Spiders ‘Smell’ Using Their Legs, Research Finds

Most spiders have eight legs, no ears and use fine leg hairs connected to nerve cells or their webs to hear sounds in their environment. But how do they smell?

A new study by an international team of researchers has found that male spiders “smell” with their legs, using olfactory hairs — wall-pore sensilla — as their “nose” to detect sex pheromones released by females.

“Spiders have always lived alongside humans, so it’s surprising how much we still don’t know about them. One long-standing mystery was related to how spiders detect smells. Now, our latest research has finally uncovered the secret,” Dan-Dan Zhang, one of the study’s authors and a sensory biology researcher at Lund University, wrote in The Conversation.

Other contributors include Gabriele Uhl, a professor of general and systematic zoology at the University of Greifswald, and Hong-Lei Wang, also a sensory biology researcher at Lund.

The discovery came after a decade of searching for the sensilla, which the research team identified and mapped.

The study, “Olfaction with legs — Spiders use wall-pore sensilla for pheromone detection,” was published in the journal Proceedings of the National Academy of Sciences.

Spiders have been evolving for approximately 400 million years and are known for their sense of vibration. Some species, like jumping spiders, also have excellent vision, but their sense of smell remained a mystery.

There was ample scientific evidence that spiders have the ability to detect odors like sex pheromones, but there were still two main questions. The first was, what is the primary olfactory organ spiders use to detect scent? The second was built upon previous studies suggesting that spiders did not have wall-pore sensilla, leading scientists to wonder how they were able to smell anything.

“Our study has solved these long-standing questions. We discovered previously overlooked wall-pore sensilla on the walking legs of male wasp spiders (Argiope bruennichi) and demonstrated that they can use them to detect airborne sex pheromones with high sensitivity,” Zhang wrote in The Conversation. “We showed that the wall-pore sensilla are not unique to wasp spiders but are prevalent across the spider tree of life.”

The team looked at female and male A. bruennichi spiders using high-resolution electron microscopy. They found that all male spiders’ walking legs had thousands of wall-pore sensilla and discovered specific features of the sensilla. They determined that they are distinct from those of insects and other arthropods.

The sensilla are found on the upper region of the legs of male spiders, close to the body, in areas that rarely come into contact with the surface as spiders walk, mate or capture prey.

“This distribution pattern already suggested the role of wall-pore sensilla in detecting airborne odours (olfaction). Interestingly, wall-pore sensilla were found exclusively in adult male spiders, not in juvenile males or females, which strongly indicates their function in mate searching and recognition,” Zhang and colleagues wrote in The Conversation. “A. bruennichi is one of the few spider species in which the chemical structure of the sex pheromone has actually been identified. Female spiders release gaseous pheromones that attract males from a distance.”

The researchers tested whether a pheromone compound would trigger a response in the wall-pore sensilla. They used a microscope to observe live male spiders, placing a recording electrode into one of their wall-pore sensilla.

“We then exposed each sensillum to a puff containing the pheromone compound. We found that even a tiny amount of the pheromone compound — just 20 nanograms — was sufficient to elicit a clear response as a burst of activity in neuronal cells from a wall-pore sensillum, and the response became stronger as the dose increased. We consistently observed the response of wall-pore sensilla to the pheromone compound, regardless of which leg pair was tested,” the researchers wrote.

Contact and non-contact areas on body appendages of a male A. bruennichi. Slow-motion footage (500 frame per second) of a male walking on a horizontal skewer. The video highlights that only the distal leg segments make contact with the surface. PNAS

The results demonstrated that the olfactory sensilla of spiders are extremely sensitive in comparison with insects’ most responsive sex pheromone communication systems. Spiders have thousands of sensilla on each of their walking legs, enabling males to pick up even faint traces of airborne sex pheromones.

To get a better sense of the broader world of wall-pore sensilla, the research team looked at 19 other species of spiders across 16 families. They found that, in most species, males have wall-pore sensilla.

They also discovered that “basally branching spider groups” like basal trapdoor spiders in Asia did not have sensilla. The pattern the researchers uncovered suggested sensilla have evolved independently within spiders multiple times, but had been lost in some lineages.

“Our study paves the way for exciting future discoveries about how spiders perceive the world through olfaction. Many intriguing questions await further investigation,” the trio of scientists wrote in The Conversation. “How do female spiders smell without wall-pore sensilla? And beyond sex pheromones, what other chemicals can spiders detect and how are these relevant to their behaviour and ecology? Also, what is the molecular and neural basis of spiders’ olfaction? Finally, how has the sense of smell evolved across the vast diversity of spider species?”

“These questions set the stage for an exciting new chapter in our understanding of spider biology,” they said.