In the original Alien, the facehugger had to break through Kane's helmet to get at him. When we see the edges of the glass, they appear to be melted by acid. I have heard that facehuggers can spray acid when they need to latch onto something. But if this is so, then how do the facehuggers know what is artificial and what is part of the host? I don't think it would risk killing its target like that.
There is another suggestion that is simply broke through the glass, but the same instance applies for the acid attack. For the facehugger to strike hard enough to shatter the glass, it would have to hit that hard on the first try. How would it know weither or not to hit with that much force? If it did that every time, then the host's face would surely suffur bone fractures to their face.
My question is, how do facehuggers break through obstructions such as Kane's helmet without injuring or killing the host?