If you find yourself outside this time of year, you might come across one of the world's more intriguing insects: praying mantises. Despite their saw-like arms and alien eyes, they pose no threat—unless of course you happen to be an insect, gecko, or hummingbird.
It’s not that the praying mantises of the America are more abundant in late summer and fall, says Sydney Brannoch, a mantis expert at the Cleveland Museum of Natural History. It’s just that they’re more noticeable.
“Right now, they’d be looking for mates, so the males might be a little bit more active, more prone to flying around,” says Brannoch, who is also a doctoral student at Case Western Reserve University. The animals have also spent all summer hunting and growing, repeatedly shedding their exoskeletons as they reach lengths of up to half a foot.
Now, as the days shorten in the Northern Hemisphere, they have only two things on their mind: food and sex.
Mantis Mates
And sometimes those interests intersect—brutally. Praying mantis courtship can be a dangerous affair; females have been well-documented biting off the heads and eating other body parts of the males that they mate with. However, the frequency of such violence may be just a tad overstated.
“First of all, not all praying mantis species cannibalize their mates,” says Brannoch.
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“Maybe if the female is starving or if the male irritates her, she might engage in that behavior. But they don’t always do it.”
In fact, of the species that exhibit cannibalism of their mates, studies have shown that the females make a meal out of the males between just 13 and 28 percent of the time.
Interestingly, getting eaten may not be as bad as it sounds.
A study published in 2016 found that when female Chinese mantises consume their mates, they acquire important amino acids that are then incorporated into the eggs they lay. They also appear to lay twice as many eggs after cannibalizing a male than they normally would.
So while the male would probably be better off to live and mate with multiple females, at least it seems his nutrients give his DNA an elevated chance of getting passed on to the next generation. (Related: Praying mantis looks like a flower—now we know why.)
Bird Brains
In some cases, male praying mantises actually comprise a significant portion—if not a majority–of a female’s diet during breeding season. However, these insects eat many other animals as well. Including birds.
Believe it or not, praying mantises have been officially recorded eating birds in every continent except Antarctica. A study published in The Wilson Journal of Ornithology in 2017 compiled 147 cases in which the insects have eaten birds, in 13 countries on six continents. Altogether, 12 different species of mantises have been seen eating 24 types of birds, with hummingbirds as the most common prey.
The mantises have a particularly gruesome way of eating their prey: head-first. In many cases, the insects first pierced the birds’ heads and then feasted on their brains. (See also: Praying mantis devours hummingbird in shocking photo.)
The most prolific of these insects is the Chinese mantis (Tenodera sinensis), which has been introduced to the American Northeast. In this region, there are 25 recorded incidents of the creatures eating birds. Ruby-throated hummingbirds (Archilochus colubris) are the most common avian victims.
More widespread through the United States, however, is the aforementioned European praying mantis (Mantis religiosa), which is likely the most well-known and well-studied of these creatures.
The study recorded other animals that the hungry insects eat: “frogs, lizards, salamanders, newts, shrews, mice, snakes, tiny soft-shelled turtles, and even once a small bat.”
Clearly, these insects are voracious predators, but can a praying mantis hurt a human?
The short answer is, it’s unlikely. Praying mantises have no venom and cannot sting. Nor do they carry any infectious diseases. And while some varieties like the East African species known as Leptocoloa phthisica can grow nearly 10 inches long, their mouths are still rather small.
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Which means that even if a praying mantis were to bite your finger, their jaws are not powerful enough to cause serious injury.
Ears and Eyes
Another special thing that praying mantises can do is to hear approaching bats. While they're flying—they indeed have wings that are normally folded up, and not visible while they are sitting and waiting to pounce on unsuspecting prey—they can detect ultrasonic frequencies that bats use to hunt.
Using an ear-like organ in the center of their chest, they can hear sound above 20,000 hertz, just outside the range of human hearing. If they detect the series of chirps that bats use to make their final approach toward a bit of prey, they can change their flight path, dropping into a downward spiral that helps them avoid being caught, Brannoch explains.
Besides this talent for hearing, these animals have incredible eyes. Unlike all other insects, they can see in three dimensions. And yet, their 3D vision works completely differently from our own, says Vivek Nityananda, an animal behavior expert at Newcastle University upon Tyne in north England.
In a study published earlier this year, Nityananda and his colleagues fitted praying mantises with tiny 3-D goggles, not unlike the ones humans wear to the movies. The differently colored lenses allowed the researchers to show the insects two images at once and then record their reaction.
In the end, it became clear that a praying mantis’s 3-D vision is based on movement.
“Our stereo vision relies on matching the light and dark patterns of a scene and using the difference between the views of our eyes to tell depth,” says Nityananda.
This allows humans to judge depth. But for a sit-and-wait predator like a praying mantis, the background image isn’t nearly as important as the part of the picture that’s moving. After all, it could be lunch.
“Mantises must judge how far away their prey is before attempting capture and without moving, since moving would alert the prey,” says Nityananda.
“Stereo vision is a good solution to doing exactly that.”
It’s pretty incredible that praying mantises can see in 3D so well, a task that is computationally expensive for primates and more complex animals. But mantises, which only possess about one million neurons—compared to that 100 billion or so found in the human brain—have figured out a way to do it more efficiently.
Optical Illusion
Another oddity about mantis eyes: It appears that they have pupils, and can follow you with their eyes as you move. But this is a bit more complicated than it seems.
“Mantises have these big compound eyes, and they’re made up of thousands of light receptors called ommatidia,” says Brannoch.
What mantises don’t have are pupils, which animals like mammals use to focus light on the retina and form an image in the brain.
When you look at the insect’s eyes, the dark spot you see moving is actually just more of those light receptors that, because of the angle, are absorbing all the wavelengths of light—which is why they look black. Meanwhile, the light receptors all around the dark spot are reflecting certain wavelengths, which is why the rest of the eye appears green, white, brown, or purple.
Therefore, the mantises pupil is actually a bit of an optical illusion. No wonder scientists call it a pseudopupil. And thus, they cannot focus their eyes on one point in the same way that animals with pupils can.
No doubt further studies of their vision and behavior will reveal future wonders.
“They are charismatic and beautiful animals, and we have a lot to discover about them,” Nityananda says.
