The Reapers take the form of which race they're using to create it. The Human-Reaper larva looked like a human because thousands of humans were processed in order to create it.TheDoctor455 said:Actually, I kinda thought ME2 touched on their true motivations... (massive spoiler warning)
when it was revealed that the collecters were melting the colonists down to make a human reaper. Now, this is either, as EDI suggested, their means of reproduction... or it is their strange way of "preserving" a sentient race... and was it just me... or did the human reaper change gender based on what gender Shepard was? Made me think that they intended to base the human reaper's AI off of a slightly altered version of Shepard's personality...
And they are not preserving sentient races. When a Reaper is activated, he/she inherits only the memories of the thousands of beings that were used to create it. The Reaper itself is still a machine, but a machine with thousands upon thousands of memories.
I'm guessing that's why The Collectors wanted Shepards body, since basing the AI of the Reaper on a very smart human with a powerful personality that managed to defeat Saren, his Geth alies and Sovereign is a smart idea.