New research shows these ferocious insects don’t just hunt like robots.
Cara Giaimo – Science – 14 May 2020
“A praying mantis watches intently as a fly bobs by. In less than a blink, she’s snatched it up. When the tape is played back in slow motion, we see the mantis pause and calibrate, almost like an experienced baseball catcher who has realized she’s dealing with a knuckleball.
It’s an impressive highlight reel. As detailed in a paper published this week in Biology Letters, it’s also evidence that mantises strike less like automatons and more like active hunters, calibrating their attacks to more efficiently capture their prey as it flies by at different speeds.
Predatory animals are traditionally divided into two categories based on how they catch their meals. The first group, pursuit predators, run down their prey. The action heroes of the animal world, they tend to be flashy and attention-getting, whether they’re large like cheetahs or tiny like robber flies. Researchers have uncovered “extraordinary examples of how flexible their pursuit can be,” said Sergio Rossoni, who performed the new study as a master’s student at the University of Sussex, and is now a zoology doctoral student at the University of Cambridge.
The second group, known as sit-and-wait predators, skulk until the time is right, and then, bam — they strike. In the past, such predators were “thought to be quite stereotypical in their behavior,” Mr. Rossoni said, almost like windup toys. Researchers had described praying mantis strikes in particular as always occurring “at the same rate with the same movements,” he said.
Recently, though, this view has been called into question. Work on mantis shrimp, which deploy an ultrafast punch to smash their prey, has shown that they are able to vary their strike speed, and a 2016 study of praying mantises found that they displayed flexibility when “catching” stationary bugs. Mr. Rossoni and his then-supervisor Jeremy Niven, a zoology professor at the University of Sussex, decided to test praying mantises further, and see whether they varied their approach with slow or speedy prey.
For his experiment, Mr. Rossoni placed one Madagascan marbled mantis at a time on a raised platform underneath a bright light. (Other species preferred to hunt upside-down, which made filming difficult.) He then swung a target — either a dead bug, or a bead that looked like one — toward the mantis on a transparent wire.
The target could move at three speeds, each meant to approximate a different type of mantis prey. The slowest, 200 millimeters per second, is the average flight speed of a fruit fly. The fastest, 730 millimeters per second — or a little over one and a half miles per hour — mimicked a blow fly.
He put eight different mantises through dozens of swings, filming each with a high-speed camera. He then analyzed the insects’ recorded moves.
The strike of a praying mantis has two phases. In the first, the approach phase, a mantis extends its arms up and outward. In the second, the sweep phase, the mantis scoops the prey out of the air and pulls it in to eat.
Mr. Rossoni and Dr. Niven found that the mantises did indeed adjust their strike speed, according to how quickly the target was moving. Most of that modulation occurred in the approach phase — when presented with a slower target, the mantises would raise their limbs more slowly or pause in the middle, in a zombielike pose.
And if they initially miscalculated the speed of their prey, the mantises would often “correct their own mistakes” with a similar pause, Mr. Rossoni said. “Considering that some of the strikes are less than a tenth of a second, this is quite extraordinary.”
It also adds to a growing conversation about what insects — from wasps that can logically infer to ants that can roll down inclines — are capable of.
“Historically, they were viewed more as almost miniature robots that were following very simple sets of rules,” Dr. Niven said. “I think that there is new research coming out that suggests that that rule book might be much more complicated.”