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I can recommend the setting book 'Into the Darklands' for this.
That was one of the things I bought for my brother along with the first two 'second darkness' AP's a few years back. Gave him the 'gift of gaming' for him and his university flatnates. He studied environmental sciences back then so we had a lot of discussion as to monster ecology. Vestigial eyes were discussed, changes in pigmentation, skin translucency, bioluminescence, the kinds of micro-organisms that formed the lowest rung on the food chain and the prevailence of scavenging organisms.
Wikipedia has a pretty good list of Cave dwelling critters, and some good links to more info on this. But I'll summarize for those who want a quick starting point.
Underground critters can be divided into Troglofauna and Stygofauna. The former living in dry cave systems and the latter in underground rivers and such.
Keeping the Troglo / Stygo prefix the same (Denoted by X) they are then divided into:
1) X - philes
These complete a large part of their life cycle underground but are exposed enough to surface tunnels to have retained the features to allow them to life on the surface. Many species of chiropteran (Bats) are the classic example of this.
(Due to its proximity and number of accessable surface passages, Nar-voth creatures would largely be this type)
2) X - xenes
These creatures are known as 'cave guests' and can spend a part of their life cycle underground, but they are not adapted to survive their entirely. Hibernating mammals such as bears are an example of this.
(These are the sort of beasties you'd find guarding the surface passageways)
3) X - bites
These are the archetypal subterranian creature and the kind that spends their entire life cycle underground. Usually descended from X-philes, these are the ones like the pale cave salamander or the eyeless cave fish whose lack of exposure to the surface ecology causes them to adapt in weird and wonderful ways.
(classic example of this are the Troglodytes which were once lizardfolk who fled underground and adapted, and this also factors in the various crotters that come up from the lower levels)