a venomous stonemite  

This is from the perspective of a solo ranger that rarely goes far up any of the 3 paths and drops the expedition when it starts to respawn with too many stonemites.

Stonemites summon and these little devils sure pack a punch.
When soloing the hynids, pookas, nocs and cragbeasts, I treat these with respect and avoid them like the plague.

Killing the non-stonemites is done by tagging, snaring, dotting, then root and shoot, repeat upon root break at ~60% health and in 2 minutes or less, voila.... a healthy chunk of xp.

Stonemites are much like the hynids and pookas, they can spawn in any of the locations.
The 2 spawns on the flat part where it splits into 3 tunnels are frequently stonemites, or can have stonemites respawn.
Up the left tunnel, at top of the incline where it turn right towards a stoneworker model, the left hand spawn CAN be a stonemite but the right hand spawn is always a noc or ukun.
There is generally (always?) a roaming kyv in the left tunnel who can path quite far down towards the flat area and the 3-way split.
Up the middle tunnel there are 2 spawns before reaching a sort of flat area and a bend in the tunnel.
These 2 are generally stonemites, hynid or pooka, and any one of the 3 can respawn there.
There is one roamer in the middle tunnel who paths down just a little bit past where the first spawn on the incline is, that roamer can be stonemite, pooka or hynid.
Up the right hand tunnel there are 2 spawns before reaching a flat area and a bend to the left.
The first and second spawns can be stonemite, pooka or hynid.
There is always a roamer in the right hand tunnel and for me it has always been a flinty cragbeast.
Respawn time on all mobs appears to be 15 minutes.
If there are no stonemites in those locations when I zone in, that gives me 8 mobs to kill, 10 if i'm lucky on the roamer's pathing before respawns occur.
Some expeditions I've had to drop right away since there were 4 or more stonemites right at the first spawn points.
I'm guessing that you could get 3 or 4 stonemites every 15 minutes just around the 3-way split.
It seems that the further in you go, the more mastrug, noc and aneuk you're likely to encounter, so my gut feeling is around where the animals spawn is your best bet for stonemites.
It appears to me that more animals spawn close to the 3-way split than further in.
Again, I don't solo very far in so my perspective may be skewed.
For me, plenty of stonemites around where it splits into 3 tunnels, if there's none there now, there will soon be when there's respawns. :)