在2014年6月21日的托福閱讀考試中有這樣一道題：變態(tài)的兩棲動物。針對這道托?？碱}，出國留學網(wǎng)(www.liuxue86.com)小編來為大家普及一下關(guān)于變態(tài)的兩棲動物的背景知識，這樣有助于考生在面對這類題目時方便作答。小編在此提醒大家：標題出現(xiàn)術(shù)語時不要緊張，第一段一定會有相應概念解說。需要通讀首段和其余各段首句后進行解題，避免造成一知半解，文章結(jié)構(gòu)不清等狀況。
托福閱讀真題再現(xiàn)：
版本一：M什么的cycle從larva到adult 有的larva也會不到adult
版本二：metamorphosis，有frog 和butterfly 的例子。
文章開始說這種類型的生物挑選不同的niche 在不同的變態(tài)時期有困難。後來提到adult 有為後代挑選habitat 的義務。有些時候adult 吃是為了繁殖的需要。還有些物種變態(tài)到一個時期就不繼續(xù)變態(tài)了，就是不進化到adult，然而他們已經(jīng)成熟到可以繁殖了。當然這些發(fā)生在比較極端的環(huán)境，像什麼高海拔什麼的
版本三：
Metamorphosis(變態(tài)的兩棲動物)
第一段說變態(tài)就是juvenile變成pupil，這過程中會有一個protection，可能會花費幾個小時或幾天。然后又從pupil花幾天變成成蟲。(這里有題)青蛙會從蝌蚪(tadpole)變成青蛙，蝴蝶會從毛毛蟲(larva，復數(shù)larvae)變成蝴蝶，這就是變態(tài)(囧)。
第二段介紹了變態(tài)的優(yōu)缺點，優(yōu)點是可以有效地分配自己的life cycle，應對不同時期的不同環(huán)境。缺點是變態(tài)需要很詳細的規(guī)劃，而且會消耗大量能量。那么為什么還要選擇變態(tài)這種方式呢?(這里有題，問寫這句話的目的是什么)
第三段說青蛙的變態(tài)，因為青蛙的卵是在 比較惡劣的條件下誕生的，所以青蛙會把自己的體力大部分消耗在繁殖和產(chǎn)卵這部分life cycle上。
第四段說蝴蝶其實也是這樣，不贅述了。
第五段說還有一些在好一點條件下誕生的卵會在juvenile階段就sexual mature而且不再變成成熟形態(tài)。
最后一段就說什么情況下生物會選擇metamorphosis呢?如果juvenile階段的環(huán)境不好，那就很可能會選擇metamorphosis。
新東方富亦聰解析：
標題出現(xiàn)術(shù)語時不要緊張，第一段一定會有相應概念解說。需要通讀首段和其余各段首句后進行解題，避免造成一知半解，文章結(jié)構(gòu)不清等狀況。
相關(guān)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.
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