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UBerserker wrote: Mercenaries could be creatures of different races (which explains why some of them in Terraniux are fatter or slimmer)

I'd attribute that to usual physiological variations within individuals of the same species.

A document written (I believe) by UsAaR33 for Oldskool suggested that the Mercenaries hailed from a world (or cluster of worlds) orbiting Vega, and named them as such as denizens of that star system. As far as I know this has been the community's most notable attempt to give these guys any backstory, or at least in what has been made public. What we know of them from Epic remains limited to what is said in the games, and that isn't much beyond their activities and namesakes. I've written supplemental material extensively for many of the races of Unreal intended to coincide with planned releases, but this has not yet been made available.

It seems that in terms of what is official and what has been provided by Epic, they were just another species non-indigenous to the planet. I attribute their lack of appearances in the mother game and somewhat arbitrary placement in RTNP to the possibility they were either a late addition to the game or were considered by the developers to be less versatile as an enemy type (or both).

A lot of the maps with them were removed from the main game.

......................

Another one of these questions I've had for years.. was there any reason given as to why the Skaarj queen looks/is so different from the other Skaarj? She has legs like a t-rex/ostrich plus mantis forelegs, is 20 ft tall, and makes freaky elephant-like noises.

It's almost like she's not even the same species but some kind of hybrid.

Come to think of it.. how does she spawn pupae anyhow? I don't remember ever seeing egg sacks anywhere in the original game, and what the heck are pupae doing so far away from her bunker/spawning room/secret Skaarj nail salon in places like Sunspire? Seems a bit dangerous to transport their kids around like that.

She looks like some overgrown Pupae... there's no info whatsoever on the matter though. I guess it's just a chance-based evolution, hell Skaarj don't even have wings either aside the Warlord.

Queens don't spawn Pupae, it's never seen or explained how it's done. Born Pupae are inside those wall nests in the MotherShip/Crypts e.g. those square-shaped holes.
And Pupae are valuable assassins for the Skaarj I suppose.

Hm aren't all Skaarj technically overgrown pupae? I always thought the wall holes were where they put them in after they hatch, like bee larvae (and then feed them with that purple goop stuff seen on the crypt maps).

What about pupae to other Skaarj types? Never seen cocoons or anything either, do they just morph into Skaarj? How do they even select the type/caste it's gonna be?

It depends on how they evolve and how they're going to fare in life. I'm pretty sure they let them hatch inside those holes, or those Pupae just enjoy staying in their wall hatches in the crypts.
Crypts were created to seal Skaarj and their stuff after the first invasion.

Still, it's never explained how do they evolve. The red or green eye coloration is also not explained.

The eyes could simply be variation, like brown or blue eyes in humans.

Wait, so the crypts were built by the Nali then?

By Nali yes, then the Skaarj turned them into outposts upon their second invasion.