Adventitious refers to something that is added from an outside and often unexpected source rather than being an intrinsic element: Adventitious ant shelters involve incidental or adventitious ant-nest sites that are associated by chance and are not an integral part of plants.

Such incidental nest sites can be divided into convenient categories as follows:

1. Preformed cavities in live branches and stems, excavated by wood-boring beetles and other insects and then later occupied by ant colonies.
2. Cavities in stems and branches that are naturally hollow or contain a pith soft enough to be easily excavated by ants which includes grasses, sedges, composites, and other herbaceous and shrubby plant forms and so a great variety of ants occupy them.
3. Natural or preformed cavities in the bark of trees; such as, pine trees in the southern United States that shelter an entire fauna of ant genera that nest adventitiously in the bark.
4. Roots of epiphytes or the tangled root systems of orchids, gesneriads (mostly tropical herbs or shrubs), and other tropical epiphytes are ideal nest sites for ants.
5. Galls (abnormal swelling of plant tissue) formed by cynipid wasp larvae, or Cynipidae, a family of gall wasps, hymenopteran insects in the super-family Cynipoidea that produce galls on oaks, which have been observed in Europe and North America.
6. Earthen or carton nests constructed vertically against the sunken portions of tree trunks by a few ant species in the New World tropics because the trees provide a partial wall of solid wood which provides some protection for the ant colonies.

None of these diverse structures appears to be "designed" to accommodate ant colonies and all of them are ordinary anatomical features of the plants that the ants exploit, apparently in a unilateral manner.

In contrast, the domatia do appear to serve as ant nests because they are featured by cavities that form independently of the ants.

So, it is adventitious when roots and tubercles absorb nutrients from waste material carried onto the cavities, and even holes or thin windows of tissue through which ants can more conveniently enter and leave their colonies are essentially provided naturally by the plants, but not to entice ants as with the domatia.