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?<\/p>\n
A new study by an international team of researchers has found that male spiders \u201csmell\u201d with their legs, using olfactory hairs \u2014 wall-pore sensilla \u2014 as their \u201cnose\u201d to detect sex pheromones released by females.<\/p>\n
\u201cSpiders have always lived alongside humans, so it\u2019s surprising how much we still don\u2019t know about them. One long-standing mystery was related to how spiders detect smells. Now, our latest research has finally uncovered the secret,\u201d Dan-Dan Zhang, one of the study\u2019s authors and a sensory biology researcher at Lund University, wrote in The Conversation.<\/p>\n
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.<\/p>\n
The discovery came after a decade of searching for the sensilla, which the research team identified and mapped.<\/p>\n
The study, \u201cOlfaction with legs \u2014 Spiders use wall-pore sensilla for pheromone detection,\u201d was published in the journal Proceedings of the National Academy of Sciences.<\/p>\n
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.<\/p>\n
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.<\/p>\n
\u201cOur 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,\u201d Zhang wrote in The Conversation. \u201cWe showed that the wall-pore sensilla are not unique to wasp spiders but are prevalent across the spider tree of life.\u201d<\/p>\n
The team looked at female and male A. bruennichi spiders using high-resolution electron microscopy. They found that all male spiders\u2019 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.<\/p>\n
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.<\/p>\n
\u201cThis 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,\u201d Zhang and colleagues wrote in The Conversation. \u201cA. 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.\u201d<\/p>\n
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.<\/p>\n
\u201cWe then exposed each sensillum to a puff containing the pheromone compound. We found that even a tiny amount of the pheromone compound \u2014 just 20 nanograms \u2014 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,\u201d the researchers wrote.<\/p>\n
The results demonstrated that the olfactory sensilla of spiders are extremely sensitive in comparison with insects\u2019 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.<\/p>\n
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.<\/p>\n
They also discovered that \u201cbasally branching spider groups\u201d 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.<\/p>\n
\u201cOur study paves the way for exciting future discoveries about how spiders perceive the world through olfaction. Many intriguing questions await further investigation,\u201d the trio of scientists wrote in The Conversation. \u201cHow 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\u2019 olfaction? Finally, how has the sense of smell evolved across the vast diversity of spider species?\u201d<\/p>\n
\u201cThese questions set the stage for an exciting new chapter in our understanding of spider biology,\u201d they said.<\/p>\n