In my latest game (v5. 30, conventional start), the Invaders are doing some very odd things. They've got jump point survey ships now, so they don't have to rely on a wormhole opening up to access a system. Of course, this means that when I discovered a wormhole in the third system I encountered, they immediately started sending enormous numbers of ships to earth.
However, once they arrive, they do nothing. They just sit in orbit and eventually disappear. It's rather frustrating as they cause lots of additional intervals, which take forever to process, and I can't blast them out of the sky yet as I haven't researched meson cannons. The only aggressive action I've seen them take is that occasionally one of their survey ships will ram a freighter to destruction. That's it.
I'm unsure whether this is a bug or intentional, to prevent the Invaders from killing off early empires too quickly.