Insects

No other group of land-dwelling animals has as many members as the class Insecta. More than 800,000 species of insects have been described—about 300,000 of these are beetles (order Coleoptera)—and some experts believe there may be more than 30 million different species. Insects are the earth’s dominant animals. The success of this group is due partly to its tremendous adaptability and huge variation in life styles. Insects live in almost every habitat, from steamy tropical jungles to cold polar regions. The ability to fly has allowed them to spread to new, unexploited habitats and to escape from predators. Many millions of years ago, it helped them to disperse and has given them greater access to food and more desirable environments.
It is possible that the great increase in the numbers and variety of flowering plants during the Cretaceous period also contributed to the enormous success of insects. Most flowering plants are dependent upon insects for pollination.

The locust has the anatomy of a typical winged insect, with a head, thorax, and abdomen. The thorax has three segments, to which the three pairs of legs and two pairs of wings are attached. A chambered heart forms part of the dorsal blood vessel, which pumps blood forward with the aid of valves. Food is taken through the mouth and is digested in the crop, the pyloric ceca, the midgut, and the hindgut The first ganglion of the ventral nerve cord forms the brain. Spiracles (not shown) open out in the cuticle of each segment. Air passes into them and carbon dioxide is released from them. The air is conducted by tracheae (ducts) to the blood, into which the oxygen diffuses.

General features

The exoskeleton, which covers the entire insect body, is composed of chitin and hardened by proteins. It is made up of several parts: the tergum, covering the back; the sternum, on the underside; and two pleura, which link the tergum to the sternum. The pleura are considerably thinner than the rest of the skeleton.

The body is clearly divided into a head, thorax, and abdomen. The head consists of five or six segments, but they are fused together and are not obvious in the adults. Typically, the insect head bears a single pair of sensory antennae, one pair of compound eyes, which may be color sensitive, and one ocelli, or clusters of light-sensitive cells.

A characteristic unique to insects is that the thorax is composed of three segments, each bearing a pair of walking legs. The legs are modified in different species for grasping, swimming, jumping, or digging. The winged insects (subclass Pterygota) also bear a pair of wings on the upper surface of each of the second and third thoracic segments. The abdomen is made up of 10 or 11 segments connected by flexible membranes. The eighth and ninth segments—also the tenth in males—bear the genital appendages.

An insect’s heart lies in the upper part of the body cavity within the thorax and, in most species, in the first nine abdominal segments. The blood circulates in a blood cavity, called the hemocoele, and bathes all the tissues.

Respiration in most insects takes place by means of a system of internal tubes called tracheae, which open to the exterior via paired openings called spiracles. Oxygen diffusion along the trachea is sufficient to meet the demands of the insect at rest. During activity, however, air is pumped in and out of the trachea system by the expansion and collapse of air sacs (enlarged parts of the tracheae). The expansion and collapse of the trachea are controlled by movements of the body. The spiracles can close to prevent water loss.

The principal excretory organs of insects are the Malpighian tubes, which open into the hindgut and the rectum. Uric acid, dissolved salts, and water are drawn from the hemocoele; the fluid in the tubes then passes into the rectum, where useful salts and water are extracted before the waste products are excreted with the feces.

The insect nervous system is much like that of other arthropods, with a brain and a system of linked individual and fused nerve bunches called ganglia connected to a ventral nerve cord. Apart from the eyes, sense organs occur all over the body, but most are concentrated on the appendages. Such sense organs include receptors sensitive to chemical signals and hairs that are sensitive to touch.

Dragonflies (A) have densely veined, primitive wings that cannot be folded across the back. But more highly evolved insects, such as wasps (B) and beetles (C), have developed structures at the wing base which allow the wings to be folded across their body, and the wing venation is reduced. The two wings on either side of the wasp’s body are hooked byfrenal hooks. In beetles, the front pair of wings have hardened and become elytra, to form a wing case. They fly only with the hind wings.

