新托福听写训练 第7期（在线收听）

Higher animals like dogs, horses, even human being look like their parents when they are born. There are some minor changes that occur as these animals grow. But the basic form and structure stay the same. Now this is called direct development, which is quite different from metamorphosis. When lower animals like ants, flies, or sea urchin emerge from the eggs, they don't look at all like their parents. Only after series of major changes do they carry on the characteristics of their parents. Butterfly and moth provide the best example of what biologists call complete metamorphosis because they pass through four distinct stages of extreme changes. Here is the illustration of butterfly at 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 months in this stage, a butterfly larvae spins the cocoon of silk around itself. A larvae in the cocoon is also called pupa. So it is known as the pupil stage. The pupil stage is the period of inactivity when larvae structures evolved adult structures. Depending on the species, the pupil stage can last anywhere 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 a twig or tree land and pumps blood into the trunk and wing until they are full size and strong.Now some insects like grasshoppers and roaches have only 3 stages, egg, larva, and adult. When an animal skips the pupil stage, it is called incomplete metamorphosis. It's not only insects that undergo metamorphosis. Frogs and toads are the most< animals >well know example of other form changing animal. Frogs lay their eggs in water and the eggs hatch into tadpole. A tadpole looks more like little fish, because it has tails and no legs. But it gradually loses its tail and develops into four-legged frog.