The life cycle of an insect is often very complex, and the organism passes through various stages before it becomes an adult — that is, sexually mature. This change in form is known as metamorphosis, and each time an insect changes, it undergoes a process known as molting. Sometimes there is very little change in the organism at a molt, but sometimes the changes are profound, as when an adult butterfly molts from its cocoon.
The number of molts during the life of an insect ranges from three or four to more than 30, and in general, the more primitive the insect, the greater the number of molts. One of the major functions of molting is that it allows the insect to grow. Because a sclerotized cuticle is very rigid, an insect cannot grow without shedding the cuticle. Once the insect is freed of its old cuticle, its body can expand before the new cuticle hardens. Molting also serves two other important functions: it enables the insect to alter its cuticular structures and allows it to get rid of waste materials that have accumulated in the old cuticle.
Molting begins with the epidermis cells secreting a new cuticle underneath the old one. When the new cuticle is formed, the old cuticle becomes detached from the body. When the insect is completely free within the old cuticle, the process of molting is complete; the actual emergence of the insect from the old cuticle is known as ecdysis. At first the new cuticle is soft and can stretch, but soon it becomes hardened.
The molting process is controlled by certain hormones. The process is initiated by a molting hormone that is secreted by the prothoracic glands in the first portion of the thorax. A second important hormone, the juvenile hormone, is secreted by the corpora allata, a pair of nerve masses in the head. This hormone is produced throughout all the immature stages of the life cycle, and when it is no longer secreted, adult characteristics are allowed to develop. The insect's brain controls the corpora allata directly and the prothoracic gland indirectly by means of another hormone.
The most primitive insects, which are always wingless, are practically the only insects that do not undergo metamorphosis. When these insects emerge from the egg, they are exactly like the adults except that they are smaller and sexually immature.
The winged insects are divided into two groups with respect to their metamorphosis. In one group, called the Hemimetabola, the young that hatch from the egg appear similar to the adults but are smaller, sexually immature, and lack functional wings. In all of these insects, except for the mayflies, the wings do not develop until the final molt. This type of metamorphosis is sometimes called simple, or incomplete, metamorphosis, and the young are called nymphs.
In the second group of winged insects, called the Holometabola, the young that emerge from the egg are called larvae, and they bear no resemblance at all to the adults. They may also differ from the adults in their feeding habits, habitats, and other characteristics. After several molts each larva changes into a pupa, which is the resting stage of the life cycle. A pupa does not feed and generally does not move about, but it undergoes many internal changes and finally molts to produce the adult. This type of development is known as complete metamorphosis.
Stages of the Life Cycle
All insects hatch from eggs. Most insects lay their eggs, but in a few species, including some cockroaches, bugs, and flies, the eggs are retained in the body of the female until they hatch. While inside the female's body, the embryos may be nourished either by material inside the egg or by nutrients from fluids inside the female's body.
Insect eggs are relatively bulky, with a high content of nutritive material called vitellus. The external covering of the egg, called the chorion, is thick and protects the egg in several ways. It keeps the egg from drying out and helps protect it from frost and drought. A spongy layer of air spaces in the chorion allows an exchange of gases, permitting the embryo to breathe.
The size and shape of insect eggs vary considerably. Some are so small that they cannot be seen by the naked eye while others may be nearly half an inch (13 mm) long. In many butterflies and moths the eggs are spherical or oval and brightly colored, but in the majority of insects they are elongate, sausage-shaped, and dull in color. Often, the eggs have ridges or grooves that may extend as winglike projections, as in the eggs of many flies. Insect eggs also vary greatly in color.
Some insects, including mosquitoes, lay their eggs in water, but many insects deposit eggs on plants or on the ground. Many moths, butterflies, lacewings, ant lions, and other insects attach their eggs to leaves, depositing them in neat stacks. Lacewing flies attach their eggs by a stalk. Cockroaches and mantids protect their eggs by enclosing them in a frothy mass inside protective capsules called oothecae. Locust eggs are deposited in the soil and are protected by egg pods. Other insects conceal their eggs from predators by hiding them in crevices in plants or by pushing them into plant or animal tissues.