Insect flight

One of the most successful adaptive features of insects is flight. Most pterygotes have wings, although wingless insects whose ancestors had wings occur in this group. Various species, such as ants (order Hymenoptera) and termites (order Isoptera), have wings only during certain stages of their life cycle; others, such as fleas (order Siphonaptera), have completely lost their wings as a result of their parasitic life style.
The earliest insect wing is thought to have been a fan-shaped membranous structure with heavy supporting veins. Modern wings, however, are more highly specialized structures, composed of two pieces of opposing cuticle, which are separated in places by veins. These veins support the wing and provide it with blood. Primitive wings such as those of dragonflies (order Odonata) have large numbers of veins that form a netlike pattern, but subsequent evolution has favored a reduction in venation.
Many insects have two pairs of wings. These may move independently, as in damsel flies (order Odonata), or they can be hooked together so that they move as a single structure, as in many hymenopterans, such as bees and wasps. Coleopterans (beetles) have undergone a further change—the first pair of wings has been hardened to form the wing cases, called elytra. These wings form leathery covers that protect the beetle’s body. Insects of the order Diptera, which includes all the true flies, have a pair of forewings only. The hind pair has been reduced and modified into club-shaped structures called halteres, which act as organs of balance.

The action of flight is fairly straightforward. An upward beat is produced by the contraction of vertical muscles in the thorax. When these muscles contract, the thorax flattens, causing the wings to move up. A downward stroke is produced either by the contraction of muscles attached to the wing base or by the contraction of horizontal muscles in the thorax. The thorax arches upward, causing the wings to move down. This up-and-down movement on its own does not provide enough impetus for flight, however, and the insect’s wings must also move back and forth, resulting in a wingbeat that forms a figure eight or an ellipse, which is tilted at an angle to the vertical.

The number of wingbeats per second varies greatly from between 4 and 20 for butterflies (order Lepidoptera) to 190 beats per second in bees, and up to 1,000 per second in a gnat. The mode of flying also varies in different groups. Butterflies tend to have a slow, fluttering style, whereas bees and flies can hover and dart. Flight is controlled by a complex interaction of feedback from sensory hairs on the head, stretch receptors at the base of the wing, visual cues, and the wing muscles themselves. There is no nervous center for the control of flight. Flight speed appears to be controlled by the flow of air against the antennae or by sight.

The large compound eyes of flying insects provide good eyesight for feeding. The eyes of some flies each consist of about 4,000 lenses packed together, which are not always of equal size—those of the horsefly are larger on the front and upper parts of the eye.

Life cycles and development

Most insects lay eggs. Once hatched, the primitive wingless insects, such as silverfish, gradually mature through a series of molts. They do not change form, but simply grow larger with every molt. The winged insects, however, undergo a metamorphosis in which they change form. These insects are divided into two groups: the Endopterygota, such as flies and butterflies, whose larvae do not resemble the adult and whose wing development is internal, appearing only in the final stage of metamorphosis, and the Exoptery-gota, such as cockroaches, grasshoppers, and some bugs. These insects hatch as miniature versions of adults, with external wing buds but without mature sexual organs, and they gradually develop by incomplete metamorphosis (hemimetamorphosis).

The basic life style of insects can be modified by various factors. Aphids, for example, exhibit parthenogenesis (reproduction by females without fertilization by males) in favorable weather conditions. Unfertilized eggs laid in the autumn hatch into wingless Ovoviviparous females, which do not lay eggs but give birth to broods of similar females. The cycle continues until conditions become less favorable, when one generation produces males along with winged females, and they mate.

Many insects have complex life cycles especially parasitic insects. Parasitic wasps, for example, have a form of reproduction called polyembryony, in which the embryonic cells give rise to more than one embryo. This means that from one egg deposited in the body of a host, many larvae can be formed, and the resulting embryos can use the host’s body as both a refuge and a food source.
A great variety of parasitic insects exist, from blood-suckers that live permanently on a host during their adult lives to the parasites that visit a host only to feed. Some, such as mud daubers, are parasitic in the larval stage only. The egg-laden female wasp finds a spider, which it paralyzes with its sting. It then builds a nest of mud into which it puts the spider. Finally, it lays an egg on the spider’s body and seals up the nest When the egg hatches, the larva has a ready source of food until it transforms into a pupa.

