In the underground animals assume characteristics of troglomorphism. In complete darkness they do not need eyes and instead of eyes, animals develop their scent and touch senses. Also, they loose their skin pigment and become colorless. Their feelers where their touch senses are located elongate. Underground animals are modest food consumers, they have slow metabolism and elongated life span even by several times.  

According to the Adjustment Degree:

• troglobites (cave dwellers) - species that are completely adjusted to underground habitats so that in normal conditions they could not survive outside the underground. Aquatic troglobites are called stygobionts;
• troglophiles (cave lovers) – species that can normally survive in some habitats on the surface, but can also in generations live in the caves as well (i.e. they normally feed and breed);
• trogloxenes (cave guests) – species that appear regularly in the underground but at least a part of their living functions they can perform only outside. Usually this refers to food; and
• incidetals– all other species that are frequently found in the underground. Historical fauna

We can consider the subterranean fauna of Dinaric region with surrounding areas not only as natural but also as both historical and cultural heritage (B. Sket). It has historical significance because cave animals were firstly discovered and examined at the far northwest pars of Dinaric Karst, in Slovenian Karst. Research extended to south parts as well by building road communications. Somewhat further to the far southeast of Dinarids, in Skoplje Basin in Macedonia, interstitial fauna was discovered for the first time in this region.  

Numerousness and Spread of Subterranean Fauna

There are two greater types of underground fauna:

• interstitial fauna, found around the world, in sand-banks of rivers or in sands at beaches. There, it fills the little channels among fine gravel and grains of sand that are splashed with water. Those are very tiny animals of millimeters length;
• cave fauna, found quite narrowly, mostly connected to Karst.

Dinaric Fauna

Karst caves with fauna can be found on all continents, but caves in Dinaric Karst are far the richest with fauna in the world! Until now, only in the underground water of wider Dinaric area (i.e. between Sava River and Adriatic Sea) almost 400 species were described, what is considerably more than anywhere else in the world. Similar is with terresterial cave species but data about them are somewhat less precise. Riches of the Dinaric subterranean fauna were proved during recording the richest cave systems in the world. Professors Boris Sket and David Cuvler managed to collect data for 20 caves throughout the world which determined 20 or more species of troglobites. Among them are even six Dinaric, out of which five are in Slovenia. The Postojina-Planina system at that time had 84 troglobites, and Vjetrenica was in the second place with around 60 species. Later research showed that there is considerably bigger number of species in Vjetrenica.

Vjetrenica

According to data that became known in the meantime, Vjetrenica is populated with 92 troglobites species what places it in the first place in the world. Here is an overview of its representatives:

• Entrance fauna: the most important for it is wall fauna, that is together with others represented by: butterfly (Triphosa sabaudiata) dipterous (Limonia nubeculosa), pseudospider (Nelima troglodytes), grasshopper (Troglophilus genus) and cave cricket (Dolichopoda araneiformis), diplopoda (Apfelbeckia landefeldii).
• Terresterial fauna in Glavni kanal: coleopter (Antroherpon apfelbecki), trechinae (from Neotrechus genus), insects Collembola and (Diplura), centipede (Typhloglomeris caeca), pseudospider (Dinaria vjetrenicae) and other
• Veliko jezero is a big and in some places deep basin with clay floor in which together with the "tinies" the following animal live: prawn (two species of Troglocaris genius) and at least two among the biggest species of blind schrimp (Niphargus vjetrenicensis, N. trullipes).

Small streams in the upper parts of Vjetrenica that flow partially on loam and partially on sand and rocks, are populated with, among the others, the following animals: cave pill-bug (Monolistra hercegoviniensis) and tiny snail (Lanzaia vjetrenicensis), and cave tube-worm (Marifugia cavatica).
"Cave hygropetricum" is inhabitated with coleopeter Hadesia vasiceki, and Schaefern's schrimp (Typhlogammarus mrazeki).

• In Donja Vjetrenica (Lower Vjetrenica) one can sometimes see fish Popovo Minow (Delminichthys ghetaldii, syn Phoxinellus g.). The most distinctive is Niphargus balcanicus.

• Shrimps are found near rocks, and swimming further is Troglomysis vjetrenicensis; this is the only habitat known where this specie can be found! There are also smaller blind schrimps (Niphargus  spp.), and tiny Copepoda.
• In Potok of Absolonov kanal the most present is Typhlogammarus mrazeki, but there are also other types of scrimps and pill-bugs (Monolistra hercegoviniensis), prawn from Troglocaris genius, and snails from genius Orientalina and Human Fish (Proteus anguinus).