Imago

Imago is a boss in Metroid: Zero Mission. It is a giant fly-like creature encountered in Ridley's Lair. It attacks Samus Aran for destroying its eggs. In the Japanese website of Zero Mission, the Imago and its previous forms are given the title Giant Evolving Bug.

Description
Imago's larval form, the Kiru Giru, hailed from Norfair, and Samus encounters four of them during the course of her Zero Mission. The first two that are encountered are free-roaming, and are able to be killed by damaging their softer underbellies, either by shooting from under the larva using the Wave Beam, or by placing Bombs in the larva's path. While traveling through a Norfair tunnel infested with parasitic Tangle Vines, she comes upon a third larva that appears to have been almost fully consumed. A fourth larva was also ensnared by the vines, and is defeated by freezing the winged Ripper found near it, and then using the Ripper as a platform to shoot the vines holding the larva up on the ceiling with Missiles. As the battle continues, the larva will regularly shoot waves of spores at Samus. The larva eventually plunges into a stone platform, causing the ground to cave underneath it, and revealing a passage to Ridley's Lair. If timed right, the larva will kill the Ripper as it falls.

Very shortly thereafter, Samus sees the larva digging deeper underground, having hardened its shell into a cocoon. Samus returns to Norfair and follows the larva's tunnel, only to find that it has left its cocoon. Samus enters a new chamber lined with eggs, and accidentally trips over one close to a blocked-off Super Missile Tank. This maddens the Imago, now a full-grown adult, and it attacks Samus.

Combat
Imago attacks by flying around and attempting to impale Samus with its deadly stinger. This can be easily avoided, though, as Imago flies at a height that allows Samus to simply duck or use the Morph Ball to pass under it. Once she deals enough damage, the Imago's stinger is destroyed, and its only mode of attack is to ram its opponent. Nearby, though, there is an organic wall blocking access to a Super Missile tank; if Samus goes near it, the enraged Imago rams the wall by mistake, destroying it, and gives Samus access to the power-up while ending its own life.

Interview
"Are the Kiru Giru captured by the Karamutsuta (Tangle Vines) in inner Norfair and the Kiru Giru Imago in Ridley the same life form? If so, it has amazing tenacity..."

Yoshio Sakamoto: "They're the same life form. But I don't know if it has tenacity. Samus only coincidentally narrowly saved the Kiru Giru captured by the Karamutsuta (Tangle Vines) from certain death, and afterward, the reason Kiru Giru that became an Imago attacked Samus was because Samus had trampled on the eggs it had created."

Trivia

 * According to the Metroid Manga, the adult Imago isn't a part of Zebes' natural ecosystem, but a mutant beast created by Mother Brain
 * The battle with the second free-roaming Kiru Giru is very similar to the battle with Crocomire in Super Metroid. Coincidentally, he was a planned boss for Zero Mission but was dropped.
 * The music heard in the rooms of the first three Kiru Giru sounds very similar to the battle music of Spore Spawn and Botwoon of Super Metroid. As well, like Botwoon and Spore Spawn, the ensnared larva releases small spores. They more resemble Botwoon's spores, as Spore Spawn's flew around the room in different directions.
 * There is a glitch where if Samus attempts to kill the Imago using a Spin Jump Attack, it will stop moving and force the game to be reset.
 * Imago is the scientific word for the adult stage of an insect, such as a butterfly.
 * There is a way to skip Imago via an alternate route.