Insects

Insect Facts

Classification:

Kingdom: Animalia

Subkingdom: Bilateria

Phylum: Arthropoda

Subphylum:Hexapoda

Class: Insecta

Orders: Common orders include Coleoptera (beetles); Diptera (flies, mosquitoes); Hemiptera (true bugs); Hymenoptera (ants, bees, wasps); Buttodea (cockroaches, termites); Lepidoptera (butterflies, moths); Odonata (dragonflies, damselflies); Orthoptera (crickets, grasshoppers, locusts, katydids)

Geographical location: Every continent except Antarctica

Habitat: Mainly terrestrial, some aquatic, primarily freshwater

Gestational period: Highly variable; some insects produce one, two, or several generations per year; conversely, relatively large insects may take more than one year for larval development

Life span: Highly variable; adult mayflies live less than one week, while queen termites have been known to live for more than twenty years

Special anatomy: Major regions include the head, featuring one pair of antennae, the thorax, with three pairs of legs and up to two pairs of wings, and the abdomen, housing spiracles and genitalia; in some insects, one or both pairs of wings are modified for functions other than flight, such as protection or balance

The history of insects dates to the Devonian period, about 400 million years ago. Numbering more than one million species in the twenty-first century, insects are the most diverse class of animals on Earth. Insects populate almost every habitat except the deep oceans and permanently frozen land masses. They may be specialized to live underground, in live or rotting trees, in fast-flowing rivers, or stagnant puddles. Parasitic insects live attached to the outside (hair, skin) or inside (stomach lining, respiratory tract) of other animals. Crafty insects modify their habitat by building their own houses; for example, spittlebugs mix air into slimy anal secretions to form shelters; bagworms spin silk to hold leaves together around them; and some African termites glue soil particles together with saliva into colossal nests measuring six meters high and nearly four meters across.

Indeed, many estimates suggest there are as many as 5–10 million more extant but undescribed species. The great biological mass of insects makes them a crucial part of many ecosystems, especially as they often form basic levels of food chains. They display a wide range of morphologies and specialized evolutionary adaptations.

Insect Anatomy

Insects are invertebrates noted for their jointed exoskeletons. Chitin makes this armor especially strong. The exoskeleton functions much like the internal bones of vertebrates, serving to protect internal organs and as sites of muscle attachment. The exoskeleton limits the overall size that insects can attain and makes growth energetically costly. The insect must shed its old exoskeleton and form a new one to increase in size. The old exoskeleton may be eaten to reclaim some of its nutrients. While a freshly molted insect is waiting for its new outer covering to harden, it is particularly vulnerable to attack from predators. Molting insects tend to be white; the exoskeleton will also develop color as it hardens.

Along the relatively straight digestive tract of an insect, there are regions specialized for food storage, grinding, chemical breakdown, and nutrient absorption. Before undigested remains leave the insect, water, and salts are reclaimed. Insect waste, termed frass, is thus very dry. This last step of water conservation is very important to insects because they lose water rapidly due to their small size.

The insect respiratory system is also adapted to save water. The tracheal system is a collection of tubes allowing gas exchange from the outside environment directly with individual tissues. The openings in the body wall, spiracles, can be closed to conserve water between breathing cycles.

The circulatory fluid of insects, hemolymph, functions in keeping tissues moist and transporting nutrients and hormones. There are no red blood cells or other oxygen-carrying molecules because the insect respiratory system is totally separate from the circulatory system. The circulatory system is open. Hemolymph enters a series of chambers, collectively termed the heart, through holes called ostia. Muscular pumping of the heart propels the fluid through the aorta toward the insect's brain. Near the brain, the hemolymph flows out into the general body cavity or hemocoel. Hemolymph directly bathes internal organs on its way to and from the legs and wings, starting the cycle again by entering the heart ostia. There are distinct veins in the wings. Insects take advantage of fluid circulation in the wings to transfer heat, warming themselves or cooling off when needed.

