Biology 生物研究 bio 人請進, 關於電影[異形2 Aliens] [***20分***]

I type better in English so may I answer in English first? I did not only copy and paste. I just paste some pictures to help explaining things I typed myself. I will look at this problem in a comparative anatomy perspective.
Hypothesis 1: It's a Diplobiontic creature
Diplontic life cycle
If you know about the life cycle of animals and plants there are diploid stages (2n) and haploid stages (1n). Diploid in human start as an embryo, through childhood into adult. Haploid(1n) stage is very short, only sperm and egg(gametes).

圖片參考：http://upload.wikimedia.org/wikipedia/commons/thumb/2/23/Gametic_meiosis.png/250px-Gametic_meiosis.png

Haplontic life cycle
In some animals like jelly fish, the body of the adult animal is actually 1n. So female and male jelly fish are the same genetic makeup with the gametes (sperm and egg). The 2n stage (zygote) is very short.

圖片參考：http://upload.wikimedia.org/wikipedia/commons/thumb/7/7f/Zygotic_meiosis.png/250px-Zygotic_meiosis.png

Diplobiontic life cycle
And there is a third type of life cycle that the 1n and 2n stages are all obvious. These are more common in plants called algae and bryophyte (fern allies). They grow one body that is 1n, and another body that is 2n. Sometimes this 2n is a parasite on its own 1n parent.

圖片參考：http://upload.wikimedia.org/wikipedia/en/thumb/8/86/Sporic_meiosis.png/250px-Sporic_meiosis.png

So back to your story, your alien may be a 1n (gametophyte) stage where you say 幼蟲, and after it has mated and made a 2n (sporophyte) where you say 成蟲. It is a very different looking animal and their relationship is not parent and child. It's something in between.
Hypothesis 2: It's a parasitic creature that needs an intermediate host to mature
This example is the Chinese Liver Fluke. The eggs are passed out into the water, eaten by snail. Inside the snail it grow into the next stage, bursted out the snail shell, and infect a fish by digging into its muscles. When human eat the fish it completes the lifecycle by infecting the intestine, and finally make its way into the liver.

圖片參考：http://upload.wikimedia.org/wikipedia/commons/thumb/1/1f/Clonorchis_LifeCycle.gif/280px-Clonorchis_LifeCycle.gif

So in your story, human is just one more host to complete this complex lifecycle. Considering your alien only needs one host (human) instead of three (liver fluke), that's a simple lifecycle indeed.
My choice and why:
I think you probably would like the Hypothesis 2 because it is what you describe with chinese words like 幼蟲, 成蟲. And very likely the novelist/movie makers got the Alien idea from these parasitic life cycle too. But personally I like the Hypothesis 1 because it is exotic. Not a lot of people are aware of Diplobiontic creatures, and people keep arguing in 知識+ about if sperms and eggs are animals. So this might as well be your next discussion topic!