Pheromones are chemical substances released by an organism that influence the behavior or physiology of other individuals of the same species. These chemical signals allow communication within a species without direct contact or sound. Like many other animals, spiders extensively use these chemical messages to navigate their world and interact with conspecifics. This system of chemical communication plays a role in various aspects of spider life.
Pheromones in Spider Reproduction
Reproduction is a primary context for pheromone use among spiders, facilitating mate attraction and selection. Female spiders commonly release sexual pheromones, which can be airborne to attract males from a distance or contact-based, deposited on silk or their cuticle, to stimulate close-range courtship. These chemical signals convey information about the female’s reproductive status, including maturity, receptivity, and species identity, helping prevent inter-species mating.
For instance, the female desert spider secretes a pheromone called 8-methyl-2-nonanone, which strongly attracts males and initiates courtship. Similarly, female orb-weaving spiders, such as the wasp spider Argiope bruennichi, emit volatile sex pheromones like trimethyl (2R,3S)-methylcitrate from their bodies and silk to lure males. In species like the garden cross spider Araneus diadematus, males are attracted to odors of adult females, with candidate compounds including sulcatone and various long-chained alkanes and alcohols.
Male spiders are often most attracted to adult virgin females or juvenile females nearing their final molt. This preference suggests an advantage for the first male to mate, possibly due to sperm priority. Male spiders, like wolf spiders, also produce pheromone blends to entice females and trigger courtship. Female spiders can control pheromone emission, such as by manipulating the pH of their silk in false black widow spiders, influencing attractiveness and mating timing.
Upon detecting a female’s pheromone, males engage in species-specific courtship rituals, involving leg-waving, abdomen-swinging, and web-plucking. If receptive, the female positions herself for mating; otherwise, she may signal disinterest by shaking her web or moving away. These chemical cues ensure males identify a suitable mate, promoting successful reproduction.
Pheromones for Social and Defensive Behaviors
Beyond reproduction, spiders use pheromones for social interactions and defensive strategies. Aggregation pheromones draw spiders together, supporting communal behaviors like overwintering or forming shared webs for communal hunting in social species. The African social funnel-web spider Agelena consociata uses volatile pheromone cues that affect aggregation, although individuals may not discriminate between their own and unfamiliar conspecific nests.
Alarm pheromones serve as warning signals, released when a spider perceives a threat or is injured, alerting nearby individuals. These signals can induce defensive responses, such as raising front legs, retreating, or coordinating collective actions within a group. Social spiders, including the African social velvet spider, use alarm pheromones to communicate the presence of predators, enhancing collective safety.
Spiders also use territorial pheromones to establish and defend their living spaces. By marking webs or surrounding areas, spiders communicate ownership and deter intruders. These signals can convey information about the spider’s size, sex, and reproductive status, allowing other spiders to assess whether to engage in a territorial dispute. For instance, males of the orb-weaving garden spider may deposit pheromones on a female’s web to indicate presence and intentions.
Some spider species use pheromones to attract prey, releasing volatile chemicals that mimic the scent of food sources. These pheromone-laden silk threads can lure insects into traps, maximizing hunting success.
How Spiders Detect Pheromones
Spiders possess specialized sensory adaptations to detect chemical signals. Their primary mechanisms for sensing pheromones involve chemoreceptors, sensitive structures found predominantly on their legs, particularly on the tarsi (leg tips) and pedipalps. These receptors allow spiders to “smell” or “taste” chemical cues in the air, on silk threads, or other surfaces.
Research has identified specific types of sensilla, hair-like sensory structures, responsible for pheromone detection. Male spiders, for example, use thousands of “wall-pore sensilla” on their walking legs to detect airborne sex pheromones. These sensilla are positioned to avoid constant contact with surfaces, ensuring detection of volatile chemical signals from a distance.
Another type, “tip-pore sensilla,” found on the lower legs, is involved in contact chemoreception, allowing spiders to “taste” chemical cues directly from a substrate. These male-specific wall-pore sensilla are absent in females and subadult males in many species, highlighting an evolutionary adaptation linked to mate searching. The sensitivity of these sensilla is remarkable, with some studies showing a clear neural response to as little as 20 nanograms of a pheromone compound. Spiders often enhance pheromone detection through specific leg movements, such as tapping or waving their legs in the air or on surfaces. These movements help collect and channel airborne chemical molecules towards chemoreceptors, or directly sample contact cues from the substrate.