INSECTS

                                                                                              INSECTS                                                                                                                                                                          Insect, common name given to any animal of a class belonging to the arthropod phylum. The insects are the largest class in the animal world, outnumbering all other animals. At least 800,000 species have been described, and entomologists believe that as many or more remain to be discovered. The class is distributed throughout the world from the polar regions to the tropics and is found on land, in fresh and salt water, and in salt lakes and hot springs. The insects reach their greatest number and variety, however, in the tropics. In size, the insects also exhibit great variation. Some small parasitic insects are less than 0.025 cm (less than 0.01 in) in length when fully grown, whereas at least one fossilized species related to the modern dragonflies is known to have had a wingspread of more than 60 cm (24 in). The largest insects today are certain stick insects about 30 cm (about 12 in) long and certain moths with wingspans of about 30 cm (about 12 in).

Insects also are the most highly developed class of invertebrate animals, with the exception of some mollusks. Insects such as the bees, ants, and termites have elaborate social structures in which the various forms of activity necessary for the feeding, shelter, and reproduction of the colony are divided among individuals especially adapted for the various activities. Also, most insects achieve maturity by metamorphosis rather than by direct growth. In most species, the individual passes through at least two distinct and dissimilar stages before reaching its adult form.

In their living and feeding habits, the insects exhibit extreme variations. Nowhere is this more apparent than in the life cycle of various species. Thus the so-called 17-year locust matures over a period of 13 to 17 years. The ordinary house fly can reach maturity in about ten days, and certain parasitic wasps reach their mature form seven days after the eggs have been laid. In general the insects are very precisely adapted to the environments in which they live, and many species depend on a single variety of plant, usually feeding on one specific portion of the plant such as the leaves, stem, flowers, or roots. The relationship between insect and plant is frequently a necessary one for the growth and reproduction of the plant, as with plants that depend on insects for pollination. A number of insect species do not feed on living plants but act as scavengers. Some of these species live on decaying vegetable matter and others on dung or the carcasses of animals. The activities of the scavenger insects hasten the decomposition of all kinds of dead organic material.