Assassin bugs tap spiders to distract them before a lethal strike

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Assassin bugs live up to their name. The insects expertly stalk and feed upon other small invertebrates, jabbing them with a venomous proboscis. Some species even hunt spiders and use a strange trick to gain the upper hand.

Using their antennae, assassin bugs tap spiders, which appears to discombobulate the arachnids long enough to let the bugs make a toxic strike, researchers report September 29 in the Biological Journal of the Linnean Society. The findings provide insight into some of the sophisticated hunting tactics that predators evolve when targeting dangerous prey (SN: 8/4/21).

Thread-legged bugs are a subfamily of particularly gangly assassin bugs, and some species spend their lives in a place most insects avoid: spider webs. The bugs silently creep along the spider’s silk, taking care to make their vibrations seem benign before violently dispatching the web’s architect, seizing the spiders with their front legs and injecting them with venom. 

While watching two species of Stenolemus assassin bugs hunt spiders, ecologists Anne Wignall and Fernando Soley took note of the bugs’ habit of lightly knocking their antennae on spiders once the bugs were within striking distance. 

“It struck us early on that tapping prey was a really strange thing to do,” says Wignall, of Massey University in Albany, New Zealand. Spiders could easily defend themselves in a lethal fashion. “Watching [the bugs] spend so much time and effort on stealth, only to essentially tap [the spiders] on the shoulder was absolutely fascinating.”

To figure out why the bugs tap, Wignall and Soley, of the Organization for Tropical Studies in San José, Costa Rica, tested the behavior of 30 web-building cellar spiders (Pholcus phalangioides) in the laboratory. The researchers replicated the bugs’ antennae tapping by gently brushing the spiders’ leg with a dog hair. After the tapping, the team measured the spiders’ responses to a vibrating tuning fork placed on the web, mimicking a struggling insect. 

Tapped spiders were far less aggressive than those that weren’t tapped, fully ignoring the fork four times as often. Tapped spiders also attacked the fork about 25 percent as often as their untapped counterparts. 

Wignall thinks that the assassin bugs are reducing spiders’ aggression levels by imitating the types of physical touch that these typically solitary spiders experience near fellow spiders. “Whenever they do come across another spider, it’s usually because it’s a close relative such as a sibling in the nest, or a potential mate. Both of which are situations in which aggression would not be a good idea,” she says. 

Invertebrate zoologist Ondřej Michálek of Masaryk University in Brno, Czech Republic, who was not involved with the study, says that the researchers have a “completely valid hypothesis.” Many spiders avoid cannibalism by using special tactile signals that could be copied by adroit predators, thereby deceiving the arachnids, he says. 

Going forward, Wignall wants to understand the tapping in greater detail, determining how many taps are sufficient to pacify a spider and the number of spider species that the bugs are manipulating.

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