When you are in Norfair after defeating Kraid, you will eventually encounter a larva that you cannot pass, he blocks the way. When you have the Varia suit, you will be able to go around. You will obtain the Wave Beam as well. You will reach a room that is more or less impossible to get out. The door turns to gray behind you. The path above you is blocked by a larva. You can destroy it with the Wave Beam by getting underneath it (on the level below) and shooting its belly. After you defeat it, you will move to the east, only to find a larva charging at you. To defeat it, fire Missiles, Charged Beam, or the regular Beam multiple times to push him back. After he passes the little plant (long and oval shaped on the ground; you can walk through it), morph into your ball and wait. He will buck up and start to charge. Just before he hits you, plant a bomb and roll to the east. If timed correctly, the bomb will explode under his belly. You will be able to tell if it works as he will take damage. Repeat this until he dies.