More or less. They don't drop it, it just gets constructed, and to construct it you need vision I think. You have 20 seconds to kill it before units pop out. They pop out one at a time, so it's best to just hit the worm until it's dead before dealing with the units. I still have a lot of trouble with it, personally, but I'm not that good. This is just what I've read on the subject.
If you have two siege tanks blanketing your main, and a siege tank at each expo, they will take out the nydus before it finishes building, so long as they are in range.