pp test1 Listening Q191-196

[molecular]

191-196 Animal Physiology: Metamorphosis
Higher animal like dogs, horses even human beings look like their parents when they are born. There are some minor changes that occur as these animals grow. But the basic form of the structure stays the same. This is called direct development, which is quite different from metamorphosis. When lower animals like ants, flies and sea-urchins emerge from the eggs, they don’t look at all like their parents. Only after series of major changes do they take on the characteristics of their parents. Butterfly and moth provide the best example of what biologist call complete metamorphosis because they pass through 4 distinct stages of extreme changes. Here is the illustration of butterfly at the each of the four stages of complete metamorphosis. The first stage is the egg. Butterflies usually lay their eggs on plants and these plants provide food when they transform into larvae, the second stage. After about month in this stage, a butterfly larva spins the cocoon of silk around itself. A larva in the cocoon is also called pupa. So it’s known as the pupal stage. The pupal stage is the period of inactivity when larva structure evolves into adult structure. Depending on the species, the pupal stage can last anywhere from a few days to several months. When the pupa finally mature into the adult butterfly, it pushes its way out of its cocoon and cross onto the twig or tree land and pump the blood into the trunk and wing until they are full size and strong. Now some insects like grasshopper and roaches have only 3 stages—egg, larva and adults. When an animal skips the pupal stage, it is called incomplete metamorphosis. It’s not only insects that undergo the metamorphosis. Frogs and toads are the most well known example of other form-changing animal. Frogs lay their eggs in water and eggs hatch into tadpoles. A tadpole looks more like little fish because it has the tail and no legs. But it gradually loses its tail and develops into four legs frog.

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