Most insects leave their eggs once they are laid. A few flies, however, brood their eggs until they hatch, and earwigs care for their eggs by periodically turning them over and licking them. The honeybees and other social insects show the greatest degree of maternal care for both the eggs and the developing young.
The amount of time it takes for an insect embryo to develop inside an egg varies greatly. In some species it may take only a few days, but in others it make take months. When the embryo is fully developed and ready to emerge, the chorion of the egg breaks open.
Nymphs, unlike many larvae, often live in the same habitat as the adults and eat the same kind of food. In aphids, where many wingless adults are produced during the summer, it is very difficult to tell the nymphs from the adults.
In some insects the nymphs are aquatic while the adults are not. In these species the nymphs usually have special structures, such as tracheal gills or swimming legs. In the water bugs, where both the adults and nymphs are aquatic, the differences between the adults and nymphs are very small.
Insect larvae, which vary tremendously from one species to the next, are divided into four types. One type of larva moves about on three pairs of thoracic legs and is known as a campodeiform larva because it resembles the primitive wingless insect Campodea. Many beetle larvae, notably those of ladybirds and ground beetles, are of this type.
Caterpillars represent the second type of larva. These larvae, known as eruciform larvae, spend most of their time feeding on vegetation and move sluggishly from one meal to the next. These larvae have well-developed thoracic legs and a large baggy abdomen that is supported by several pairs of structures known as false legs or prolegs.
Larvae that live inside their food material have no need to move about and have short thoracic legs. These larvae, known as scarabaeiform larvae, become more obese than the caterpillars and their head and thorax appear very small compared with their large abdomen. The larvae of May beetles and stag beetles are of this type.
Larvae that have lost all traces of legs are said to be apodous. These include the larvae of some beetles, such as the weevils, most of the Hymenoptera (ants, wasps, bees, sawflies, and others), and all of the flies. Some fly larvae, however, do have prolegs on the abdomen. The term "maggot" is applied to a special kind of fly larva that has a small pointed head, mouth hooks, and a blunt posterior end with spiracles. The term "grub" is a less precise term and is generally applied to any larva that is barrel-shaped without a definite front or rear end, lacks legs, and is covered with spines or similar structures.
Although some pupae, notably those of caddis flies, mosquitoes, and some Neuroptera (lacewings, ant lions, and others), may swim or actively crawl about, most pupae appear to move only when the adult has fully formed within the pupal skin and has not yet broken out of it. Although most pupae appear to be inactive, they are undergoing complex biochemical and structural changes; the larval structures are being resorbed, and adult structures are being formed.
The pupa is roughly the same shape as the adult, with the head, legs, and wings enclosed in a covering called the pupal skin. If these different structures are covered separately so that the legs, for instance, can move, the pupa is said to be exarate. If the appendages are held down and cannot move, the larvae is said to be obtect. If the pupa is equipped with mandibles that can cut a way out of the pupal skin, it is considered to be decticous, but if it lacks mandibles for escaping from the covering, it is said to be adecticous. Decticous pupae occur in certain primitive families of insects, and exarate adecticous pupae are characteristic of most beetles, Hymenoptera, fleas, and some flies. Obtect pupae are most characteristic of moths and butterflies.
Many pupae, in addition to being enclosed in a pupal skin, are protected by an outer covering that is constructed by the larva before it pupates. Usually, the protective outer case is made of wood or some other material bound together by silk and is called a cocoon. The term "chrysalis" is limited to the all-silken cocoons of moths and butterflies. Most cocoon-building larvae secrete silk from special glands related to the salivary glands.