The Insect Diet

Insects display a diverse range of diets, often highly specialized to certain ecological niches. Phytophagous insects feed on living plants; for example, caterpillars are well-known leaf eaters. Other insects feed on plant roots, shoots, flowers, stems, or fruits. When a large group of phytophagous insects occurs in the same area, the crop or forest damage that they cause can be extensive. Yet there is quite another side to the relationship between insects and plants. Insect-pollinated plants have evolved odors, shapes, and ultraviolet color patterns to attract these important visitors. When the butterfly, bee, or other pollinator stops to drink the plant's rich nectar, pollen from the male part of the flower sticks to the insect's body. When it visits another flower of the same species, it deposits the pollen on the female floral organ. Without this interaction, beans, tomatoes, tea, cocoa, and many other plants could not reproduce.

Other insects are scavengers, serving an important purpose by recycling nutrients found in dead plant and animal matter. Termites are a nuisance when they infest a house but are invaluable at breaking down dead wood in nature. Dung beetles have the curious habit of forming animal feces into balls and rolling them away to feed their young. Carrion feeders, such as flies and beetles, assist in the decomposition of animal corpses. A succession of flies and beetles reduce a corpse to bones.

Predatory insects hunt and kill to eat. Some predators use powerful mouthparts to tear apart their prey. Other predators inject digestive enzymes into their prey, digest them externally and suck out the liquefied tissues through a specialized beak. The first pair of legs of the praying mantis are striking examples of raptorial modifications for grasping and holding struggling prey. Some insects have a varied diet, such as dragonflies, which in their aquatic nymph stage, may eat crustaceans, tadpoles, and even small fish. Adult dragonflies eat bees, butterflies, and mosquitoes.

Parasitic insects, such as mosquitoes, typically ingest host blood, mucus, or tissues, often with minimal direct irritation or harm to the host. Trouble begins if there is a heavy parasitic burden (a large number of parasites per host), or if the parasitic insect transmits disease-causing organisms to the host. Ectoparasites live on the outside of their hosts. In this category, fleas and lice are adapted to avoid detection by the host and deter removal during normal grooming processes. Endoparasitic fly larvae live in the digestive or respiratory tract, mainly in livestock. Endoparasites of invertebrates, however, often kill the host because there is little difference in size between the parasite and host. Species of endoparasitic wasps inject their eggs into caterpillars. When the larvae hatch, they tunnel farther into the host and feed off of its tissues. Upon completion of larval development, the wasps emerge from the caterpillar, killing it in the process.

Some parasites spend their lifetimes closely associated with the host, but others only briefly visit the host. On the time continuum, lice not only form a constant, more-or-less permanent association with a single host, but many generations of lice may inhabit the same host. However, these parasites are transferred from host to host during mating, nesting, or other close contact between individuals. Unlike lice, fleas leave the host frequently between bloodmeals. On the other end of the spectrum, mosquitoes normally require multiple hosts to complete a single bloodmeal; they aim to move on before they can be slapped by hand or tail.

Insect Behavior

Much of what insects do, they do by instinct, a genetically preprogrammed response to environmental stimuli, nonetheless amazing in its elegance and effectiveness. Antennae and setae, hairlike projections through the exoskeleton, serve as two of the many receptors of external stimuli. The necessity of keeping these sensory organs clean is evidenced by preening, commonly using mouthparts and legs. Major categories of environmental stimuli signal friend or foe.

Many insects locate opposite-sex conspecifics, individuals of the same species, by sight. Color patterns, especially in the ultraviolet (UV) spectrum, attract potential mates. For example, fireflies (Coleoptera) use visual recognition of patterns of light flashes. In some species, females use signaling to attract males. In other species, the male and female signal to each other with the proper code and response. Females of the genus Photuris mimic the flashing patterns of other firefly species. When the male approaches, they become prey.

Sound production and reception are also used for mate location. The sound generated by a female mosquito's beating wings in flight is picked up by the male mosquito's antennae. Male homopterans and orthopterans produce songs of courtship. Cicadas use abdominal muscles and a resonating chamber for sound production. Grasshoppers and crickets rub their wings and or legs together. Sound receptors, "eardrums" of females in these orders, are found on their forelegs or first abdominal segments. Sound production and reception vary with temperature.

