Big spiders may frighten haunted house visitors this week, but spiders themselves would do well to fear assassin bugs.
A new study has just revealed the bugs' devious and deadly tactics. Like nightmarish bass players, assassin bugs pluck spider silk in webs, mimicking the movements of exhausted, stuck prey. When the hungry spider eases in for what it thinks is a sure meal, the assassin bug taps the spider, and then grabs, stabs and eats it.
The hunting technique, described in the latest Proceedings of the Royal Society B, exemplifies what's known as "aggressive mimicry," when a predator advertises its presence, yet uses deception to lure in its prey.
For the study, Anne Wignall, a researcher in the Department of Biological Sciences at Macquarie University, and co-author Phillip Taylor analyzed the assassin bug's hunting method.
They began by collecting spiders, as well as assassin bugs, on the grounds of their university. They also captured vinegar flies and aphids — usual spider prey — and leaves to simulate debris falling into a spider web.
Spiders and their webs were placed on a vibration-isolating table. Vibrations made in the webs by stuck flies, aphids, spiders-seeking mates and the falling leaves were all recorded and measured. The scientists also measured vibrations made by assassin bugs that plucked the web silk.
"Spiders tended not to respond to leaves and female spiders responded to male courtship by entering a characteristic copulatory position," according to the researchers.
Spiders reacted to the assassin bug silk plucks, however, as they did to struggling prey. Measurements of the pluck vibrations even showed that the "bad vibes" were nearly identical to those made by tired, stuck prey.
"To a spider, the vibrations generated by bugs may resemble small or exhausted prey that cannot mount high amplitude or high frequency struggles," Wignall and Taylor wrote.
Once the spider comes to investigate the plucking, which it senses by touch, the assassin bug will "tap the spider a few times while moving into position and attacking," Wignall noted.
Assassin bugs stalk spiders, too.
"The assassin bugs that stalk the spiders will tap the spider gently and very carefully with their antenna while they slowly move into position above the spider's body to attack," Wignall explained. "Once in position, the assassin bug will quickly stab the spider with its mouth part, called a proboscis or rostrum."
"The spider will usually stop struggling within 10 seconds of being stabbed by the assassin bug," added Wignall.
Studies on the behavior and biology of certain species of assassin bugs, such as those from South America, could help to wipe out Chagas disease, a parasitic infection.
According to the Howard Hughes Medical Institute, the protozoan parasite that causes the disease — and is spread by South American assassin bugs — affects millions of people each year, leading to severe chronic illness and tens of thousands of deaths.
"Anyone can be bitten by an insect while on holiday and unknowingly contract a parasitic disease," said Momar Ndao, laboratory director of the National Reference Center for Parasitology. "For example, people with Chagas disease — caused by a parasite transmitted through the bite of the South American assassin bug — can remain symptomless for two to three decades, while the parasites slowly invade the body's organs."
But some assassin bug species, such as those nicknamed "termite eaters," can benefit humans. These cunning pest-killers use dead termites to trick live ones into approaching them. This talent could make these assassin bugs a handy, natural form of pest control.