The process of metamorphosis may be continuous or it may be interrupted by periods of diapause. True diapause is more than a mere slowing down of the rate of development — it is a complete state of suspended animation that requires a specific stimulus to make the insect become active again. The main function of diapause is to enable the insect to survive any period of unsuitable conditions. In winter it is called hibernation and in summer it is called estivation. Diapause may occur during any stage of an insect's life cycle. Though periods of quiescence occur in some insects with incomplete metamorphosis, true diapause is mainly a feature of insects with complete metamorphosis.
A common example of diapause occurs in the pupae of many moths and butterflies, which normally spend the winter in exposed places. If the pupae are taken indoors and kept at a relatively high temperature, they will not complete their development because they require the stimulus of a frost to break the diapause. Other factors that may act as stimuli to break the diapause in insects are light, humidity, and mechanical stimuli. Fleas, for example, develop to the point of becoming adults within the pupal skin, but they do not emerge until vibrations indicate the proximity of a suitable host on which the adult can feed.
Adaptations and Behavior
Adult insects with chewing mouthparts, such as cockroaches and ants, are general feeders, eating a wide variety of plant and animal matter. Plant-eating and parasitic insects are the most restricted feeders, sometimes feeding on only one species of plant or animal. Many carnivorous insects eat other insects, including members of their own species. A tremendous number of insects at all stages of their life cycle are saprophagous, feeding only on the products of organic decay.
Very few insects can digest cellulose, the material that makes up the cell walls of plant cells. Termites and a few other insects have certain cellulose-digesting protozoa in their digestive tract. Other plant-eating insects do not digest cellulose, but either suck the plant sap or digest only the contents of the cells, excreting the cellulose undigested.
Some insects, such as houseflies, predigest their food by pouring saliva over it before it is eaten. Another type of predigested food is honeydew, the sweet excretion of aphids. Honeydew is rich in carbohydrates as well as amino acids (components of proteins) and a few trace elements. It is eagerly sought by many insects, who sometimes stroke the aphids to hasten the excretion of honeydew.
Insects are preyed upon by a wide variety of animals. Some insects depend on their speed and agility for escaping from predators and many have various defense mechanisms for warding off their enemies. Some insects are protected by spines, stings, and other devices, but most insects rely heavily on their coloration to protect them from predators. Coloration is said to be a passive defense, while spines and similar devices are said to be active defense mechanisms.
Adaptive coloration is generally divided into two broad categories: cryptic, or concealment, coloration, and aposematic, or warning, coloration. Cryptic coloration makes an insect less conspicuous against a particular background either by closely imitating the background or by confusing the outline of the insect against the background.
Aposematic coloration may be divided into two types. One is a simple device to frighten off potential enemies by invoking an alarm reaction. The second type of aposematic coloration is called mimicry. Generally, there are two major forms of mimicry. Batesian mimicry, which is named for the English naturalist Henry Bates, is thought to give protection to an innocuous insect that assumes the color and pattern of a dangerous or unpalatable species. Müllerian mimicry, which is named for the German entomologist Fritz Müller, refers to a more general situation in which a number of different insects share a common appearance and are said to derive some advantage from this in that the attacks of predators are shared by all of them.
Both chewing and sucking insects use their mouthparts for defense, and the so-called bite of an insect is often a piercing or stabbing with the proboscis. Stinging is another familiar type of insect defense, in which the female insect inserts her modified ovipositor into the tissues of an animal and releases certain toxic substances. Stinging is characteristic of certain bees, ants, and wasps, which have ceased to use their ovipositor for the purpose of laying eggs but use it as a weapon against insect enemies or other, larger animals that may threaten them. Stinging is also used for immobilizing prey.
Many insects also have special glands that are derived from the cuticle and that produce a variety of poisonous chemical substances. Many caterpillars have barbed hairs, called urticating hairs, which are connected to glands that produce substances irritating to the skin. Other glands, called repugnatorial glands, produce substances with an unpleasant odor that repels other insects. These so-called stink glands are most commonly found in bugs, beetles, and caterpillars. Bombardier beetles expel a fluid that may contain formic acid or iodine; the fluid is expelled from their abdominal glands with such force that it appears as a cloud of mist. Reflex bleeding is a curious active defense mechanism of certain beetles. When attacked, the beetles squeeze out drops of blood through the joints in their body or legs. The blood has such a nasty taste that the predator is repelled.
