A male spider of a web-spinning species faces a dilemma. How can he convince a female on her web that he’s a mate, not a meal? The males of many spider species succeed by plucking a mating song on the female’s web strings. First, the male must announce his presenceknocking on the door as it were. If he gets no response, he’ll keep trying for days, sometimes waiting until the female completes her last molt and becomes sexually mature.
When he finally receives his invitationa series of twitches and jerks of the web from the femalehe approaches carefully. Because web-spinning spiders see poorly, the male continues his plucking patter right up to mating. Each species has a specific way of announcing and approaching, so the female knows the vibration comes from a male of her species.
Or so she thinks. Certain spiders, called pirates, creep onto the web of a waiting female of a different species. There they vibrate their bodies and twitch their legs in a precise imitation of a male seeking to mate. When the fooled female approaches, the intruder turns the tables and eats her. Here the communication has gone awry for the receiver. The sender benefits with a meal. Mimicking spiders, which belong to a variety of species, can also mimic the motions of a struggling insect caught in the prey spider’s web. Despite the fact that scientists call this deceptive signaling, pirate spiders have no conscious intent to deceive, and although they can tailor their tactics depending on the species under attack, they can only prey upon a small range of species.
Other species, such as the Mexican Cupiennius salei, spin no webs, but still sense the world via vibrations. Like their web-spinning relatives, Cupiennius sits and waits for prey to pass by. But it only emerges from its banana-leaf hideout at night. How then to “see” prey? The final segment of the spider’s leg acts a little like a shock absorber, bending slightly to take up vibrations of whatever substrate the spider rests on. An insect walking by inadvertently signals its presence with a signature vibration. To the spider, this says “dinner time.”
Detecting the dinner bell among a cacophony of vibrationsrain drops, large animals, even cars and humansis no easy task. Scientists think the spider depends on the precise frequency of vibrations. Cars shake the leaf at a low frequency, cockroaches at a much higher one. The spiders detects food in somewhat the same way we distinguish a dinner bell from the rumble of a passing bus.
With such a finely honed sense of vibrational frequency, it makes sense that the spiders can detect mates. But Cupiennius carries vibrational signaling far beyond mere mate detection. If a male catches a whiff of female scent, he begins a frenetic courtship dance, shaking his backside like an eight-legged Elvis. He sends vibrations, a sort of love poem, throughout the leaf in pulses punctuated by pauses. A female politely “listens” to the whole poem before responding with a short shake, turning on the porch light as it were. If the poem, however, is foreignthe product of a different speciesthe female remains silent. By recording the vibratory love poems of half a dozen closely related Cupiennius species, scientists detected differences in the lengths of pulses and pauses. Playing back the vibrations to a female, the scientists never saw her respond except to the poem of her own species.