Text 15 arthropoda

The name arthropoda means literally "jointed feet". In terms of numbers of species and individuals, the arthropods are the dominant forms of animal life today. Over 500,000 species have been described and it is believed that there are many more to be found and identified. Typical forms include the insects, spiders, crabs, lobsters, centipedes and millipedes. The arthropods represent the highest, form of life to be found among invertebrate animals, (animals without backbones).

The most striking characteristic of the arthropods is the fact that they have jointed appendages. Their bodies are bilaterally symmetrical and segmented. They have a tubular digestive system and a ventral nerve cord.

The arthropods are divided into five main classes. In our study, we will consider the three most important ones — the Insects, Crustaceans and Arachnids.

Class Insecta

The insects are highest in the scale of invertebrate development. These animals are found almost everywhere - in air, water, soil and even as parasites within or on the bodies of animals and plants. They outnumber all other animals known to man. The scientific study, of insects is known as entomology.

The body of an adult insect is divided into three regions: head, thorax, and abdomen. All insects, at some time in their life history, have three pairs of legs. Trachea or air tubes are the organs of respiration; and Malpighian tubes are the characteristic organs of excretion. The mouth parts of insects are adapted for various functions, such as biting, chewing, piercing, and sucking. Adult insects, typically, have one pair of antennae, and two pairs of wings. There are various insects which do not possess all these typical characteristics. Some types, such as the flea and the louse, have lost their wings; and the flies have only one pair of wings. The scale insects during part of their life histories have no appendages, being little more than sacks, filled with eggs.

Metamorphosis

In their life histories, insects pass through several distinct stages. This process is called meta­morphosis (meaning "change of form").

Metamorphosis is said to be complete or indirect when the insect passes through four stages in its life history - egg, larva (caterpillar), pupa, and adult. The adult first lays an egg, which develops into a worm-like stage called a larva. Examples of larvae are the maggot of the housefly, the grub of the Japanese beetle, or of the honey bee, the wriggle of the mosquito, and the caterpillar of a butterfly. The larvae eat a great deal and in this stage many of them do serious damage to crops. The larva then secretes a case that completely surrounds it, and goes into an inactive period. The organism is now in the pupa or chrysalis stage. Frequently a cocoon is also present about the pupa. Finally, the adult emerges from the pupa stage. This type of development is characteristic of bees, beetles, flies, mosquitoes, etc.

Metamorphosis is said to be incomplete or direct when the insect undergoes a less marked series of changes. In direct metamorphosis the insect passes through only three stages-egg, nymph, and adult. The nymph which hatches from the egg looks much like the adult except that it is smaller and may not have any wings. The nymph grows into an adult by shedding its external skeleton. This process which is known as molting, may take place many times before the insect becomes an adult.

Tasks

1. Write down whether the following statements are true or false. Give extensive answers.

1. The arthropoda are divided into three main classes.

2. Crustaceans outnumber all other animal known to man.

3. All insects possess all their typical characteristics.

4. In complete metamorphosis the insect passes through five stages in their life history.

5. Moulting may take place many times before the insect becomes an adult.

2. Complete the sentences using the text and translate them into Russian.

1. The arthropoda represent … .

2. … as parasites within or on the bodies of animals and plants.

3. The month parts of insects are adapted … .

4. … the wriggle of the mosquito and the caterpillar of a butterfly.

5. The nymph which hatches from the egg looks much like … .

3. Answer the following questions on the text in written form.

1. What are arthropoda?

2. What are their characteristic features?

3. What is the body of an adult insect consist of?

4. What does metamorphosis mean?

5. When metamorphosis in said to be a) complete or indirect; b) incomplete or direct?

4. Translate a passage into Russian in written form.