Mobility and Flight
In general, insects with well-developed legs and chewing mouthparts are active predators and run about rather than fly. Some flying predators, however, such as dragonflies and robber flies, have powerful legs for seizing and holding their prey. Chewing insects are generally more active than sucking insects, which tend to remain in one position when they are not flying from one meal to the next. Thus, on a single plant there may be hundreds of aphids sucking the plant sap and remaining stationary while some of them are being devoured by roving predators, such as the larvae of ladybird beetles, lacewings, and syrphid, or flower, flies.
The sedentary habit is carried to the extreme in the female scale insects, which are active only during their first nymphal stage and then spend the rest of their lives attached to a plant. Male scale insects have wings and fly about actively. A modification similar to that of the female scale insect occurs in parasitic insects, notably the female of the fly Ascodipteron, which lives buried in the skin of a bat. This female does develop wings, but upon reaching a host, she loses her legs and wings and buries herself in the host's skin while her abdomen enlarges into a huge sac filled with eggs. This enlargement of the abdomen, known as physogastry, occurs in many insects. The best-known example of this phenomenon occurs in the female queen termite, whose abdomen may reach a length of several inches and contain hundreds of eggs. Other examples of physogastry are seen in jigger fleas, queen ants, and many unrelated insects that inhabit ant nests, including various beetles and flies.
Insects fly by beating their wings up and down. This motion is controlled by the powerful indirect muscles of the thorax, which are stimulated by the nervous system. The nervous system also stimulates the direct muscles, which twist and flex the wings to provide control and direction. The number of wing beats per second varies greatly among insects — it is directly related to the strength of the wing muscles and the weight of the insect. Butterflies, which generally have large wings and light bodies, may beat their wings from 4 to 20 times a second while flies and bees, which have heavier bodies and smaller wings, may beat their wings 100 times a second. It is estimated that mosquitoes may beat their wings up and down as much as 1,000 times in a second.
Although many insects are capable of sudden bursts of speed, they generally cannot sustain them for long periods. The greatest speed at which an insect can fly is about 35 miles (56 km) per hour, a speed that can be attained by certain dragonflies. Most common insects may reach a speed of only about 5 to 8 miles (8–12.8 km) an hour.
Although many insects tend to remain in one region, others may become widely dispersed by means of periodic migrations. This tendency to disperse into larger and more varied environments has been one of the main factors in the successful habitation by insects of so much of the earth's usable surface. Most insects migrate by flying from one area to another, but a few species, including the processionary moths and the army and driver ants, travel in groups over ground.
Migratory movements generally take place early in the insect's adult life, as soon as it is ready for sustained flight. The number of insects involved in a migration flight is generally very large. A locust swarm often consists of a billion individuals and a flight of painted lady butterflies in California was once estimated to consist of 3 billion individuals.
The dispersal of insects by air is sometimes divided into two types, active and passive, although some authorities disagree with this distinction. Passive migrations occur in many small insects, such as aphids, scale insects, and caterpillars, which may be carried up into the air and transported considerable distances by storms or by the general movement of air currents. The migration flight of desert locusts is also an example of passive migration. The actual flying movements of the insects are aimed at keeping the swarm together while the swarm as a whole is carried by the wind. The migration flight ends when the swarm arrives in an area of low atmospheric pressure, which indicates that rain may soon fall, making the ground suitable for the locusts to deposit their eggs. An example of active migration is the migration of butterflies, which takes them from one breeding ground to another or away from a breeding ground when the weather is too cold or during a drought. The butterflies are not simply carried by the wind, but fly in a definite direction at certain times of the year. Sometimes, there is a reverse migratory movement in which the same individuals or those of the next generation return to the original site. The North American monarch butterfly is probably the most famous insect migrant. In the fall, swarms of monarchs fly from the northern United States and Canada to the Gulf Coast and Mexico and during the trip they alight and feed daily. In the spring they fly in the reverse direction, heading north, and they do not stop to feed along the way.
