Metroid Husk

Metroid Husks are the organic materials discarded by Metroids, usually when the creatures have undergone a metamorphosis. Various types of husks, shed skin and membranes are seen throughout the Metroid series.

Metroid II: Return of Samus
On planet SR388, a Metroid will shed twice in its lifetime, each time when it drastically alters its physiology. The first husk is the entire membrane of its larval form. Prior to its metamorphosis, a Larva will cease to levitate and lay on the ground, flashing brightly until the Alpha Metroid emerges from the topside, leaving an empty and shattered membrane. These discarded husks are present in nearly every area of the planet and their presence usually means there are Metroids in close proximity. The membranes are seemingly soft and viscous on the inside, as Samus' body will slowly sink through it if she stands on top of their opening.

The second husk consists of the body of a Gamma Metroid. It is discarded in its entirety when a Zeta Metroid breaks out from the former's backside, akin to an insect emerging from its pupa. Like the previous shedding, the Metroid is initially flashing brightly prior to the transformation as well. The process is done in midair, which causes the empty body of the Gamma Metroid to fall to the ground off-screen. Contrary to the membrane of the Larva Metroid, this second husk does not remain in the area and vanishes after metamorphosis.

Samus will only witness the actual shedding of both husks once. The first Gamma and Omega Metroids did not discard any material from their Alpha and Zeta Metroid stages, respectively; instead, their growth occurs in the form of rapidly changing back and forth between the two forms for a few seconds before attacking Samus.

Metroid: Samus Returns
In the remake of Metroid II, the husks are quite different and more varied. A Metroid will now leave a husk behind for nearly every stage of its life cycle. All shed material features an overall dark yellow color.

When the Larva Metroid begins to transform, it lays on the ground as its membrane and mandibles harden and turn into a dull color. The Alpha Metroid within then struggles to break free, briefly levitating the larva's entire body before it finally shatters the topside and shakes off the husk. This is the only metamorphosis witnessed by Samus, due to all the others occurring off-screen.

As the discarded material is left on the ground, a bright, opaque, yellow fluid leaks and spreads out from it. Interestingly, the Alpha Metroid discards not only the prior form's membrane, but also the flesh and fangs underneath it, contrary to Metroid II where only the membrane was shed. They are named in the Map key of Samus Returns. If Samus obtains Metroid DNA and inserts it into the Chozo Seal without having exterminated all Metroids present in the area, the location of a husk corresponding to any surviving individual will be marked on her map; curiously, the discarded membrane emits energized steam as long as its respective Metroid remains alive.

Every Metroid stage following the larval phase will also shed their entire skin and/or body in order to reach the next form, all the way to the fully grown Omega Metroid. However, only the larva's husk is seemingly durable, as all others' will crumble and disintegrate at the slightest touch during their respective cutscenes; some will even disintegrate on their own. Due to this, along with the larvae's husks being located in the background, none of the Metroids' shed skins can be interacted with during gameplay.

Due to the Queen Metroid's presumably old age and the last surviving Infant Metroid's very young age, they are the only two stages in the species' life cycle to not be seen shedding any organic material in the game.

Metroid Prime series
In the first game of the series, the highly mutated member of the species known as the Metroid Prime discards its first form's exoskeleton during its confrontation against Samus on Tallon IV. However, this shedding is not the result of a metamorphosis, but rather a consequence of being heavily injured by her.

At the end of their first battle, the Metroid Prime, exhausted, falls into a deep pit. Samus jumps down and discovers the creature's body at the bottom where it is leaking out copious amounts of Phazon from all sides. Both its helm-like structure and face are absent; in their place is instead a black, gaping hole. Soon after, the organism's Core Essence, featuring the aforementioned missing face, flies out of the hole and the second battle begins. The now empty black shell of the Metroid Prime remains in the room and can be climbed on by Samus.

In Metroid Prime 3: Corruption, numerous husks identical to the Metroid Prime's exoskeleton are found embedded into the floors and walls of the Genesis Chamber on Phaaze. Though never confirmed, it is likely they belong to other Metroid Primes that have evolved from the lesser mutants present on the planet. Unlike the exoskeleton shed by the creature on Tallon IV, these appear to be dried and maintain the helm-like structures, which implies that Phaaze's Metroid Primes molted out of their husks in an entirely dissimilar process as well as under different conditions.
 * "These husks are identical to the bioform you encountered within the crater on Tallon IV."

Metroid Fusion
After the Restricted Laboratory from the Biologic Space Laboratories research station was breached and destroyed, Samus finds several large, empty yellow membranes resembling a larval Metroid's littered throughout Sector 1. Eventually, Samus finds a massive, torn membrane in the Docking Bays, before she is confronted by its former occupant: an Omega Metroid. The creature grew from an Infant Metroid that escaped the Restricted Lab. The massive husk is crushed by the Metroid jumping into view.

Trivia

 * As previously mentioned, the original Metroid II features only two detailed metamorphoses that result in the creation of a husk, specifically in the Larva-Alpha Metroid and Gamma-Zeta Metroid transitions. This served to emphasize on the drastic changes in physiology between the three main Metroid body types: the jellyfish, arthropod, and theropod forms. All other transitions that do not leave a husk are relatively simple in comparison, which reflect the core form they keep (example: Alpha and Gamma Metroid possess the arthropod form).
 * The remake Samus Returns removes this emphasis altogether by making every Metroid stage discard a husk from the previous form.
 * The fact that the Omega Metroid in Fusion shed nothing but Larva Metroid membranes throughout its life cycle suggests that the organism skipped the Alpha, Gamma and Zeta Metroid stages. Instead, it remained in its larval phase, growing larger and shedding several membranes until its Omega form molted out of the last one seen in the Docking Bays. This may have been the result of the rapid growth process created by the BSL's scientists and hinted at by Adam.
 * Massive Metroid larvae have been encountered in the past, prior to the events of Metroid Fusion. However, they reached their gigantic sizes through other means.


 * Three out of the four nuclei present inside a Larva Metroid's membrane seemingly behave like stem cells during the creature's transformation into an Alpha Metroid. This comes from the fact that only a single nucleus remains after the metamorphosis, as the other three were undoubtedly used to create the Metroid's new body.