Class Insecta

Almost three-quarters of all animal species on Earth are insects. Although insects are not found in salt water, they live in almost every freshwater and land habitat. Insects range from the fairy fly, only 0.2 mm long, to the African Goliath beetle, over 10 cm long. Insects are the only invertebrates that can fly.

Insects include bees, ants, beetles, butterflies, fleas, lice etc. The insects constitute the largest class, by far, of the arthropods. In fact, there are more species of insects than of all other animals combined. About 800,000 are known, of which about 275,000 are beetles. Most insects are terrestrial and most breathe by means of trachea.

The evolutionary success of insects is due in part to the characteristic structure they share with other arthropods – exoskeletons, segmented bodies, and joint appendages. A key to their success is that most insects are small and have adapted to specific habits and needs. In many cases this allows multiple species of insects to exist without competing with one another for scarce resources. Their ability to fly has also added to their evolutionary success.

Insects are both harmful and beneficial to human society. Only about 1 percent of insect species are destructive to crops and property. Harmful insects include household pests, such as termites; crop and livestock pests, such as boll weevils; and hosts of disease-causing organisms, such as mosquitoes infected with parasitic protozoa.

Many insects, on the other hand, are beneficial to human society. Insects pollinate fruit trees, flowers, and many field crops. Bees produce honey and beeswax, silkworms form cocoons from which silk is spun, and lac insects provide the raw material for commercial shellac. Some kinds of insects are natural enemies of destructive insects. For example, the larvae of certain wasps feed on caterpillars that destroy plants.

The characteristic features of insects: three main divisions – the head, the thorax, and the abdomen; three pairs of legs; one pair of antennae and two kinds of eyes. The simple eyes detect light and darkness the compound eyes detect shapes. One more characteristic feature - a set of mouthparts similar to those of the lobster. In the more primitive insects, such a grass-hopper, the mouthparts are used for handling and masticating food but in the more highly specialized groups, the mouthparts are often molded into sucking, piercing, slicing or sponging organs, many of which are exquisitely adapted to the nectaries (nectar- holding organs) of special flowers.

On the head are antennae, compound eyes and mouthparts. An insect’s mouthparts are modified for different methods of feeding. Butterflies have mouthparts shaped like coiled tubes, which they uncoil and use to suck nectar from deep within flowers. A praying mantis tears its food apart with mandibles. Flies have no mandibles and therefore lap up food with a tonguelike lower lip.

Three pairs of legs and any wings the insect has are attached to the thorax. The legs are usually adapted for jumping, walking or running.

Most adult insects have two pairs of wings made up of light strong sheets of chitin; the veins in the wings are chitinous tubules that serve primarily as braces. The wings of the various orders of insects have evolved separately from one another. In some, such as the fleas and lice, they have been partially or totally lost returning the insect to the condition of its wingless ancestors.

The abdomen may have as many as eleven segments. The abdomen houses the heart, the respiratory and excretory organs, and the spiracles through which the insect breathes. In respiratory, tracheae carry air to body cells. All insects have an open circulatory system, through which blood flows into large open spaces surrounding the organs.

Incomplete Metamorphosis

Insects begin their lives as tiny eggs. However, most insects change from one kind of body form to another during their lives. This change in body size and shape is called a metamorphosis. This process happens in two general ways.

In some species, the egg hatches into a young insect that looks very much like the adult insect, only smaller. This development process is called incomplete metamorphosis. For ex., grasshopper egg hatches into a small grasshopper called a nymph. As the nymph grows, it sheds its old exoskeleton a number of times. Each time it develops a new, larger exoskeleton. Eventually, the nymph becomes an adult. The adult insect reproduces and lays eggs.

Complete Metamorphosis

It is another developmental process found in some insect species. During the complete metamorphosis, the young insect looks very different from its adult form.

For example, the complete metamorphosis of a butterfly involves 4 stages of development. First, a butterfly egg hatches into a caterpillar (larva). The next stage is called the pupa stage. During it, the larva is covered by a shell, inside which it gradually changes into an adult.

Date: 2014-12-22; view: 949