For virtually any sensory system and behavior used by predators to detect prey, prey insects have evolved counterstrategies or defensive ploys. For predators relying mainly on vision, insects possess physical properties and behaviors either to avoid being seen or to maximize being seen. For predators that rely primarily on sound, prey have counterbehaviors to minimize sound generation, to evade sound-emitting predators, or to counter with effective sounds of their own. For predators using olfaction as their primary searching sense, prey have evolved systems to reduce their own odor, to mask it, to mimic the odors of unsuitable prey, or to blunt sensory orientation with allomones and aposematic odors. As a rule, insects supplement general defenses and behavioral strategies with multiple suites of defenses directed toward specific sensory systems.
Hiding from predators is a nearly universal tactic of insects. Even well-defended insects such as stinging wasps conceal their nests within dense vegetation, among roots, or in holes. Aposematic insects such as Dasymutilla occidentalis tend to rest in concealed places during periods of inactivity and run and hide under leaves or among vegetation when an approaching potential predator is sensed. Toxic butterflies often rest with wings folded and in among vegetation that hides them from view. Many other insects are masters of concealment and are so cryptically colored and patterned that finding them in a photograph has become an educational and entertaining challenge for children and adults alike. Some insects are concealed only during particular times. Caterpillars are commonly concealed on bark or in the ground while at rest, but are more apparent while feeding on leaves.
Concealment and hiding take many forms. Less obvious than daytime concealment against visual predators, but an equally frequent and effective defense, is use of time for concealment. Looper caterpillars (Geometridae) and others that are cryptically colored and concealed feed on leaves during daylight. At night when they are not cryptic or concealed from spiders, beetles, ants, and predators that search for prey mechanically and by olfaction and vibration, some conceal themselves by terminating feeding and hanging below the vegetation on long silken threads. An extreme of concealment is used by mayflies, which "conceal" their adulthood by reducing their adult life to only about a day, just enough time to mate, lay eggs, and die.
If an insect is capable of flying, jumping, running, crawling, or dropping to safety, escape is often the first response to detection by a predator. Nevertheless, not all insects can, or do, attempt to escape when approached by predators. Many larvae live in confined spaces and can move little or slowly, and none can fly. These individuals must rely primarily on other means of protection such as concealment, crypsis, or chemical defense. Other species rely first on their aposematism and/or noxious nature for protection and only attempt escape secondarily.
When concealment and escape fail, most insects resist and fight back by biting with mandibles, kicking and struggling, stinging, and releasing allomones, with varying success. Leg spurs and spines are used effectively by some large moths, including sphinx moths in the genera Manduca and Eumorpha and cockroaches (Archimandrita marmorata), to defeat the grasp of even large potential predators. These sharp spines not only can painfully pierce skin but also can anchor strong kicks to free the slippery insect from grasp. Some male wasps possess either sharp genitalia or separate "pseudostings" that are jabbed into grasping predators. Jabbed predators might mistake pseudostings for actual stings of female wasps and release the male.
Pain is the early warning system to indicate that bodily damage is occurring, has occurred, or is about to occur. Bodily damage is a serious threat and risk to an organism's ability to survive, feed, and reproduce. When given a choice between a meal with accompanying pain (plus perceived bodily damage) and the loss of a meal, predators often opt for the latter. The venomous stings of wasps, bees, and ants are legendary for their abilities to cause pain and deter predation. Spiny caterpillars and an assortment of biting bugs and beetles, including assassin bugs (Reduviidae), giant water bugs (Belostomatidae), water scorpions (Nepidae), and predaceous diving beetles (Dytiscidae), also produce painful venoms. Allomones can be effective by causing immediate pain. Examples are formic acid, sprayed by ants in the subfamily Formicinae, carabid beetles, and notodontid caterpillars, and quinones, released by tenebrionid and carabid beetles. Although bites and kicks might induce pain, their overall effectiveness relative to venoms and allomones suffers from lesser ability to produce pain and from predator familiarity with them and their expected effects.
Stereotyped warnings are used to threaten and intimidate predators. Paper wasps (Polistes spp.) on their nest face large adversaries with raised wings, waving front legs, abdomens curved toward the predator, and wings flipped, fluttered, or buzzed. These threats inform the predator that it is spotted and an attack will ensue if the advance continues. Hissing cockroaches (Grophadorhina portentosa) threaten by hissing, which resembles the defensive hiss of a snake. Many flies and harmless bees and wasps buzz loudly when grabbed. These aposematic buzzes sound similar to those of painfully stinging honey bees and wasps and often serve as effective warnings.
