After you defeat an Ensnared Kiru Guru, you are able to reach the Ridley Region.

Explore Ridley

From the bottom of the elevator to Ridley, go down to the bottom of the room. Go through the left door and save the game, then go through the door on the left.

There is a door on the left that you can't open with your current weapons, so go up through the opening in the ceiling.

Go up the right side of the room and you'll eventually see the ensnared Kiru Guru from earlier. You can't actually progress farther, so you need to go back to the elevator up to Norfair.

Norfair

Go through the door on the left and go through the tunnel. You'll notice that the Kiru Guru, which had been lodged in the floor, is no longer there, and there is now a hole in the floor. Go down through the hole. Make your way down and you'll be back in Ridley.

New Area of Ridley

Go as far down as you can go, and you'll see the shed shell of the Kiru Guru. Go through the opening in the left wall.

Go to the left and you will fall through Pit Blocks.

Go to the left and you will bump into some eggs along the ground. You can shoot the eggs to get them out of the way. When you reach the left side of the room, Imago will attack.