Unit 12. Insects.

WARM-UP: Name the insects in the picture

VOCABULARY: Learn the following words and word combinations.

Invertebrate thorax abdomen to chew wing to suck antenna (pl. antennae) fly flea lice (pl. louse) larva (pl. larvae) caterpillar pupa (pl. pupae) bee beetle to undergo molting to scent palp cuticle claw jaw to swallow intestine ant

Guess the meaning of the following words.

Trachea; to bite; cocoon; mosquito; egg; external skeleton; segment; exoskeleton; parasite; animal; plant; respiration; breathing; excretion; muscles; muscular contraction.

Practice the following for pronunciation.

invertebrate [ın´vз:tıbrət]

thorax [´Өo:ræks]

abdomen [´æbdəmən]

trachea [trə´ki:ə]

antenna [æn´tenə]

flea [fli:]

louse [laυs]

larva [´la:və]

caterpillar [´kætəpılə]

pupa [´pju:pə]

cocoon [kə´ku:n]

cuticle [´kju:tıkl]

jaw [dʒo:]

READING: Read the text and do the tasks that follow.

The insects are highest in the scale of invertebrates development. These animals are found almost everywhere – in air, water, soil and even as parasites within or in 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 no 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.

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 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 (naiad), and adult. The nymph which hatches from the egg looks much alike 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.

May-beetles appear in spring, in the month of May. They are vegetable-eaters. The beetle’s body consists of three clearly distinctive parts – head, thorax, and abdomen. The head carries a pair of feelers (antenae), the eyes and mouth parts. The thorax is composed of three segments and bears three pair of legs and a pair of hard forewings which enclose the two membranous hindwings – the organs of flight.

On each side of the segmented abdomen are small openings used for breathing. The body ends with the anus.

Beetles have three pairs of segmented (jointed) legs, growing out of the thorax. The two main pairs of wings, the forewings, which are hard, and the membranous hindwings, are also used as means of locomotion.

The eyes of the beetle are compound, made up of a large number of separate units. Each unit can see only that part of the object that is directly opposite it. The eyes are not big but can only distinguish things at short distances.

The feelers (antennae) serve as the organs of smell and enable beetles to scent food a considerable distance away. Having landed on a tree the beetle begins to run about the leaves touching them with the palps, segmented appendages to its mouth-parts, which serve as the organs of touch.

The May-beetle has a hard external cuticle. This is composed of a substance known as chitin, which is excreted by the skin cells. The cuticle serves as protective armour.

Where the segments join, the cuticle is thin and flexible, allowing the beetle to move its head, thorax, abdomen, legs, feelers and palps.

The cuticle is not only the protective armour, it also serves as an exoskeleton, to which the muscles are attached.

Different sections of the beetle’s skeleton and body, as well as the legs, are set into motion by muscular contraction.

The beetle takes a leaf between its legs which end in claws, and bites off a piece with its jaws.

The jaws, two hard, toothed plates, are found on each side of its mouth.

The upper and lower lips hold the food, which is immediately swallowed.

The beetle is unable to chew, therefore quite large pieces of leaves enter the intestine.