All but extinct the Rachni are almost a complete mystery. Little trouble was taken to understand them, and no quarter given during the Rachni wars. The queen is at the centre of a hive mind, held together by singing. They are capable of ship-building and FTL travel and despite other limitatons are a fierce and implacable foe when roused.
Rachni Workers
AGI 2, APP 0, BLD -4, CRE 0, FIT 1, INF -4, KNO 0, PER 0, PSY -1, WIL -1
STR -1, HEA 0, STA 5 (8/10/15), UD 1 (Bite x6), System Shock 5
H2H 1, Any relevant tech/building skills 3
Rachni workers can explode as a suicide attack (or when killed) as a 5m radius, Intensity 6 toxic explosion.
Rachni Soldiers
AGI 1, APP 0, BLD 1, CRE 0, FIT 2, INF -4, KNO 0, PER 1, PSY -1, WIL -1
STR 1, HEA 0, STA 35 (32/50/85), UD 8 (Claw x18), System Shock 5
H2H 3, Small Arms 2, Combat Sense 2, Notice 2, Tactics 2, Dodge 2
Rachni Soldiers can spit acid, which bypasses shields and does x25 damage.
Rachni Brood Warriors
AGI 1, APP 0, BLD 2, CRE 1, FIT 3, INF -3, KNO 1, PER 1, PSY 0, WIL 0
STR 2, HEA 1, STA 45 (42/65/110), UD 11 (Claw x26), System Shock 6.
Biotic Strength 3
H2H 4, Small Arms 3, Combat Sense 3, Notice 3, Tactics 3, Dodge 3
Warp 3, Stasis 3, Barrier 3
Brood Warriors can spit acid which bypasses shields and does x30 damage.
Rachni Queen
AGI 0, APP 0, BLD 9, CRE 3, FIT 3, INF 3, KNO 3, PER 1, PSY 3, WIL 3
STR 6, HEA 3, STA 85 (102/145/230), UD 21 (Claws x41)62.43*50
The Queen can spit acid which bypasses shields and does x50 damage.
The Queen is considered to have all skills at 3 and all Biotic skills at 3. Her Biotic Strength is considered to be 8.
Postmortem Studios 
The evil bug chase is always a fun thing to do. I have found memories of being a bug hunters, pouring verious chemical on them, setting them on fire, even going nuclear aliens style. Well … kinda, in my mind.
They sure make for a great antagonist in the post sheppoccalypse. I always had my doubts about them, mostly in me3 where they are clearly indoctrinated.
I wondered if the queen could give new eggs, not tainted by the indoctrination effect.
In either way, it could cause for rebellious offspring and future queen could be unrully. Some groups might be interested in controlling them using reaper tech. This is an interesting avenue.
A crazy idea here. What about that very same mission on noveria but in reverse? You know the mission where the rachnie are in thrall with serberus. Hmm, why not a mission where the queen took over Assari personnel and use them as infiltrated units or the way that EDI is using the cyborg “corpse”. (BTW EDI has the most magnificent camel toe I ever saw in a game! Greyson academy entrance shot must be the nicest on of all!) Funny thing if you could have a “relation” with the queen thrall … woa this is getting weird.
While the stats are a great way to resume an encounter, it may be an idea to compile a behevioral stereotype of each subspecies of rachnie rather than just an attack pattern.
It is clearly something lacking in their regards from the ME game. They are only depicted while fighting. Not when foraging or doing their stuff while other are around.