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In order to become a butterfly, a caterpillar's body dissolves almost completely and is rebuilt from its own juices.
为了变成蝴蝶，毛毛虫的身体几乎完全分解，并利用这些液体物质重塑身体。
As inconvenient and even downright dangerous as this process sounds, it's actually quite common.
虽然这个过程听起来既麻烦又危险，但它却非常普遍。
Butterflies are just a few of the 800,000 insect species that transition from larvae to adults through complete metamorphosis.
有80万种昆虫会经历从幼虫到成虫的完全变态发育，其中蝶类昆虫只占很小的比例。
That's approximately 85% of insects and 70% of all known animal species.
这80万种昆虫大约占全部昆虫的85%，占所有已知动物种类的70%。
But how exactly does a caterpillar become a butterfly?
但毛毛虫到底是如何变成蝴蝶的呢？
When a caterpillar hatches from an egg, it has none of a butterfly's physical traits.
当毛毛虫从卵中孵化出来，它并不具备任何蝴蝶的身体特征。
It does have groups of cells inside its body called imaginal discs that will eventually become its butterfly parts.
但它的体内拥有一些细胞，称为成虫盘，成虫盘最终会发育成蝴蝶的身体部分。
For now, these cells remain dormant.
现在，这些细胞处于休眠状态。
Juvenile hormone suppresses their activity, and prevents the caterpillar from beginning metamorphosis too early.
保幼激素抑制着这些细胞的活动，避免毛毛虫过早开始变态发育。
Immediately after hatching, the caterpillar begins to feed,
从卵孵化后，幼虫立刻开始进食，
building up fat until its rigid skin, called a cuticle, becomes too tight.
积累脂肪，直到坚硬的表皮开始绷紧。
At this point, a hormone called ecdysone triggers the cuticle to shed, or molt.
此时，蜕皮激素开始分泌，促进表皮脱落。
As the caterpillar grows, it usually molts four times.
在发育过程中，毛毛虫通常需要蜕皮四次。
Then, when it's nice and plump, the caterpillar's levels of juvenile hormone drop,
之后，表皮变得漂亮丰满时，毛毛虫体内的保幼激素水平下降，
which triggers it to stop eating and moving.
这会使毛毛虫停止进食和活动。
A final burst of ecdysone prompts the caterpillar's cells to begin to self-destruct.
蜕皮激素最后一次分泌促使毛毛虫的细胞进行自我分解。
Soon, the muscles, fat, and other tissues are almost entirely liquefied,
肌肉，脂肪等组织很快就会接近完全液化，
though the imaginal discs stay intact and begin to grow.
但成虫盘依然保持完好，并继续发育。
At the same time, a second skin layer called the pupal cuticle forms underneath the first.
与此同时，第二层皮肤蛹会在第一层皮之下形成。
One more molt exposes the hard exterior of the pupa.
再一次的蜕皮使蛹的坚硬外壳裸露。
Besides the imaginal discs, only a few tissues are spared,
在成虫盘周围只有很少的组织得到保留，
including parts of the respiratory system, the heart, some abdominal muscles, and the mushroom bodies of the brain.
其中包括部分呼吸系统，心脏，一些腹部肌肉和脑蕈形体。
The caterpillar juice then fuels the development of the imaginal discs into eyes,
幼虫解离形成的液体为成虫盘的发育提供能量，使其发育成眼睛，
antennae, legs, wings, genitalia, and other body parts.
触角，腿，翅膀，外生殖器以及其他身体部分。
Once its new body is built, the insect molts one last time, shedding the pupal cuticle.
一旦新的身体形成，会进行最后一次蜕皮，即脱掉蛹。
From there, it's free to fly away a new butterfly.
自此，一只新蝴蝶诞生了。
Even after such a dramatic transformation, the butterfly does retain some memories from its caterpillar days.
即便经历了如此巨大的变化，蝴蝶依然能够保留部分毛毛虫时期的记忆。
It's likely the mushroom bodies of the brain carry important knowledge from the caterpillar over to the adult butterfly.
这很可能是由于脑蕈形体将幼虫时期的重要知识传递给了成年蝴蝶。
How did such an involved developmental process come to be? We don't know for sure.
为何会有这样的发展过程出现呢？我们目前仍不确定。
The leading theory is that the caterpillar is actually a drawn-out version of a life stage
主流理论认为，毛毛虫实际上是某个生命阶段的延迟，
that takes place inside the egg for some other insects.
而对于其他昆虫，这个过程通常发生在卵中。
According to this hypothesis, over millions of years, the larvae evolved the ability to eat and live outside the egg.
根据这个猜想，在数百万年间，幼虫进化出了吃掉卵壳和在卵壳外生存的能力。
Regardless of how complete metamorphosis originated,
不管完全变态发育是如何开始的，
it's become part of the life cycles of a dizzying number of insect species.
它成了大量昆虫生命循环的一部分。
Still, plenty of species get along perfectly well with a simpler developmental process.
对许多物种来说，更简单的发育过程才是它们所适应的。
What survival advantages might complete metamorphosis provide to make up for the added hassle?
完全变态发育对生存有什么好处，才能弥补这种额外的麻烦呢？
For one, it keeps larvae and adults from competing for the same habitats and food sources.
其一，它使得幼虫和成虫免于争夺栖息地和食物。
And while the pupa may seem vulnerable,
虽然蛹看上去很脆弱，
this immobile stage can actually be a good way to pass parts of the year when food is scarce.
这种静止的状态可以很好地帮助幼虫渡过食物匮乏的时间。
To us, a butterfly's metamorphosis might sound as fantastic as a phoenix rising from its ashes.
对我们来说，蝴蝶的变态发育如同凤凰涅槃一般奇妙。
But these transformations are taking place all around us, all the time.
但是这些转化一直都在我们身边进行着。
From the Hercules beetle to the honey bee to the garden ant,
从独角仙到蜜蜂，再到蚂蚁，
countless squishy larvae dissolve and emerge as armored, aerodynamic, and nimble adults.
不计其数脆弱的幼虫经过分解发育成外表更为坚硬、呈流线型的敏捷成虫。
