Add me to those who have struggled with manual focus using a Hi-Def camcorder. After a while I've come to a pact with myself, at least for the time being: If there's no compelling reason to use manual focus I let autofocus handle it, as it always seems to do better than me. Maybe that's the coward's way out, but it keeps my frustration levels down.
One place where manual focus is a necessity is where you want to change focus in one continuous shot, between something closer up and something farther away. For example in a drama movie where you're seeing the victim with the background blurred and then the focus changes to show the stalker lurking in the background. Your spider video is another prime example. The subject is so small, and you're moving to follow it, and autofocus can't figure out what the subject is supposed to be. My Canon has a neat little feature which allows you to "point" to the subject that you want to focus on by touching it on the touch screen. The target becomes surrounded by a green rectangle that moves with the subject as it moves and/or you pan. I was skeptical at first, but it really works amazingly well. The magic of microprocessors. I suppose that no self-respecting pro is going to use auto-focus, but it's surely a lifesaver for this amateur.
In manual mode it helps to understand the relationship between aperture (F-Stop) and depth of field. If it's not important to you that the background (or foreground) necessarily be out of focus, a greater DOF will make it easier to keep your subject in focus. It gives you more breathing room. There are, of course, tradeoffs; you need more available light if you use a higher F-Stop. And aperture isn't the only thing that affects DOF. Maybe you're aleady familiar with this. If not just Google "aperture vs depth of field" for tons of information.