Gamma Metroid

The Gamma Metroid is the fourth stage of a typical Metroid's life cycle. They appear in two games in the Metroid series, as an enemy in Metroid II: Return of Samus, and a cameo in Metroid Fusion. The Gamma stage is not very different from the previous Alpha Metroid; both have the closed membrane as the belly and a plated exoskeleton on top. Whereas the Alpha Metroid had small barb-like growths along its sides, they have now grown into longer limbs. The previous form also had only a pair of single eyes, and now they have multiplied into a pair of groups of three. The creature's forehead horn has become larger and more well-defined. The Gamma Metroid is capable of paralyzing enemies with a small field of static electricity emanating from its lower membrane. This lightning attack can deflect Missiles as well as deal damage to Samus. Gamma Metroids can withstand ten missiles, assuming it doesn't deflect any.

Samus Aran encountered sixteen Gamma Metroids during her mission to SR388.

Manual
"After molting from the lesser mutation, the Metroids will grow much larger, gaining the power to attack with lightning bolts."

Nintendo Power issue 31
"These creatures are much larger than the Alphas and they attack intruders with stinging lightning bolts. Hit them quickly."

Trivia

 * Hopping Metroids bear a similar physical structure to Gamma Metroids. In fact, the former may potentially be a Phazon-mutated strain of the latter.

Gallery
Metroid Gamma