Blue Gender/Fridge

Fridge Brilliance

Why are the Blue so strong and hard to kill? Because they're based on (and possibly may be) insects, and insects are animals known for being capable of lifting many times their own weight! Also, some insects can sustain harsher injuries or conditions than humans can despite their small size, so imagine how hard it would be to kill one roughly the size of a house!
The Blue's cores are possibly some kind of central nerve system which explains why the Blue die the moment they get damaged; they literally cannot move with their nerves disabled.
With their hard shells and large bodies, it wouldn't be a surprise that the reason Blue cannot swim is due to their enormous density (creatures need to have the same density as water to float or swim properly).

Everyone on Second Earth, no matter how geographically different, can understand each other and speak the same language. What do you think happened to all of the different languages people used to have?
Even without the Blue finding their position, the refugees in episode 4-5 will most likely die from malnutrition or starvation, considering they have limited source of food they can find from rundown grocery stores (which will run out eventually) and the Blue making it near impossible to leave without getting killed. Really, their demise was basically a Mercy Kill (minus the whole getting slaughtered and being eaten to death thing).
Because a Sleeper's B-Cells makes them all insane and delusional from overuse, Second Earth is basically weaponizing mentally disabled individuals!

A Blue's weakness is their core, so why not train some pilots or soldiers to be snipers, who can shoot the Blue's weak-point from a safe distance?
In episode 4, Marlene calls out Yuji for firing a gun in a Blue-infested city, saying that it puts everyone at risk. If that's the case, why not equip their guns with silencers, just in case they need to defend themselves but not wanting to risk attracting the Blue's attention?

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