Layers, roto, motion tracking. If you don't have any idea of what I am talking about start here: Basic Workflow
You need to track your head from the point where you want spiders to start appearing, use that tracking info to position a null that you can use for maintaining position of your spider layers. duplicate your footage layer and Roto any parts of your face that you want to hide the spiders as they emerge, then create a bunch of spiders. I'd use a particle system with a custom particle to make the spiders, then lock the spider layer to the null by parenting.
The key to making this work is pre production planning. You have to get suitable footage to make this work well.
Once you get the layers all set up you will need to do some color grading to make things look real. This is not a single click with a plug-in project.