Unit seven

I. Pre-reading Activities.

A. Practise the pronunciation of the following words and memorize them. • muscles /ˈmʌs(ə)l/ • appropriate / ə'prəuprɪət / • adaptation /ædæp'teɪʃ(ə)n / • innervate /ˈɪnəveɪt/ • contribute /kənˈtrɪbjuːt/ • conserve /kənˈsəːv/ • flexible /'fleksəbl / • pressure /'preʃə / • contraction /kən'trækʃ(ə)n / • abdominal /æb'dɔmɪn(ə)l/ • perceive /pəˈsiːv/ • elaborately /ɪ'læb(ə)rətlɪ /

B. Look up the meaning of the following words in the dictionary and memorize them.

• exoskeleton • tracheae • muscuskeleton • fiber • neurons • aerodynamic • thorax • axis • vessel • vortex • mitochondria • tissue

С. Discuss the question.

- How does the body structure determine the flying function of insects?

II. Reading Activities. A. Read the text below using a dictionary.

MUSCLES AND INSECT FLIGHT

Encyclopedia of animals

Flight is an adaptation that has contributed to the great biological success of insects. In many flying in­sects, the striated flight muscles are attached not directly to the wings but to the flexible portions of the exoskeleton that articulate with the wings. Each contraction of the muscles produces a dim­pling of the exoskeleton in association with a downstroke and sometimes, depending on the exact arrangement of the muscles, on the upstroke as well. When the dimple springs back into its resting position, the muscles attached to it are stretched. The stretching immediately initiates another contraction, and the cycle is repeated. The deformation of the cuticle is transmitted as a force to the wings which beat—so fast that we may perceive the sound as a musical tone. In the common blowfly, for instance, the wings may beat at a frequency of 120 cycles per second. Yet, in the same blowfly, the neurons that innervate those furiously contracting flight muscles are delivering impulses to them at the astonishingly low frequency of three per second. The mechanical properties of the musculoskeletal arrangement provide the stimuli for contraction, by stretching the muscle fibers at a high frequency. But the nerve impulses are needed to maintain it.

Flight muscles must be kept at appropriate operating tempera­ture if they are to function. You have probably noticed the constant twitching of the wings of such insects as wasps even when they are crawling instead of flying. Probably this behavior is necessary to keep the temperature of the flight muscles high for instant readi­ness. You may also have noticed that the bodies of many moths are quite furry. Moth fur serves the same function as fur in a mammal, to conserve body heat. When the moth awakens and prepares for flight, it shivers its flight muscles at a low frequency to warm them up, constricting its abdominal blood vessel to keep the heat in its thorax. Gradually the frequency of the shivering increases until, at a critical moment, the moth spreads its wings and hums off into the darkness.

Insect flight muscles in action have a very high metabolic rate, perhaps the highest of any tissue anywhere. Accordingly, these mus­cles contain more mitochondria than any known variety of muscle, and they are elaborately infiltrated with tiny air-filled tracheae that carry oxygen directly to each fiber.

Many insects have special adaptations to rid the body of the excess heat produced by the flight muscles. The rapidly flying sphinx moth, for example, has in its abdomen what amounts to a radiator - a great blood vessel that carries heat from the thorax, where it is generated, and emits it into the cool of the night.

Just how insects fly has been an aerodynamic mystery. Insect flight involves much more than just flapping the wings up and down. The flapping motion of the insect wing changes direction and speed, and upstrokes alternate with downstrokes at very high rates. At each shift of stroke, the wing rotates about its long axis, and tilts to just the right angle for the new direction of motion.

Somehow insects are able to create lift that is 20 or more times their body weight. In 1996 Charles P. Ellington and his col­leagues at the University of Cambridge in England reported that as insects flap their wings downward, some of the air flowing over the wings rolls up along the front edge forming a vortex (like a whirlpool) that becomes larger as it moves along toward the tip of the wing. The vortex generates a low pressure region above the wing that sucks the wing upward. This generates an extra lifting force.

B. Reading Comprehension.

- What is flight?

- How does temperature affect the function of the flight muscles?

- How do the flight muscles work?