Insects

An insect is an invertebrate animal with six legs and a body that is clearly divided into three main segments. The heads of most insects have a pair of antennae, compound eyes, and large jaws. Insects inhabit nearly every part of Earth and make up the most numerous class of living animal. They are considered to be the most successful group of living creatures ever to have lived on Earth.

Insects belong to the phylum Arthropoda and make up its largest class. Well-known examples include bees, ants, butterflies, grasshoppers, beetles, moths, mosquitos, and the house fly. Insects are so diverse that there is probably no typical insect, although there is a basic insect anatomy or structure. An adult insect has three distinct body segments—the head, thorax, and abdomen. The head contains the sense organs like antennae and eyes, as well as three pairs of mouthparts that are adapted for either biting, chewing, puncturing, or sucking, depending on the species. The thorax has three segments, each of which has a pair of legs that are used for walking and clinging. If an insect has wings, they are attached at the thorax. Also located at the thorax are small tubelike openings called trachea that insects use to take in oxygen and expel carbon dioxide. Some of these openings are also found on the forward part of the abdomen. Since insect tissue gets oxygen directly through these tracheae, the circulatory system is fairly simple. The insect abdomen is used primarily for reproductive purposes. Digestion occurs in a three-part gut and wastes are excreted out of very specialized organs called Malpighian tubules. Named after the Italian anatomist (a person specializing in the structure of animals) who discovered them, these function like kidneys and remove waste from the insect's system. Insects also possess an exoskeleton, a hard outer support structure that protects their soft internal organs and provides some protection against predators.

Insects have been described as the dominant form of life on Earth. One writer even argues that if visitors from other planets came to Earth and studied its life forms, they might want to communicate first with an

insect because of their staggering diversity and overwhelming numbers that the insect represents. One reason for this steady success is the small size of insects. A small insect does not need as much food as a large animal does, so it is easier for an insect to find food. Insects also have highly specialized mouthparts and digestive systems that allow them to consume almost any plant in existence. If necessary, however, they will eat anything they can find. An insect's small size also allows it to hide more easily from its enemies, thus possibly enabling the insect to avoid predators and live longer.

Another reason for their success could be the distinct advantage having wings provide insects. Being able to fly means insects can easily escape earthbound predators, but it also confers the advantage of being able to leave a certain habitat if it proves difficult or dangerous to live there. Flying also allows insects to populate other habitats that most animals would have a hard time reaching. Altogether, their small size and flying ability gives insects a competitive advantage in the struggle for survival.

Another major reason that certain insects prosper is because they are social insects. Although some live independently of others, some insects live together in what are called colonies. A colony is a group of animals that live together and share work and food. Honeybees, termites, ants, and wasps are social insects. In a colony, an individual insect does not have to provide itself with food and shelter, since all the tasks needed for living are carried out by different members of the colony. Each member has a specific job to do, and it performs only that role and nothing else. For example, in an ant colony, a queen ant lays eggs and contributes to the growth of the colony; soldier ants guard the colony and do the fighting during an attack; nurse ants tend only to the queen, her eggs, and the larvae; and worker ants locate food outside the colony and bring it back for all. All of the roles are performed totally by instinct. The system works because each insect in a colony spends every day of its life doing the job it must do and nothing else. Besides being social, insects also have a staggering reproductive capacity. For example, if all the eggs of a single fly were to survive and reproduce through only six more generations, there would be more than 5,000,000,000,000 flies.

In many ways, insects are mankind's most aggressive competitor. They will eat crops as well as stored food. They can swarm and consume every green thing in sight. They also can destroy paper, wood, and cloth. They bite humans and other animals and transmit diseases.

On the other hand, insects are important and necessary pollinators of flowers and crops, and many beneficial insects (like the ladybug and praying mantis) attack or destroy many insects harmful to humans or crops. Bees provide honey as well as pollination. Overall, human beings have now learned that we cannot defeat or even diminish the range, extent, and diversity of the insect population. However, humans can control insects' negative effects by learning more about their habits, needs, and life cycles.

[See alsoInvertebrates ]