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CALIFORNIA WILD

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Preying Mantids
Hiding in Plain Sight

Roberta Brett

I never enjoyed the prospect of rising early during my college years. But a student in the biological sciences needs time in the field and, driven by "the early bird catches the worm" syndrome, I seemed to end up most weekends doing just that. In those days, I was highly motivated. As a formidable requirement to pass a general entomology course, I had to collect and identify 50 families of insects.

And so, on this particular autumn Saturday, the chilly dampness of night still lingering, I pulled into a solitary gas station not far from my destination, Del Puerto Canyon in the Central Valley of California, a traditional collecting spot for beginning bug hunters. While waiting to refuel, I carefully examined a nearby juniper bush for prospects. Poking between its prickly green spines, I found myself returning the stare of a praying mantid. Its large eyes held my gaze and followed my movement. As I reached to collect it, it struck out with its large front legs. Lucky for me I was not a small insect.

Mantids are easy to recognize because of their most impressive distinguishing feature: their front, raptorial legs. No longer efficiently used for walking, these legs are highly modified for capturing prey. Alternating rows of long and short spines along the femur and tibia snatch and impale prey with raptor-like speed. A single curved spine at the tip acts as a grappling hook to help snag a victim and bring it to the waiting jaws. The mantids' mandibles, although small and delicate-looking, are strong enough to pierce human flesh and can reduce even large prey to fragments.

At rest, the front legs are held in a position we like to call "prayerful." This characteristic pose inspired Linnaeus to name an Old World species, Mantis religiosa. Its pious appearance may have earned it the title "praying" mantid, but "preying" mantid would be a more accurate term. As orthopterist Ashley B. Gurney wrote in 1951, "the only thing mantids would seem to pray for is a square meal."

That square meal usually consists of insects and other arthropods, though mantids are not adverse to eating vertebrates. Well-documented cases describe how praying mantids have devoured prey sometimes larger than themselves, including other predators. Small lizards, frogs, snakes, and mice have fallen victim to mantids, and very nearly a hummingbird who was stunned but escaped unharmed thanks to human intervention. (The human would probably have stayed out of it, if it had been the other way round.)

These mainly diurnal insects are generalist predators making a living by the "lie and wait" approach to hunting. They feed only on living prey. Mantids are convincing mimics of their surroundings, employing magnificent camouflage. Species that live in rocky substrate have short stubby wings and are mottled brown like the stones around them. Those that live in the forest canopy resemble green leaves or twigs, bark, or lichen. Some sway from tree branches when disturbed as leaves wave in a breeze. On the ground, brown dead-leaf mantids disappear among the dry leaf litter.

Other mantids increase their chances of catching a meal by perching on or near a flower head where insects are likely to visit. As juveniles, their coloring may even match the color of the flower. The flower mantid of southeastern Asia is white if it develops on a white flower, pink if it develops on a pink flower. Unable to sustain strong flight, mantids do not take prey while on the wing but will hang upside-down from branches or cling to grasses as they patiently wait for their next meal to come along. Only a few ground-dwelling species such as Amantis reticulata from Brunei and the minor ground mantid (Litaneutria minor) native to California, have enough speed to aggressively run down prey.

Mantids spring into action from their well-camouflaged state when their keen vision has been piqued. Large compound eyes dominate the relatively small triangular-shaped head and provide excellent stereoscopic vision. Three simple eyes, or ocelli, which resemble tiny marbles, are located between the large eyes and provide ancillary visual information aiding in the discrimination of light and dark.

Thanks to its highly mobile head, which can turn through more than 180 degrees, a mantid can readily spot any movement. The precision and coordination necessary for a mantid to capture its prey comes from a combination of acute binocular vision and a series of stiff hairs located on its neck. The eyes operate in tandem, combining the images they receive and enabling the mantid to fixate on an object. If the striking distance is too far, more than ten centimeters for most species, it will close in, moving imperceptibly slow. (If a creature should get too close, nearer than 1.5 centimeters, the mantid will instead employ defensive strikes in an attempt to push the intruder away, even if it might otherwise be prey.)