Insect flight involves a wing movement in the shape of a figure eight. The downward beat results from the contraction of muscles at the wing base and the longitudinal muscles, which also raises the tergum. An upward beat is achieved by the contraction of the vertical muscles.

Feeding adaptations

The mouthparts of insects show great variability. In some, they are adapted for sucking, in others, for piercing and biting. In still others, the mouthparts are modified for chewing, crushing, and tearing.

Chewing mouthparts are found in carnivorous and herbivorous insects. This is regarded as a primitive condition, and is characteristic of insects such as dragonflies, grasshoppers, and beetles, in both the adult and larval stages. Butterflies have chewing mouthparts during the larval stage only.

The mouthparts of bees and wasps are modified for biting and sucking, as well as for chewing. In the case of honeybees, sucking is necessary for the collection of nectar, while chewing ability is necessary for the manipulation of pollen and wax.

The piercing mouthparts of insects that feed on plant juices, such as aphids, penetrate plant tissues, and the insect feeds on the sap. The mouthparts of other insects, such as houseflies, are modified to form a proboscis, which is used to soak up liquid food partially digested by enzymes in the saliva.

A honey bee colony consists of workers, drones, and a queen, each with distinct roles. The queen controls the colony and lays up to 2,000 eggs a day. Drones mate with the queen, and workers forage, build combs, and attend to all other aspects of the colony’s life.

Social organization

Communal living is developed to the highest degree in termites, ants, bees, and some wasps. The social life of honey bees, for example, is controlled by a rigid division of labor, and different ranks, or castes, of bees perform different functions within the colony.

A typical colony consists of about 60,000 workers, which are all sterile females; 200 male drones; and a single queen, whose function is to lay eggs. The main purpose of the drones is to mate with the queen. The duties of the worker bees alter as they become older. Newly emerged workers stay in the hive and clean and feed the queen, drones, and larvae. At this stage glands in the workers’ throats produce “royal jelly,” which is fed to very young larvae for the first four days of their lives and continually to larvae destined to become queens. After about two weeks, when the wax glands have developed, the workers’ duties change to cell construction and cleaning, as well as receiving stores of pollen and nectar from foraging bees. Three weeks later, the worker becomes a field bee, with large pollen baskets on the hind legs. Workers born in the spring live for about eight weeks, but those emerging in late summer or autumn live through the winter. Workers kill or drive drones from the hive in the autumn to conserve food stocks.

The queen maintains unity in the colony by the production of chemicals called pheromones that influence the behavior of the bees, largely by inducing the workers to nurse the larvae. As the queen ages, her pheromone production diminishes, and the workers start producing new queens by feeding certain larvae solely on royal jelly. At some stage in the colony’s life, the old queen leaves with about half the workers. At this time, a new queen emerges and the other developing queens are usually killed. The new queen mates with the drones during a nuptial flight and returns to head the colony.

A bee hive is made up of a series of vertical wax combs, with hexagonal cells on both sides. Bee larvae mature in the cells. Drone cells are larger than those in which workers develop, and the queen cells are more saclike. Some cells are constructed for storing pollen and honey for use during bad weather or during the winter.

One of the most astounding features of bees is their ability to communicate the position of a rich food source to other workers by means of a special dance. On returning to the hive, a foraging bee alights on a flat surface and begins to dance in relation to the position of the sun. She moves in a straight line, wagging her body. If she moves upward, the food source lies toward the sun; downward means that it lies away from it. By moving at angles to the left or right, she indicates that the bees must fly with the sun on their left or right, respectively. The speed of the dance indicates the distance of the food. The other workers follow the progress of the dance by touching her, and then fly off toward the food.

