Flood stalker form

The Flood Stalker Form is a Flood Pure Form that specializes in stealth and agility. It is apparently made from the re-casted bone of dead enemies. It will frequently mutate into either a Ranged Form or a Tank Form when the need arises.This form sits low beneath your field of view and their attacks can go widely unnoticed until you look down. They are able to crawl on walls and ceilings like a spider with relative ease and are capable of enormous jumps much like the normal parasitic combat forms. A possible flaw in the Halo 3 AI is that it has been observed to rarely attack the player and normally prefers to mutate itself into one of the two other Pure Forms. It rarely attacks the player off of the levels Cortana and Halo

How to Defeat Stalker Forms
Stalker forms are very weak, so a single blast from a shotgun, a good swipe from an energy sword or a long burst from an assault rifle will kill it. However, they are incredibly fast and agile, so great accuracy is needed. If you can get up close, a single melee attack from any weapon can usually bring one down. Any projectile weapon will do, but grenades do a much better job, because their spindly physique makes them harder to hit. When playing on higher difficulties, they have a frustrating habit of mutating when they're stuck. The grenade appears to be absorbed but does not detonate, making it useless. If you hit them with an Incendiary Grenade or a Flamethrower, they will instantly jump away and scream. Then they will die after a few seconds of burning. The name "Stalker" is represented quite well on both Cortana and Floodgate as they blend in quite well with the surrounding Flood ridden walls.

Trivia

 * Stalker Forms are more likely to mutate into one of the other Pure Forms than to attack in open combat against a player.
 * The Stalker Form is similar to a spider.