The direction of the raptorial strike is determined by a series of stiff hairs on the mantid's neck that are in continual contact with its head. Changes in the angle of the head affect how erect the various sensillae are. Experiments to determine the importance of these hairs have shown that if they are removed there is a 70 to 80 percent reduction in accuracy. Removing hairs on only one side makes things even worse. Generally, these insects are very efficient. Eighty- five percent of the time, a striking mantid captures its prey.

Once in the mantid's deadly grasp, the victim has little chance. A few bites quickly render the prey immobile. A healthy and hungry mantid can consume up to 15 crickets a day, sometimes grabbing a second one while still munching on the first.

With the fastidiousness of a cat, a mantid will give as much attention to its post-meal ritual as it does to eating. Running forelegs through their mouthparts, labial and maxillary palps glean small bits of debris that could abrade the cuticle and damage the water-proof protection the exoskeleton provides.

Camouflage, besides enabling the mantid to hunt effectively, is also its best defense--as long as it keeps still. If it moves at an inopportune time its cover is blown. Once spotted the mantid is fairly helpless, though it does have a few maneuvers left. It will often try to bluff its way out with a variety of defensive displays. Rearing up and raising its menacing forelegs exposes false eyespots on the thorax which can confuse a predator. Extending and fluttering its wings reveals startling colors and gives the appearance that the mantid is larger and fiercer than it is. But if that fails, the mantid is an easy mark. Unequipped with caustic acids or toxic chemicals to ward off attackers, the massive forelegs and cumbersome proportions are now disadvantages to a quick escape.

One recent study, though, has revealed why mantids do have a fighting chance when it comes to avoiding some nocturnal predators. Until recently, all mantids were thought to be deaf, relying on vision as their main sense. But experiments conducted in 1986 by neurobiologists David Yager and Ronald Hoy, at Cornell University, have shown that some mantids can hear. However, their auditory system appears to be unique for they possess only one ear. Yager calls it an "auditory Cyclops." Even grasshoppers, with their tympana found on their legs, have two ears. The separation between our ears allows us to discern the direction and distance of a sound. But the single teardrop-shaped ear found in mantids is on their bellies! It is in a deep slit on the ventral side, between the second and third pair of legs. This ear cannot locate the source of a sound.

Yager and his colleagues set out to determine what benefits the ability to hear might have for an animal which doesn't produce noises either to locate prey or attract mates. They exposed mantids to a barrage of sounds of differing frequencies, and found that they are most sensitive to the ultrasonic range, between 25 and 60 kilohertz, the same high frequency of chirps produced by bats to locate prey.

In addition, they discovered that some 40 percent of mantid species had no ears, and there was a strong correlation between earless mantids and wing atrophy. Earless mantids have wings that are too short for flight. Moreover, a third of all mantid species show sexual dimorphism with regard to hearing: males with long flight wings can hear ultrasound, but flightless females of the same species are deaf.

Why the discrepancy? The researchers suspect that ultrasonic hearing provides an advantage to mantids in avoiding bat predation. Males of certain mantid species cruise the night air broadcasting sexual pheromones, a behavior that puts them at considerable risk from aerial predators. Their flightless counterparts below, hidden in the brush, aren't subject to these threats.

To test their theory, they simulated free-flight by rigging mantids to a suspended wire and turning on a small fan. Then they played the same high frequency radar-like chirps bats use to echolocate. Sure enough, when the mantid ear detected ultrasonic signals, it began evasive maneuvers similar to those used by fighter pilots. In normal flight a mantid's body is aerodynamically efficient, its wings tucked and its abdomen flat. A tenth of a second after it becomes alarmed, the mantid is already stretching out its legs and arching its abdomen. The insect stalls, then falls off, doubling its normal air speed of two meters per second as it recklessly spirals downward away from the source of the sound.

