A couple of years ago someone posted a message asking about the appearance of the Pirate Militia, here http://www.gamefaqs.com/boards/928517-metroid-prime-3-corruption/54678995. Another user theorized that their apparent slug-like appearance could be the result of their limbs having been amputated.
I have my own theory about why they look like that. I think that all space pirates are born in a slug-like larval form, perhaps even smaller in physical size than the militia. Their arms and legs don't develop until they reach adulthood, kind of like how frogs are born in a legless form and only gain their front and back legs once they have reached maturity.
If they survive long enough to be old enough to serve as Pirate Militia, they are temporarily given a cybernetic body to use and in turn become members of the militia. If they survive long enough for their normal arms and legs to develop and grow, then they obviously stop using the mechanical body and become full fledged troopers.
What do the rest of you think? --- Ever had roasted Space Pirate? It tastes just like chicken! :D
If the Space Pirates had small, sluglike forms, then why would Samus's Morph Ball cause them so many headaches? You recall in the first Prime there were reports of research disasters from trying to duplicate the tech...if there are smaller intelligent baby Pirates, they wouldn't need the Morph Ball tech? I guess the best counterargument is that the ones that small wouldn't be sentient enough to be of use. --- Usque adeone mori miserum est? After all, there's always a continue...