Since Metroids canonically have four internal nuclei, I think the nuclei are assigned a mandible, as for the big one, it probably allows them to float and drain energy.
Metroids were designed by the Chozo and drain energy. The X are energy based lifeforms.
So, I think the Chozo designed the Metroid to seek and drain any energy life form they could find, when the Chozo and Metroid weren't part of the human race in physical form or research. But, I do also think they were designed to not drain life forms with a Chozo signature, and that's also why they drain everything they encounter, as far as Samus is concerned, maybe the Chozo didn't think to give her a Chozo signature, hence why the Metroids even go after Samus in the first place, mainly because her Chozo blood doesn't give off or give a complete Chozo signature. What do you think?
Well first of all, the metroids weren't "designed" by anything. Metroids came from the Metroid Queen, who was born by the designers at Nintendo. Afterwards, the space pirates captured and tried to use the metroids as weapons.
Actually, according to the manga, the Chozo designed the Metroid as a final effort to stave off the upcoming X invasion. At their massive evolutionary rate, they would soon become powerful aggressors. Metroids are their antithesis, but before Metroid could be finished, the Space Pirates attacked and unleashed the incomplete version of Metroid. This is the Metroid we know.
And Samus has Chozo DNA in her, so technically she would have a Chozo signature. The Chozo were going to give the Metroid a limiter to attack only the X, but the Space Pirates intervened before this measure was introduced.
Which manga was this?
Nice site *bookmarks it.
It's a very nice site, actually.
be prepared for a wordy post...
What if the Metroids were actually just draining heat from your body? If they have extremely sensitive veins in their fangs, as we'll call them, and the fangs were kept at a frigin temperature due to the powerful heat-absorbing of the veins inside of the fangs, then any time they touched something with the fangs, it would cause immense pain to the prey and a massive amount of heat would be transferred (by the natural laws of science) from the prey to the Metroid's fangs, which in turn would be sucked up instantly by these mysterious veins and sent to the upper half of the Metroid.
Now here's the interesting part: if a Metroid did run on heat, all kinds of things would fall into place. Metroids go for the head, latching onto what is actually the greatest loss of heat in the human body. In 2d Metroid games, they go for the upper half, which is still the most prominent half for heat, since heat rises. That's another thing: heat rises. Metroids are like Jellyfish balloons, no? And they expand and grow as they gain more "energy." So what if that energy actually went into inflating the bubble that is their upper half? And what if that's how they fly, like a hot air balloon? They do seem to drift around. Well if heat is their primary transportation method, wouldn't that mean extreme cold could stop them dead in their tracks? Ahhhh, the true weakness: ice! Metroids kept at lower temperatures seem to be less hostile, a lack of mobility maybe due to deflation?
If cold is the Metroid's true enemy, doesn't it make sense that it's always seeking heat?
EDIT: forgot to mention, this would also explain the lack of markings. As for turning to dust, well, I think the prey would only "freeze" if they were covered in water when this happened.