Open air tests using real bats and unfettered mantids confirmed the same evasive maneuvers seen in the laboratory. Some bats would begin to pursue the fleeing mantid but would not risk crash-landing and went in search of less troublesome prey. In over two hundred tries researchers observed eleven bat attacks on hearing mantids, five of which the mantids used evasive maneuvers to successfully elude the bats. Another three survived without employing any avoidance tactics. Earless mantids were not as fortunate. Using a species of mantid known to be deaf, they witnessed six unchallenged bat attacks, five of which the bat won.

Since the mantid ear is not designed to determine the direction of the ultrasound, the success of its escape is based solely on the unpredictability of its dive. Depending on which wing stalls first, the mantid veers either to the left or right. It can also gauge the urgency of the threat and accelerate its evasive response accordingly.

Ironically, those same mantid species so adept at avoiding bats in flight, are not as well protected when stationary. Bats that glean insects from vegetation use a lower frequency of echolocation, which is only detectable by a stationary mantid at very close range, and does not give the mantid sufficient time to flee.

Of course, the mantids' most notorious meals are their mates. In art, literature, and film, the female mantid has become the personification of the demonically callous charmer, who summarily lops off her partner's head if she becomes annoyed during the mating ritual. This common misconception, that all males typically end up in the females' jaws, may in fact be based solely on the behavior of one species and some infrequent occurrences under artificial or captive conditions.

It was first described in a German text published in 1658. Then, in 1886, United States National Museum scientist Leland Ossian Howard recounted in his paper, "The excessive voracity of the female mantis," observations he made of his two pet mantids engaged in copulation:

In a few minutes, the female succeeded in grasping him. She first bit off his left tarsus, and consumed the tibia and femur. Next she gnawed out his left eye. At this the male seemed to realize his proximity to one of the opposite sex, and began to make vain endeavors to mate. The female next ate up his right front leg, and then entirely decapitated him, devouring his head and gnawing into his thorax.

By Howard's account, this unfortunate male did finally succeed in copulating, despite the absence of his head. From this single observation Howard deduced that cannibalism must be a normal precursor to copulation in mantids. But in subsequent trials that produced no headless males, Howard refuted his own claim; most of the time males were left unscathed.

Male mantids are capable of copulating without a head. Decapitation removes the portion of the brain which inhibits copulation and triggers sexual movements of the male's abdomen. Removing the male's head actually activates his sexual responses and he will vainly attempt to copulate with any long and slender object, including pencils and fingers. However, acephali is not a condition of the male's successfully mating, nor does losing his head make him more proficient. Most males mate several times during a lifetime. (In contrast, the male of the red-back spider from Australia, in order to get close enough to the female's genital opening, must throw himself directly beneath her jaws, essentially committing suicide in the process of mating.)

In laboratory tests by Philip and Nellie Rau in 1913, using Mantis carolina, the same species Howard used, not a single male was devoured during or after the 32 matings they observed. Sixty-six years later, experiments conducted by M. E. Schauff and J. C. Jones produced similar results. Out of 20 Chinese mantids, Tenodera aridifolia sinensis, only two males were eaten.

Most of the observations where male mantids have ended up as pre- or post-coital dinner have occurred under laboratory conditions, which pose several problems for the animal. Because mantids react largely to visual cues, any distraction at a critical moment can interrupt the normal sequence of events leading up to mating. Movements or shadows created by vigilant observers can distract the male's attention causing him to move hastily and perhaps miscalculate his jump onto the female. The relatively cramped and stark quarters of a tank or cage leave the male little room to approach the female without being seen and no chance to escape. In nature, the female would have already indicated her receptivity by emitting sexual pheromones. But a male suddenly thrown into a confined space with a female may find that she is hungrier than she is amorous.