Pheromones, externally broadcast chemical signals, are another means of attracting mates. In moths, glands near the tip of the female's abdomen release pheromones that are received by the antennae of the male moths. Certain male butterflies and moths have specialized scent-producing glands on their wings. The notable pair of black patches, or androconia, on the hindwings of the male monarch butterfly, secrete aphrodisiac pheromones that increase the female's receptivity to mating. Pheromones can also make already-mated females unattractive to subsequent suitors.

Members of the class Insecta display numerous protective behaviors and defense mechanisms when threatened. Walkingsticks are capable of losing part of a leg in the grasp of a predator. The amputated appendage continues to wiggle when severed, thus distracting the predator, while the walkingstick gets away. Some beetles play dead until the predator loses interest. Blister beetles are capable of reflex bleeding. They squeeze drops of hemolymph through joints in their exoskeleton. Their hemolymph contains cantharadin, which irritates and repels the predator. Bombardier beetles spray a noxious repellant from their anal glands for distances up to one meter.

Another means of defense is coloration. Insects may use camouflage to blend into the surroundings by resembling a leaf, twig, pebble, or flower. Some insects use bright colors to hide in plain sight. Large eyespots on wings can startle would-be predators or at least trick them into taking a bite out of the wing rather than from the head or body. Color patterns of orange and black seem to warn vertebrate predators that the insects would not make a good meal. Some of these insects derive chemicals from their diet that make them distasteful, even harmful, to predators. Without possessing such chemicals, mimics derive protection from their resemblance to the group that does.

Social insects exhibit the most complex behavior, approaching learning. Honeybees can communicate the direction of a food source and distance from the hive to other bees through a multipart waggle dance. Some ants are farmers, planting and nurturing fungus gardens. Mound-building harvester ants ensure that the colony's young are kept at the optimal temperature through vertical migrations. The young are transported to top levels within the mound to warm up in the morning and evening. During the heat of the day, the young are carried to lower levels to cool off. Kidnapping ants from other colonies to serve as workers in the home colony may not be considered a socially advanced behavior, but it is a complex one. The other major group of social insects, the termites, work together to build huge mounds called termitaria, some reaching heights of six meters and diameters of over three and a half meters. They engineer a series of chimneys into the structure that can be opened and closed to regulate airflow and maintain a constant temperature.

Environmental Significance

While many people think of most insects—often colloquially called "bugs"—as nuisances, they are, in fact, crucial elements of most ecosystems and, indeed, are often beneficial to humans and the environment as a whole. In addition to their roles as pollinators for many important crops, insects themselves serve as food sources for many animals, including many people around the world. It is estimated that honeybee pollination accounts for $15 billion in increased crop value annually in the United States. In addition, harmless insects, such as dragonflies, which consume pest insects such as mosquitoes, are good candidates for biological control programs, a form of pest control that takes advantage of natural predators of the pest. For example, ladybugs are commonly used by gardeners to control plant-damaging aphid populations. Biological control shows promise as a way to reduce the use of insecticides, which can have negative environmental effects, building up in food chains and harming other animals.

Though insecticides can be useful in controlling pest species that destroy crops or spread disease, their widespread use has also been linked to serious declines in the global insect population. Although insects are often inconspicuous and difficult to track, in the early 2020s, studies began to suggest that at least 40 percent of species were in decline, with some populations dropping very steeply in a relatively short period of time. In addition to insecticide use, researchers pointed to climate change, deforestation and other habitat loss, pollution, invasive species, and other factors as contributing to this trend. Scientists warned that the collapse of insect populations would have major implications on ecosystems, as insects are a vital component of food webs and fill other crucial ecological roles.

Principal Terms

Apterous: insects without wings, such as fleas

Ecdysis: molting, the shedding of the exoskeleton that allows for insect growth

Hexapod: six-footed, a general term for an insect

Ovipositor: egg-laying apparatus on the female abdomen, modified into a stinger in bees

Pupa: intermediate stage between the larval and adult stages of the life cycle

Spiracles: openings on the outside of the insect abdomen that lead to breathing tubes

Vector: transmits pathogens from one host to another

Bibliography

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