托福阅读背景材料(348)：变态的两栖动物

　　托福阅读背景知识/材料之变态的两栖动物，更多托福阅读背景材料/知识尽在新东方在线托福考试频道！

　　2014年6月21日托福阅读真题再现：

　　版本一：M什么的cycle从larva到adult 有的larva也会不到adult

　　版本二：metamorphosis，有frog 和butterfly 的例子。

　　文章开始说这种类型的生物挑选不同的niche 在不同的变态时期有困难。后来提到adult 有为后代挑选habitat 的义务。有些时候adult 吃是为了繁殖的需要。还有些物种变态到一个时期就不继续变态了，就是不进化到adult，然而他们已经成熟到可以繁殖了。当然这些发生在比较极端的环境，像什么高海拔什么的

　　版本三：

　　Metamorphosis(变态的两栖动物)

　　第一段说变态就是juvenile变成pupil，这过程中会有一个protection，可能会花费几个小时或几天。然后又从pupil花几天变成成虫。(这里有题)青蛙会从蝌蚪(tadpole)变成青蛙，蝴蝶会从毛毛虫(larva，复数larvae)变成蝴蝶，这就是变态(囧)。

　　第二段介绍了变态的优缺点，优点是可以有效地分配自己的life cycle，应对不同时期的不同环境。缺点是变态需要很详细的规划，而且会消耗大量能量。那么为什么还要选择变态这种方式呢?(这里有题，问写这句话的目的是什么)

　　第三段说青蛙的变态，因为青蛙的卵是在 比较恶劣的条件下诞生的，所以青蛙会把自己的体力大部分消耗在繁殖和产卵这部分life cycle上。

　　第四段说蝴蝶其实也是这样，不赘述了。

　　第五段说还有一些在好一点条件下诞生的卵会在juvenile阶段就sexual mature而且不再变成成熟形态。

　　最后一段就说什么情况下生物会选择metamorphosis呢?如果juvenile阶段的环境不好，那就很可能会选择metamorphosis。

　　新东方富亦聪解析：

　　标题出现术语时不要紧张，第一段一定会有相应概念解说。需要通读首段和其余各段首句后进行解题，避免造成一知半解，文章结构不清等状况。

　　相关背景：

　　Metamorphosis

　　Metamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation. Some insects, amphibians, molluscs, crustaceans, cnidarians, echinoderms and tunicates undergo metamorphosis, which is usually accompanied by a change of habitat or behavior.

　　Scientific usage of the term is exclusive, and is not applied to general aspects of cell growth, including rapid growth spurts. References to "metamorphosis" in mammals are imprecise and only colloquial, but historically idealist ideas of transformation and monadology, as in Goethe's Metamorphosis of Plants, influenced the development of ideas of evolution.

　　A. Frogs and toads

　　With frogs and toads, the external gills of the newly hatched tadpole are covered with a gill sac after a few days, and lungs are quickly formed. Front legs are formed under the gill sac, and hindlegs are visible a few days later. Following that there is usually a longer stage during which the tadpole lives off a vegetarian diet. Tadpoles use a relatively long, spiral‐shaped gut to digest that diet.

　　Rapid changes in the body can then be observed as the lifestyle of the frog changes completely. The spiral‐shaped mouth with horny tooth ridges is resorbed together with the spiral gut. The animal develops a big jaw, and its gills disappear along with its gill sac. Eyes and legs grow quickly, a tongue is formed, and all this is accompanied by associated changes in the neural networks (development of stereoscopic vision, loss of the lateral line system, etc.) All this can happen in about a day, so it is truly a metamorphosis. It isn't until a few days later that the tail is reabsorbed, due to the higher thyroxin concentrations required for tail resorption.

　　B. Butterflies

　　How does a caterpillar become a Moth or a Butterfly?

　　The answer to that can actually get quite complicated but basically what happens is this. When the caterpillar has eaten enough it turns into a pupa, more about this later on because it is different for different groups of Lepidoptera. To do this it stops eating and finds somewhere safe, here it becomes very still (pupa never eat and seldomly move at all) it then moults its skin the same as it does when it is growing only instead of another larval skin it secretes a pupal skin, (inside its old larval skin) that is much thicker and stronger. Generally this pupa then breaks out of the old larval skin, though in many moths the pupa remains inside the old larval skin, you can often find the remains of the caterpillar skin around the tail of a Butterfly pupa. All that is fairly straight forward, where it gets tricky is how the caterpillar inside its new pupal case changes itself into a Butterfly or Moth. The first thing that happens is that a lot of the caterpillars old body dies. It is attacked by the same sort of juices the caterpillar used in its earlier life to digest its food, it would not be far wrong to say the caterpillar digests itself from the inside out, this process is called "histolysis". Not all the tissue is destroyed however some of the insects old tissue passes on to its new self, the amount that does this varies between different insects, and is not very much in the Lepidoptera. There is one particular sort of tissue left, in a number of places in the insects body are collections of special formative cells, which have played no part in the insects larval life, and have stayed hidden or protected during this partial death, each of these groups of cells is called an "imaginal bud" or a "histoblast". The job of these histoblasts is to supervise the building of a new body out of the soup that the insects digestive juices have made of the old larval body. This they do using the same biochemical processes that all insects use to turn their food into part of their bodies. This rebuilding process is called "histogenesis". During this time the insect is very vulnerable because it cannot run away, and this is why insects try to choose somewhere safe to hide away when they are going through this incredible change, still I think you have to be very brave to be a Caterpillar and become a Butterfly or a Moth.

　　Larva

　　A larva (plural larvae /?lɑrvi?/) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle.

　　The larva's appearance is generally very different from the adult form (e.g. caterpillars and butterflies). A larva often has unique structures and organs that do not occur in the adult form, while their diet might be considerably different.

　　Larvae are frequently adapted to environments separate from adults. For example, some larvae such as tadpoles live exclusively in aquatic environments, but can live outside water as adult frogs. By living in a distinct environment, larvae may be given shelter from predators and reduce competition for resources with the adult population.

　　Animals in the larval stage will consume food to fuel their transition into the adult form. Some species such as barnacles are immobile as adults, and use their mobile larval form to distribute themselves.

　　It is a misunderstanding that the larval form always reflects the group's evolutionary history. This could be the case, but often the larval stage has evolved secondarily, as in insects. In these cases the larval form may differ more than the adult form from the group's common origin.