dead ants from the nest makes sense—they could spread disease—but Wilson wondered what cues the ants used to determine that an ant had died. He soaked pieces of paper in an extract made from different chemicals isolated from dead ants and gave each piece of paper to live ants.
Wilson found that the live ants treated the paper soaked in one particular chemical, oleic acid, as if it were a dead ant by carrying the paper to the refuse pile. Other items, like food, were also treated as dead ants if they were soaked in oleic acid. Even live ants soaked in oleic acid were carried off to the refuse pile—never mind that their legs were still moving!
Other Examples of Pheromone Communication
Wilson and colleagues would soon find other chemicals that act as pheromones and play important roles in ant behavior. Alarm pheromones are used to signal an emergency, like a predator digging into the nest. Because the alarm pheromone released by one individual can serve as a message to many other individuals, each of which can in turn release additional alarm pheromone, the message spreads very quickly through the colony.
Some ants also use trail pheromones to let their nestmates know where they’ve been. Trail pheromones can be used as a trail of breadcrumbs to make it easier for foraging ants to find their way home. Trail pheromones can also make finding food more efficient by helping foraging workers to take the shortest path from their nest to a source of food.
Ants generally follow the path with the strongest pheromone trail, so even if they choose their path randomly at first, ants following behind them will end up choosing the shortest path because, being shorter, more workers will have traveled along it over a given amount of time causing it to accumulate the most pheromone.
Sometimes ants from neighboring colonies find the same food. This often leads to a fight, and worker ants use alarm pheromones to recruit backup. Like baseball or hockey players clearing the bench when a fight breaks out, ants come pouring out of each nest to help in the skirmish.
Common Questions about Pheromone Communication in Insects
Moths use chemicals to find mates. There is a chemical called bombykol that attracts the male moths to the female. In an experiment, male moths that were presented with a pure solution of bombykol began rapidly flapping their wings, performing what’s known as a flutter dance—an indication of their interest in mating.
Ants that sense oleic acid believe that they have encountered a dead ant. In such a case, they drag it to the refuse pile of the colony. Pheromone communication, in this case, can be so powerful that it wouldn’t even matter if an ant is moving because if others sense the oleic acid on it, they believe it to be dead.
Ants use trail pheromones to let their nestmates know where they’ve been. Trail pheromones make it easier for foraging ants to find their way home. They can also make finding food more efficient by helping foraging workers to take the shortest path from their nest to a source of food.