Insects that perceive an approaching predator can use the elements of surprise and startle to escape. Surprise combined with rapid escape flight are often a sufficient defense. If a predator is adept at pursuit, the execution of surprise and rapid flight, followed by instantaneous concealment upon landing, becomes a powerful defense. Startle is the combination of the elements of surprise and fright. Examples of startle are dull cryptic moths, which, when detected by a bird or monkey, flash their hidden hind wings, revealing bright colors (Fig. 4) or large frightening eyespots mimicking those of an owl or large predator (Fig. 5). Desert clacker grasshoppers, Arphia pseudoneitana, are grand masters of surprise and startle. When approached by a large animal, these inconspicuous desert grassland insects suddenly jump into the air, fly away amidst a confusion of bright red wing flashes and loud clacking noise, and disappear into distant
vegetation as suddenly as they appeared. In addition to startle, this behavioral display provides the predator with a search image for red color, which can cause the predator to overlook the dull grasshopper.
Individuals in schools of fish, flocks of birds, and herds of running African ungulates present difficult targets for predators. Use of confusion of predators via mass motion is little studied in insects but likely is an important defensive behavior in some situations. The constant movement and hopping of masses of migratory locusts and the seemingly erratic circling flights of flies disturbed from a fresh cow patty are likely examples of the use of confusion as a defense.
When many individuals aggregate in a group, each member receives protection through the presence of the others. Not only does a group present fewer locations with prey but also the individuals within a group gain protection through reduction in the chance of being the chosen prey by a discovering predator. Aggregation as defense is particularly effective if the individuals are toxic or are defended as are ladybird beetles (Coccinellidae), monarch butterflies, milkweed bugs (Lygaeidae), or social wasps on a nest. In these insects, a potential predator need sample only one or a few individuals to learn the unsuitability of the whole.
Protection can be achieved by living near or associating with a defended or noxious species. The tropical paper wasp, Mischocyttarus immarginatus, prefers to make its small nest with few individuals near the much larger nest of stinging Polybia occidentals, a common social wasp. The arrangement seems to provide protection for the Mischocyttarus from vertebrate predators. No potential benefit to the Polybia has been demonstrated. Honey bees and some species of ants are known to nest in portions of termite mounds. The exact
nature of these associations is unclear and the ants generally attack termites if given the chance. The benefits to the bees and ants are more obvious; they not only share the moderated temperature and humidity environments produced by the termites, but they also gain protection and reduced risk of discovery by being in the termite mound.
Unlike a solitary individual that must detect and defend against predators alone, individuals of social species enjoy benefits of group defense. A group of many coordinated individuals can more readily detect predators than solitary individuals, can then recruit others via alarm pheromones or vibrational signals to the common defense, and can launch effective attacks en masse. Group attacks are particularly effective when individuals possess painful stings or bites and when the attackers are nonreproductive workers who can sacrifice themselves in battle with little reproductive loss to themselves or the colony as a whole. Predators confronted by a "cloud" of attackers cannot devote attention to defending against each attacker, and reduced predator vigilance enhances attacker chances of scoring an effective sting or bite. Sociality and defense are strongly synergistic and, when combined, go a long way toward explaining the success of social insects.
Overall, insects as a class have taken defensive behaviors to levels unsurpassed in number, complexity, and creative diversity within animal life.
See Also the Following Articles
Aposematic Coloration • Crypsis • Ladybugs • Mimicry • Venom • Wasps
Blum, M. S. (1981). "Chemical Defenses of Arthropods." Academic Press, New York.
Cott, H. B. (1940). "Adaptive Coloration in Animals." Methuen, London. Edmunds, M. (1974). "Defence in Animals." Longman, Essex, U.K. Evans, D. L., and Schmidt, J. O. (eds.) (1990). "Insect Defenses: Adaptive Mechanisms and Strategies of Prey and Predators." State University of New York Press, Albany. Schmidt, J. O. (1982). Biochemistry of insect venoms. Annu. Rev. Entomol. 27, 339-368.
Wickler, W. (1968). "Mimicry in Plants and Animals." McGraw-Hill, New York.
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