Xevious question

Hello all,

My first post. Nice to find people that still like to play classic Atari games.

In Xevious on the 7800 on Intermediate level well into the game an object appears out of the sky, separates and goes on each side of your ship. It looks like 2 rotating squares. There is a good screenshot of it here:

http://atariage.com/..._Xevious_14.png

It stays there about 15 seconds then rejoins into 1 object then flys off the top of the screen. A few seconds later a Garu Zakato Energy Bomber (Large black ball that explodes and sends out lots of objects in all directions) appears.

Any idea what that is and what is does if anything?

Thanks in advance