1. Testing Praying Mantises with Tiny 3D Glasses to See How They Judge Distance
In 2016, a team at Newcastle University in the UK stuck tiny 3D glasses onto a praying mantis’s head. They used teardrop-shaped blue and green filters about 7 mm (about 0.3 inches) in diameter, fixed in place with a mix of beeswax and rosin. The experiment used adult females of a species not found in Japan (*Sphodromantis lineola*) [1].
On a screen, black discs moved toward the center in a spiral. When the researchers shifted the position seen by each eye slightly to make it appear "in front of the screen," the mantis struck with its forelegs more often than when no shift was made or when it appeared further away. Although the mantis was looking at a flat screen, the glasses made it seem like an insect was within reaching distance [1].
The closer an object is, the larger the difference in where it appears in each eye. Calculating distance from this shift is called stereopsis. Nerve cells that react to this shift have also been found in the mantis brain [2]. However, this stereopsis does not seem to work the same way as in humans. Humans compare patterns of brightness between left and right images, but the mantis focuses on places where brightness changes [3]. Furthermore, striking requires both seeing the prey move and judging it is within reach via stereopsis. In this experiment, the mantis did not strike at stimuli without figure motion [4].
2. How Praying Mantises Measure Distance to Still Objects by Moving the Head Side to Side
Mantises seem to use a different method to measure distance to stationary objects. When praying mantis larvae try to move from a high vantage point and can choose among stationary objects at different distances, they usually pick the closest one. They find it easier to choose when the background has horizontal stripes and harder with vertical stripes [5].
At this time, the larva sways its head left and right, a behavior called “peering” in the Japanese source. Moving the head makes nearby objects shift position greatly and quickly in their eyes, while distant objects shift little. It is thought they use the size of this shift to know distance. To test this, researchers moved an object in front of the larva on purpose while it was swaying its head. This made the relationship between head movement and apparent shift mismatch with real distance. Yet the larva judged based on this artificial shift and got the distance wrong. Being fooled by the shift itself is evidence that the apparent shift from head sway is used directly as a clue for distance. This ability exists even in newborn larvae whose eyes are not fully developed [5].
Try this: Hold one pencil in each hand on a desk, extend your arms, and touch the tips together. Next, close one eye and try again. What happens if you sway your head left and right? Be careful not to bring the pencil tips too close to your eyes.
3. Why the Praying Mantis "Neck" Is Really the Prothorax, and How the Head Turns with It
The slender part that looks like a praying mantis’s neck is actually part of the chest, called the prothorax. The head is not covered by the prothorax and moves freely [9]. In the neck area, there are small hard plates that also serve as attachment points for muscles moving the neck [8]. Nature conservation materials note that they move their heads up, down, left, and right to measure distance with stereopsis [10].
When Japanese researchers used high-speed cameras, they found that when turning toward prey, the head, prothorax, and abdomen turned almost at the same time. The difference in start times was 40 milliseconds or less. The size of the turn was also related to each other. Direction is thought to be determined before movement starts, using visual and body position information [7]. Researchers from Kyushu University explain this flow: calculate prey position with eyes, point head and body that way, measure distance with stereopsis, then extend the claws [6].
Why does the neck turn? A natural thought is that it allows both eyes to face the prey without moving the whole body much. This is an idea read from materials, not a result tested by experiment.
4. How Praying Mantises Ambush Prey with Eyes That React to Movement
Praying mantises are "sit-and-wait" predators that do not chase prey but wait for it to come. They often use body colors that blend into surroundings [11]. Body colors vary by species and individual, including green and brown types [9].
The eye structure suits this waiting hunt. Mantises react to most moving objects of appropriate size [13], and their brains have nerve cells that strongly react to the movement of small objects [6]. In experiments, they did not strike at stimuli without figure motion. They struck only when it was moving and judged to be within reach [4]. "Waiting" also means not striking until it is close enough.
Is sitting still because their bodies are energy-efficient? A US research team measured the resting metabolism of four mantis species. The results showed no difference from the average expected for insects, and it was higher than that of spiders with similar ambush strategies. One mantis ate prey weighing 138% of its own body weight at once, and metabolism doubled about 6 to 12 hours after eating [12]. It is not simple to say they have an energy-saving constitution.