Assuming that in nature most mating occurs under cryptic cover, researchers W. Jackson Davis of the University of California at Santa Cruz and Eckehard Liske of Germany's Technical University tried a new approach. Using a video camera to capture the action, Davis and Liske were able to eliminate the opportunity for distractions. They recorded dozens of copulations and were amazed to observe an elaborate courtship ritual during which the female plays an active and positive role in mating. In a process that can take as little as ten minutes or as much as two hours, the researchers watched the male's dance-like gyrations as he slowly and purposefully approached the female, waving his antennae and moving his abdomen in a figure-eight pattern. Rather than preparing to decapitate him, the female responded by extending her raptorial legs in front of her and flattening her body against the substrate, essentially making it impossible for her to strike. Only once did the male fall prey to his mate--and she, incidentally, hadn't eaten for several days.

However, Liske and Davis did discover that sexual cannibalism occurs with greater frequency among some species of mantids than others. Mantis religiosa, in particular, engages in sexual predation more often than not and is apparently responsible for giving the whole group a bad rap. It doesn't appear to impair the reproductive capability, though: Mantis religiosa is the most widespread mantid species in the world.

After a single successful mating the female mantid will generally have enough sperm stored within her to produce several batches of fertilized eggs during her life. In a desert species from India, the female can lay between six and ten batches of eggs without further copulation, the last cluster appearing as much as 19 weeks after initial fertilization. Gravid females can produce as few as ten eggs and as many as four hundred in a single laying. While a few mantids will lay their eggs in soil, most will attach them to a twig or branch sturdy enough to support its weight. Working usually at night, she weaves her abdomen back and forth, producing a light-colored froth resembling stiffly beaten egg whites. Eventually, a pod-like structure forms. She then inserts into the foam two rows of eggs, which soon harden to form a protective cover. Usually producing a single generation of progeny, the female dies shortly after completing her egg-laying cycle.

Most female mantids desert their newly laid ootheca, but a few will remain close by. Those that abandon their egg cases tend to lay ootheca that are short and robust with a thick layer of spongy coating. Females that stay nearby lay cases that match their long narrow form and have a less dense protective covering. While a threatened mantid, guarding an egg case or not, will spread and flutter its wings to reveal warning colors, a long-bodied female positioned over a long egg case is probably better able to conceal it.

After a month, barring catastrophes, the hatchlings, looking like miniature replicas of their parents, will emerge from the relative safety of the ootheca. They usually disperse rapidly for young mantids are already fierce mini-hunters and have no qualms about devouring their own siblings.

These juveniles, as is typical of hemimetabolistic insects (those that pass through a series of growth stages), molt six to seven times prior to adulthood. The strength and durability of the exoskeleton enable the insects to occupy many different habitats, but it is too rigid and inflexible to allow for growth. Triggered by hormonal changes, as it grows larger, a mantid nymph will shed its outgrown skin with each successive molt. Molting, or ecdysis, while a normal function of metamorphosis, can be a dicey proposition. A newly molted mantid must remain immobile until the soft and delicate cuticle has hardened. This can take several hours, during which time the insect is most vulnerable. And if the exuvia or outgrown skin, is not fully shed or if the new skin dries in a contorted position the mantid's hindered movement makes it impossible to hunt.

While a wealth of knowledge exists about the lifestyle of mantids, the possible value of mantids in medical research has yet to be fully explored. We know that as early as the fifth century B.C., the Chinese were prescribing tea made from boiled mantid egg cases as a cure for asthma, gonorrhea, warts, and bedwetting. Contemporary Chinese herbalists still recommend potions made from egg cases or ground mantids to treat a variety of ailments. We'll have to wait and see what other contributions mantids may make to human welfare.

Last summer, I received a mantid from the grandmother of a budding nine-year-old entomologist. The live mantid made the long trip from Calaveras County in the Sierra foothills to San Francisco in a carefully packed box with a question--"What is its name?"--and a request that we take care of it. I assured the grandmother that we would not prematurely add the mantid to the collection. It lived out its natural lifespan, and now resides under the name Stagmomantis californica in the California Academy of Sciences' collection, adding to the treasury that will provide entomologists with yet more understanding of the remarkable praying mantid.

Roberta Brett is a curatorial assistant in the Entomology Department of the California Academy of Sciences

Spring 1997

Vol. 50:2