Social behavior in insects ranges from the tending of eggs by earwigs to the highly developed colonies of the truly social insects. There are only two groups of social insects — the termites and the group composed of bees, wasps, and ants.
Within a colony of social insects there is a clear-cut division of labor, with the members of the colony divided into two basic castes, a worker caste made up of sterile individuals and a reproductive caste. The worker caste performs many more functions and far outnumbers the reproductive caste. The workers tend the eggs, feed the developing young, forage for food, defend the nest against intruders, and clean the nest. The fertile female of the colony is called the queen, and usually all of the workers in the colony are her offspring. Her major function is to produce eggs; a singe queen honeybee may lay as many as 2,000 eggs a day. Another function of the queen is the production of a material called "queen substance," which is passed on to all members of the colony and serves mainly to prevent them from becoming fertile. The fertile male of the colony often dies shortly after mating, but sometimes remains in the nest.
Evolution Insects are believed to have evolved from millipedelike terrestrial ancestors some time before the Silurian Period, which began about 440 million years ago. The first insects had no wings; the direct descendants of this wingless group that still exist today are the silverfish, bristletails, and springtails. Some time about the Devonian Period, about 400 million years ago, winged insects appeared. The earliest winged insects had wings that stood out at right angles from the body. They could only be flapped up and down and could not be folded. Modern-day insects with wings of this type (dragonflies and mayflies) are called Paleoptera, as contrasted with all other winged insects, which are called Neoptera. The Neoptera have wings that can be folded back over the body, and many of them have structures for stowing the wings when not in use.
Fossils of early Paleoptera have been found in forests of the Carboniferous Period, which lasted from about 350 to 270 million years ago. These were the largest insects ever known, and one species, Meganeura moyni, had a wingspan of 30 inches (75 cm). The oldest Neoptera still in existence are the cockroaches, which have survived almost unchanged since Carboniferous times, although they, too, are not so large as they were then.
Insects with incomplete metamorphosis, the Hemimetabola, reached their peak in the order Hemiptera, the bugs. First came the class Homoptera, culminating in the aphids, froghoppers, and leafhoppers, which are still among the most successful insects. The class Heteroptera, the carnivorous water bugs and the more individual plant bugs, arose later.
The earliest insects with complete metamorphosis, the Holometabola, were like dragonflies in that their early stages were aquatic. Since it is believed that the ancestors of all insects were terrestrial, all of the aquatic adaptations must have arisen separately. The snake flies, dobsonflies, and other Neoptera groups formed one of the aquatic stem groups, but already a separate line of evolution had led to the Coleoptera, the beetles. The increasing aridity and desertlike conditions of the Triassic and Jurassic periods, lasting from about 225 to 135 million years ago, finally extinguished nearly all of the insects with aquatic stages in their life cycle, leaving only a small number to survive to the present. Later in the Jurassic Period, increasing rainfall favored the growth of plants, and with it, the development of plant-eating insects.
The Mecoptera (scorpion flies) of the Permian Period, lasting from about 270 to 225 million years ago, are believed to be the ancestors of a versatile group of insects known as the panorpoid complex. From this group arose not only the present-day scorpion flies, but also the caddis flies, butterflies and moths, the true flies, and the fleas. The development of the butterflies and moths is closely linked with evolution of flowering plants and they arose more recently than the other members of this group, making their appearance only in the Triassic Period. A similar association with flowering plants is found in the development of the Hymenoptera. The most primitive members of this order are the plant-eating insects with caterpillarlike larvae. Next came the parasitic members, the ichneumon flies, chalcid flies, and others. From the Cretaceous Period onward, from about 135 million years ago, social organization among the Hymenoptera seems to have evolved as an adaptation to cope with unfavorable seasonal conditions.