Brenda, the genus Caularthron belongs to the Laelinae and is related to Cattleya, Laelia and Epidendrum among others, but not to Dendrobium (which is native to Asia, the pacific islands and Australia), though I can see how the shape of the plant might look more like a hard-cane dendrobium than a cattleya relative.
There are many different plants that enlist ants to defend them against other animals. Among the most famous are certain acacias (e.g. Acacia cornigera), ferns (e.g Lecanopteris celebica) and a group of epiphytic plants known as mymecophytes (which means "ant-loving" in greek). Most of these plants can live without the ants, but grow better when inhabited by them, as the ants provide protection from herbivores and fertilise the plants with their droppings (some of the plants have specialised tissues that absorb nutrients from the dropping the ants leave inside their stems). For some plants the relationship is so important that they can't live without the ants (e.g. certain Macaranga species). Certain ant species are only found living in plants and nowhere else and some of them only inhabit a single species of plant.
Some of these plants give the ants shelter in the way of specialised organs (hollow stems and even thickened hollow spines or leaves) and sometimes food (nectar from special glands on leaf axils rather than from flowers is the most common adaptation to feed ants, though some acacias go even further and provide special protein parcels at the end of their leaf tips for the ants). The ants in return protect the plant from herbivores such as other insects and even large animals are driven off by them, including antilopes and giraffes, as some of the ants have very painful bites and stings. One plant (I can't find the exact reference for this small tree) has an obligate relationship with an ant species where the ants not only protect the plant from animals but even kill all other plants that grow within several meters of their host plant by continuously biting and injecting them with formic acid until they die, thus giving their host the best access to nutrients and light by killing all other competitors.
Among orchids there are several that often house ants within their root ball. These orchids have thin hard roots that grow outwards making a structure that looks like a sea urching/porkupine. These root balls trap falling leaves, etc... giving the orchid extra nutrients, but also provide shelter for ant colonies. This happens to all species in the genus Coryanthes (the bucket orchids). Other orchid genera that go even further and provide hollow stems for ants to live in include Myrmecophila and Caularthron. These have very thick, cane-like pseudobulbs that are hollow and have a slit/opening at their base and thus allow ants to live inside them. By the way, Caularthron and Myrmecophila are closely related genera and can be found in very much the same kind of habitat and both have their centre of distribution in central america and the caribean islands.
I hope this answers some of your questions.
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