[RPG] Do Black Tentacles Bypass DR

Pretty much - it's any spell coming from a corporeal source (you know, a dude).

Now, it can get more complicated than that with things that aren't just incorporeal but also ethereal. Only force spells affect them directly but there's a lot of spells (e.g. blink) that can be used to make someone ethereal/incorporeal and then it's fireball time.

Yes, it could, unless the stirges are flying over 20' high - at the spells centrepoint anyway.

The SRD clarifies that any spell with a radius extends into the third dimension in a similar fashion.

Black tentacles has a 20' radius of effect and only effects creatures in the area of effect, therefore as long as the stirges have room to fly above that height they're safe, otherwise they're going to get grabbed.

Since the effect is a sphere they could squeeze past in a corner if there's room, I'd give them a flying roll for this to not miss a flap and become tenticalificed.

This does create another question however, how high do spells extend if they have an area of effect but don't have a radius (looking through a load of AOE spells they seem to have done a good job of covering this actually, most spells have a height specified or are a radius effect) Maybe I need a question "how high is a square?"