Ant colonies are similar to those of bees; there is usually one queen and many female workers, with only a few males. But many ant workers are specialized to perform certain tasks only. Soldier ants, for example, have huge heads and enlarged mouthparts, and their function is to defend the colony. Other ants are workers, tending the larvae and collecting food.

The types of ant colony vary from species to species. They may be groups of hunters, food gatherers, farmers, or even stockbreeders. Army ants are hunters and swarm over victims in enormous numbers, killing them for food. Food gatherers collect plant materia! and bring it into the nest. So-called “dairying” ants live chiefly on a sugary liquid called honeydew, which they “milk” from aphids and other plant lice. Many ants are fungus growers. Inside their colonies they provide suitable conditions for the growth of particular species of fungi, upon which they feed. Leaf-cutter ants, for example, bring leaf material to the nest on which fungus grows from spores. Other ants keep aphids, which they bring into the nest in winter and put out again in the spring onto suitable plants.

Some species of ants, called slave makers, take ants of other species, which become slave-workers for the colony. Control of the slaves and the colony is generally achieved by pheromones. Ants also have been known to leave their home nest to raid another, which they move into, keeping its original inhabitants captive as slave-workers.

Ametabolous insects, such as the silverfish, develop from the egg stage into a small version of the adult. They develop without metamorphosing, in contrast to hemimetabolous insects, such as the shieldbug, which develops by incomplete metamorphosis into the adult This insect emerges from an egg, which is shaped like a barrel with a lid. The young insects resemble the adult but are wingless up to the sixth and final molt, when the beginnings of the wings develop. The horsefly is an example of complete, or holometabolous, metamorphosis. Its development occurs in three stages. The first is the feeding stage, when the hatched larva eats continually and increases in size. When it stops feeding, it becomes inactive and constructs a hardened cuticle, or pupa. Inside the pupal case, adult structures develop from embryonic forms. When the period of pupation is over, the adult insect emerges.

Economic importance of insects

Many insects are of benefit to humans. The caterpillars of the silk moth, for example, provide silk. Farmers raise these caterpillars in huge numbers, feeding them on mulberry leaves. When it is time for the caterpillars to turn into a pupa, they begin to spin a silken cocoon. To extract the thread, the silk farmer must kill the caterpillar and unwind the cocoon. Each cocoon yields about half a mile of fine silk fiber.

Another beneficial insect is the honey bee, which is often domesticated and kept in specially constructed hives, from which honey and beeswax are harvested.

Fruit trees, shrubs, and flowering plants depend upon insects for pollination. Many other insects are useful because they control biological pests. Many ladybugs (family Coccinelli-dae), for example, feed on aphids, which can severely damage both ornamental and food plants, particularly citrus fruits.

Unfortunately, there are some insects that are pests. Swarms of locusts can lay bare vast fields of crops, and the boll weevil attacks cotton plants. Some insects are dangerous because they carry disease. Malaria, for example, is transmitted by Anopheles mosquitoes. Other diseases transmitted by insects include yellow fever, elephantiasis, sleeping sickness, Lyme disease, plague, Rocky Mountain spotted fever, and typhus. The cost of drugs, vaccines, and eradication programs to control these insects and the diseases they cause is enormous.

Insects’ mouthparts differ, according to how the animal feeds. Some mouthparts are adapted for sucking, others for piercing and biting, and still others for chewing, crushing, and tearing. Although the two main types of mouthparts are those that are adapted for chewing and sucking, each order of insects has its own variation of one of these types, or a combination of both.
Molting—called ecdysis— is an essential process for growing insects because their exoskeleton is rigid and does not permit much growth. The intervals between ecdyses are called instars. The mantid, or praying mantis, has from 3 to 12 instars and takes about a year for complete metamorphosis. The insect ruptures the cuticle by swallowing air or water and contracting muscles, which increases pressure inside the exoskeleton. It then wriggles out of the old skin (visible at the bottom of the photograph).