Their only attack is shooting neutrons of varying colors (purple in The Next Wave, orange in Revenge of the Yolk). They do it every few seconds, and in Revenge of the Yolk's later waves, they're able to shoot up to three in a row.
Their appearance is simple. They are flying saucers with five windows on the top and six green lights on the bottom. In Revenge of the Yolk, they were changed to be slightly more detailed.
The Christmas edition for The Next Wave turned them into santa sleighs, while editions of Revenge of the Yolk do not alter them at all.
They appear in Bonus waves in The Next Wave, and Close Encounters waves in Revenge of the Yolk. In The Next Wave, they only have one hitpoint. After destroying them in that game, they're guaranteed to drop a Powerup and give you 10000 points. In Revenge of the Yolk, they come in four sizes - small ones with Chapter * 100 health, medium small ones with Chapter * 100 + 100 health, medium big ones with Chapter * 100 + 200 health, and big ones with Chapter * 100 + 300 health. The small ones always give 3000 points, medium small ones 4000 points, medium big ones 5000 points, and big ones 6000 points. They all have a chance to drop an Atomic Powerup. Every wave they appear in in Revenge of the Yolk, they can shoot more neutrons in a row. In the first wave, they can only shoot one, in the second two, and in the third one three. The Next Wave instead decreases the time between shooting.
Their successors can be considered to be Aliens, as they are both saucers. The alien in the UFO however isn't seen.