Insect, small, air-breathing animal characterized by a segmented body with three main parts—head, thorax, and abdomen. In their adult forms, insects typically have three pairs of legs, one pair of antennae, and in most instances, two pairs of wings. Insects rank among the most successful animals on Earth. About one million species of insects have been identified so far, which is about half of all the animals known to science. That is why for every pound of human on the earth there are 10 pounds of insects. So that is why there are many reasons why insects are so successful, their exoskeleton, their size, their body function, the way they reproduce, and their development of metamorphosis.

One of the first reason why insects are so successful because they possess a tough exoskeleton that is covered with a waxy water repellant layer. The exoskeleton of insects also has helped them survive. An insect’s external skeleton, or exoskeleton, is made of semi-rigid plates and tubes. In insects, these plates are made of a plastic like material called chitin along with a tough protein. A waterproof wax covers the plates and prevents the insect’s internal tissues from drying out. Insect exoskeletons are highly effective as a body framework, but they have two drawbacks: they cannot grow once they have formed, and like a suit of armor, they become too heavy to move when they reach a certain size. Insects overcome the first problem by periodically molting their exoskeleton and growing a larger one in its place. Insects have not evolved ways to solve the problem of increasing weight, and this is one of the reasons why insects are relatively small. But compared to animals the Exoskeletons do limit the size of animals that use them instead of it growing larger. Second, insects are small creatures and hard to see. They can live in places that are too small for other animals, and where they can also find food and protection from enemies. Some insects live between the thin walls of a leaf. Some develop within a small seed or within the eggs of other insects. Because insects are small, they need little food. A crumb provides a banquet for an insect. Small organisms, such as insects, require fewer resources per individual than larger animals. Also of their protective color. An insect may be right before our eyes, but nearly invisible because it is cleverly camouflaged like a green leaf, lump of brown soil, gray lichen, a seed or some other natural object. Some insects use bright, bold colors to send warning signals that they taste bad, sting or squirt out poison. Others have wing patterns that look like the eyes of a huge predator, confusing their enemies. Some insects also mimic bitter-tasting insects; hungry foes are fooled into avoiding them.

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Consequently, a given habitat, with finite resources, can support more small individuals than large individuals. Third, Like other animals, insects absorb nutrients from food, expel waste products via an excretory system, and take in oxygen from the air. Insect blood circulates nutrients and removes wastes from the body, but unlike most animals, insect blood plays little or no part in carrying oxygen through the body. Lacking the oxygen-carrying protein called hemoglobin that gives the blood of humans and many other animals its red color, insect blood is usually colorless or a watery green. For oxygen circulation, insects rely on a set of branching, air-filled tubes called tracheae. These airways connect with the outside through circular openings called spiracles, which are sometimes visible as tiny “portholes” along the abdomen. From the spiracles, the tracheae tubes reach deep inside the body, supplying oxygen to every cell. In small insects, the tracheal system works passively, with oxygen simply diffusing in. Larger insects, such as grasshoppers and wasps, have internal air sacs connected to their tracheae. These insects speed up their gas exchange by squeezing the sacs to make them suck air in from outside. Instead of flowing through a complex network of blood vessels, an insect’s blood travels through one main blood vessel, the aorta, which runs the length of the body. A simple tube-like heart pumps blood forward through the aorta, and the blood makes its return journey through the body spaces. Compared to blood vessels, these spaces have a relatively large volume, which means that insects have a lot of blood. In some species, blood makes up over 30 percent of their body weight, compared to only 8 percent in humans. The pumping rate of their hearts is widely variable because insects are cold-blooded—meaning that their body temperature is determined by the temperature of their environment. In warm weather, when insects are most active, an insect heart may pulse 140 times each minute. In contrast, during extremely cold weather, insect body functions slow down, and the heart may beat as slowly as a single pulse per hour. In the digestive system of insects, the foregut stores food and sometimes breaks it down into small pieces. The midgut digests and absorbs food, and the hindgut, sometimes working together with the Malpighian tubules, manages water balance and excretion. This three-part digestive system has been adapted to accommodate highly specialized diets. For example, fluid-feeders such as butterflies have a pump like tube in their throats called a pharynx that enables them to suck up their food. Most of these fluid-feeders also have an expandable crop acting as a temporary food store. Insects that eat solid food, such as beetles and grasshoppers, have a well-developed gizzard. Armed with small but hard teeth, the gizzard cuts up food before it is digested. At the other end of the digestive system, wood-eating termites have a specially modified hindgut, crammed with millions of microorganisms. These helpers break down the cellulose in wood, turning it into nutrients that termites can absorb. Since both the microorganisms and the termites benefit from this arrangement, it is considered an example of dependence. Insects have a well-developed nervous system, based on a double cord of nerves that stretches the length of the body. An insect’s brain collects information from its numerous sense organs, but unlike a human brain, it is not in sole charge of movement. This is controlled by a series of nerve bundles called ganglia, one for each body segment, connected by the nerve cord. Even if the brain is out of action, these ganglia continue to work. Fourth, insects have a large and rapid reproductive rate. Much of the success of insects results from their powers of reproduction. Most insects have short lives. They quickly become adults and reproduce. Most insects lay many eggs. Many kinds produce several generations during a season. Because insects can reproduce so rapidly and in such great numbers, they can change to meet changes in their surroundings that could otherwise wipe them out. Insects also have special methods of reproduction. For example, a female mosquito lays 200 eggs per week while a house fly lays around 300 eggs per month. For the fly, this type of reproductive rate results in about 2 X 1020 flies that are descended from each female by the end of the growing season. Associated with a rapid reproductive rate is the usually short development time of most insects. A fly can develop from egg to adult in less than 10 days. As a result fly populations will have many generations per year and the density of such populations can grow very quickly. Because of such rapid life cycles, beneficial mutations can rapidly spread through a population and enabling a species a greater probability of adapting to environmental changes. For example, the genes responsible for certain pesticide resistance in house flies, can spread through a population in as little as 1 year. Fifth, insects posses an extensive development of metamorphosis. As a result of larva to pupa to adult developmental metamorphosis, insects exhibit differential use of habitats and resources by adults and juveniles (e.g. caterpillars feed on plants, butterflies feed on nectar). This will reduce competition for resources within a species. We don’t see adults fighting with larvae for food. Metamorphosis has other consequences as well.The egg stage can be laid singly, in cluster, or in cases, and in most cases it is very well protected from harsh environmental conditions. Sixth, insects posses an incredibly wide variety of specialized and well adapted appendages for feeding and locomotion. (flight allows dispersal almost world wide with time). Insects possess all types of legs for running, grabbing, digging, crawling, and clinging etc. They also possess wings. The ability to fly allows insects to disperse from a crowded and deteriorating habitat when necessary. For example, in Africa, populations of up to a billion locusts migrate annually from unsuitable dry areas to greener areas where rains are falling. In North America, Monarch butterflies will migrate to Mexico and coastal California to avoid harsh Canadian winters. Locally, ladybird beetles migrate to mountain tops during winter and summer.
Seventh, in some groups of insects, truly social behavior has evolved. Social behavior will allow a large population to survive through difficult periods via cooperation in food gathering, food storage, temperature control, and colony