Space: Above and Beyond/WMG
The Chigs are fully human.
That's the real secret that the Aero-Tech company man alluded to when interrogating the last survivor of the Tellus attack when he said the survivor was "not the farthest man from home". The "gills" and green goo seen in the pilot and the gnarly hands, etc., are just part of their multi-layered armor. The "alien face" is a breathing mask.
The Humans and Chigs rely on a number of different FTL methods
References are made to (naturally occurring?) wormholes, "Hawking Drives" as well as relativistic travel, but it's never really made clear how people in the 'Verse travel faster-than-light, though they clearly must in order to wage an interstellar war with troops and ships moving between star systems week after week. References to FTL travel in-verse make it sound rather opportunistic, with "windows" of time during which particular journeys can be made (this probably references wormhole travel, which could be used as much as possible to save power).