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You are pretty close, actually, the caterpillars have a honey gland, an organ that secretes an amino acid and carbohydrate liquid.
你已经很接近了，事实上，毛毛虫有一个蜜腺，一种能分泌氨基酸和液态糖的器官。
The caterpillars secretes the liquid from the honey gland, rather large quantities, enough to feed several ants.
毛毛虫从蜜腺中分泌出大量液体，足够喂饱好几只蚂蚁。
But what makes this relationship obligatory for the caterpillar?
但是这种关系对毛毛虫来说为什么是强制性的呢？
Well, if the ants don’t feed regularly on the liquid from the caterpillar’s honey gland, the gland overloads and gets infected.
如果蚂蚁不定期吃掉毛毛虫从蜜腺中分泌出的液体，蜜腺就会负荷过重，受到感染。
The infection will kill the caterpillar and it’ll never reach its final stage of development, becoming a butterfly. John?
这种感染对毛毛虫来说是致命的，它永远也无法成长到进化的最后阶段变成一只蝴蝶了，John，请说？
OK, I just want to make sure I’m following here, the caterpillar needs the ants or it won’t make it to the stage where it can become a butterfly.
我只是想确认一下我听懂了，毛毛虫需要蚂蚁，否则就活不到进化成蝴蝶的阶段了。
And the ants do this because they get an easy meal out of it, right?
而蚂蚁这么做是因为它们能从中获得白吃的午餐，对吗？
But the ants don’t absolutely need the caterpillar for survival, ‘cause they can get food from other places, right?
但是蚂蚁不是绝对需要毛毛虫才能生存啊，因为它们能从其他地方弄到食物，不是吗？
So it’s still called mutualism even though it seems like the caterpillar’s getting way more out of it.
所以这种关系仍被称为互惠共生，即使看起来毛毛虫获得的利益比较多。
Oh, wait, you said they don’t have to equally benefit. Never mind, sorry.
啊，等一下，你说过它们得到的不一定是同等的好处，不好意思，忘记我说过的话吧。
Yes. But there is a type of mutualism where the relationship is necessary for both organisms to survive.
是的，但是有一种类型的互惠共生关系对两种生物来说都是生存必需的。
It’s called obligatory mutualism. And we’ll talk about that in the next class.
它叫做义务性共生，我们下节课再讲。
