7. Read and translate the text.

Text 1

Insects

A

The world's most abundant creatures are the insects, whose known species outnumber all the other animals and the plants combined. Insects have been so successful in their fight for life that they are sometimes described as the human race's closest rivals for domination of the Earth.

Despite their diversity, all adult insects share some basic external and internal anatomical features. Insects are distinguished from other members of the animal kingdom by having six legs; one pair of antennae; a ringed, or segmented, body; and three well-defined body regions. It is from the joined body rings, or segments, that insects derived their name, for the Latin word “insecta” means “segmented.”

The three main sections of an insect body are the head; the middle section, or thorax; and the hind section, or abdomen. The body is covered with a horny substance containing chitin. The protective plate also serves as an external skeleton, or exoskeleton, for the support of the internal organs.

The head bears the antennae, the mouthparts, and the eyes. The thorax has three segments; on each is a pair of legs. In winged insects the thorax also bears one or two pairs of wings. The abdomen typically has 11 segments, though no more than 10 are visible; it contains a large part of the digestive system. In females the ovipositor, or egg-laying organ, is located at the tip of the abdomen.

B

The nervous system of the insect includes a brain and a pair of parallel nerve cords, which extend along the length of the underside of the body. Along the nerve cords are a series of nerve masses, called ganglia. Each ganglion controls certain activities and is more or less independent of the others.

Air enters the body through breathing pores, called spiracles. A pair of spiracles is usually found on each of two thoracic segments and on several abdominal segments. From the spiracles, large air tubes called tracheae and smaller ones known as tracheoles carry air to all parts of the body. Some water insects breathe by means of gills. Other aquatic insects have a tube that leads to the water's surface. Certain internal parasites and very primitive insects breathe directly through the body wall.

C

Insect eyes are of two general types—simple and compound. Simple eyes, also called ocelli, are usually located in small clusters on the sides of the head or on the forehead. Although small, they may easily be seen by means of a magnifying glass. Ocelli are found in both immature and mature insects, but they appear to be more important in the mature forms. Individually these organs can do no more than detect light; however, the sensations received by several ocelli can together produce in the insect's brain an image of the surrounding area as the creature turns its head from side to side.

Compound eyes, like the sight organs of higher animals, are present in pairs. They are most common in adult insects. Some—certain mayflies (чешуйница), for example—have two pairs of compound eyes.

The eyes are called compound because each one is composed of many lenslike facets. Each of these facets—of which there are, for example, some 25,000 in a single dragonfly eye—receives a separate image. The total effect of these images is a composite picture in the insect's brain. The eyes of many insects—bees, for example—are sensitive to ultraviolet light, but insect eyes are generally less sensitive to colors at the red end of the spectrum.

D

The development from egg to adult is most interesting, especially in those insects that go through the complex changes called complete metamorphosis. The growth of insects is quite different from that of vertebrates because the insect skeleton is an external covering rather than an internal framework. Except for the pliable fold between the plates of chitinous cuticle making up the exoskeleton, there is no place where expansion can occur; thus the growing insect must periodically shed, or molt, its covering. The new skin, already formed, then expands and begins to harden.

The offspring of all insects undergo a varying number of such growth intervals before maturity. Adult insects do not grow at all. With the exception of the subimago (subadult) stage of the mayfly, only adults have functional wings. Primitive species such as silverfish mature with little change in appearance except their size. These kinds of insects are known as ametabolous insects. The immature insects of such species are called simply the “young.”

Bees, beetles, butterflies, and moths are some of the insects that go through all the changes of complete metamorphosis. They are said to be holometabolous. The young are called larvae (singular, larva). In the inactive stage immediately preceding adulthood they are called pupae (singular, pupa).

The larva hatches from an egg. Often larvae are mistaken for worms. They may be smooth-bodied, like the maggots of the fly, or hairy, like some caterpillars .

Near the end of its larval stage, the insect must find a place in which to pupate, or turn into a pupa. Beetle larvae, as well as certain caterpillars, may hollow out cells in the soil. Some caterpillars may spin silken cocoons about their bodies; some may spin bands to hold themselves against twigs or leaves.

The pupal stage is a time of tissue transformation. During this period different kinds of mouthparts, legs, eyes, and, perhaps, breathing organs must replace those of the larva. When the changes are completed, the creature bursts out of its old skin to become a fully developed insect. In this final, sexually mature state, it is also known as an imago.