5. How the Shape and Speed of a Praying Mantis's Forelegs Catch Prey and Help with Cleaning
The praying mantis’s forelegs are shaped to catch prey. They have spines, bumps, and hairs [8], and they fold to clamp onto prey. Praying mantises are thought to have diverged from a common ancestor with cockroaches about 200 million years ago [9]. Ordinary legs likely evolved into this grasping shape.
Speed has also been studied. In a large praying mantis from Madagascar (*Polyspilota aeruginosa*), the time for the claw tip segment to extend was 60 to 290 milliseconds (0.06 to 0.29 seconds). When prey moved slowly, the extending movement was also slow and sometimes stopped midway. However, success did not depend on speed [11]. They change their movement to match the prey.
These forelegs are used for more than hunting: walking, holding grass, cleaning sensory organs (there is a brush for cleaning compound eyes), counterattacking enemies, and threatening by making the body look larger [13].
6. How Praying Mantises Wrap Their Eggs in Foam That Hardens into an Egg Case
In the Kanto region around October and November, females lay eggs. The eggs are wrapped in a foam-like substance that hardens into a brown egg case about the size of an adult’s thumb. Shapes vary by species [14]. The egg case is a foamy substance made by the female, which hardens into a tough case [16]. When hardened, the foam becomes an elastic sponge containing much air, said to serve as insulation [18][19]. Some explanations say it protects eggs from heat, cold, dryness, humidity, and impact [20].
However, no paper was found this time that verified the insulation strength of egg cases by measuring inner and outer temperatures. For similar egg cases in stick insects, researchers think they likely offer benefits against parasitic insects and drying out [15]. The foam is not perfect protection. Parasitic wasps (*Ooencyrtus*) and beetles that eat eggs and case shells use the egg cases. Some aspects of their lives remain unknown [13].
SAFETY NOTE: Egg cases are brownish, thick sponge-like masses about the size of an adult’s thumb, with shapes that vary by species [14].
7. How Many Eggs Are in a Praying Mantis Egg Case and How They Hatch
The number of eggs in one case varies by source. Some say 100 to 200 hatch at once [10], about 200 for the giant mantis [18], around 300 [17], or 30 to 300 depending on species [16]. One explanation says a female giant mantis produces 3 to 5 cases when raised in captivity, totaling over 1,000 eggs [17].
How are many eggs arranged? A paper reporting on stick insect egg cases mentions that mantis-like cases are made by arranging eggs neatly and then coating the outside [15].
In spring, when it warms up, larvae emerge. First, they hang from the bottom of the case on a thread, break the membrane covering their bodies, and wait for their bodies to harden [16]. This first form is called a pre-nymph. It is said that only a few survive to become adults in the wild [20].
8. How Praying Mantis Eggs Survive Winter, Even Under Snow
Encyclopedias state that adult giant mantises are seen from late August to November and "overwinter as eggs," meaning one generation per year [9]. There are exceptions. The Japanese small mantis *Amantis nawai* overwinters as 3rd or 4th instar larvae [9].
Regarding snow, there is a legend: "If mantises lay eggs high up, there will be heavy snow." In *Nikkei Science* (1997), Sakai and Yuzawa reported that this legend has credibility [23]. On the other hand, Emeritus Professor Ando at Hirosaki University, based on 17 years of research, reported that egg position does not tend to be higher than snow depth but relates to grass and tree height, and eggs hatch even if buried in snow [21]. According to a column by the JT Life Science Museum, the hatching rate for 47 cases (about 9,000 eggs) buried in snow for about 4 months was about 98% [22]. These two conclusions disagree.
If you put an egg case in a warm room, hatching may start early, and food for the larvae may not be found [16].
9. How You Can Investigate Praying Mantises and Their Egg Cases Yourself
In winter, you might find egg cases if you look at dead grass or branches [14]. Here is what you can do.
Do not touch them; just look. If you take them home and put them in a warm room, they may hatch during winter, and food for the larvae will be hard to find [16].
Note with photos and memos what kind of grass or tree it was on and how many centimeters above the ground. Comparing this to nearby grass height lets you think about the "mantis snow forecast" story with your own data.
Stay off roads, away from water, and out of private property.
Also, the Newcastle University 3D glasses paper [1] is free online. Figure 3 shows the mantis with glasses and the apparatus; Figure 4 shows the results.