Right now I'm reading Aliens: Steel Egg and the facehuggers spit acid. I'm guessing they'll do smething similar in this movie, and the spitter part is colored pink so it doens't look like the acid is coming from nowhere.
They never could spit acid, only secrete it when attached to something. (In order to get through to the hosts face)They don't have a mouth, just the ovipositor/tube.
And if they spit acid from the pink part of them in this movie, That is disappointing indeed, and would be another f**k up related to the aliens.
Yeah in the book the acid comes from the ovipositor but I wasn't going to put it past them that they'd change that. Otherwise, I guess it's a sensory organ of some kind, it looks like it has two pits like a pit viper does. You know, it would make sense for the facehuggers to sense the heat of their prey. That could be what triggers the eggs, too.
No, what triggers the egg is changes in heat/air density near the egg. While the facehugger is still within, the egg and the facehugger are sort of one organism in symbiosis, that can surivive for an indeifnite amount of time, due to slowing down its metabolism etc.
The egg basically has its own rudamentary nervous system and its the egg that detects the prey and then via adrenaline as well as other chemicals, awakens the facehugger. The "roots"/tendrils seen below the eggs, either serve as extensions of such a nervous system or to receive nutrients from the hive itself.