C+P from my Worldbuilding thread (seems appropriate here also) pretty long, so I guess a summary would be needed if it is to be added to the OP:
Appearance: Pipra are giant terrestrial ant-like creatures measuring about 4 feet tall and 6 feet long. However these measurements vary considerably from caste to caste (explained below) as does their appearance. The most common encountered caste, the so-called Nobles, conduct trade and diplomacy with humans (and on some rare occasions elves), the Warrior caste is also encountered frequently, since they usually accompany Nobles. Nobles are relatively small (compared to the Warriors) measuring about 2-3 feet in height and 4-5 feet in length. They walk on four limbs which sprout from their thorax, the other two limbs are used for grasping, having pincer like extensions at the “wrist” which can rotate freely in 360 degrees. As with all castes of Pipra (or so it is believed) neither the Nobles nor the Warriors have any photo-receptive organs. It is believed that the Pipra primarily use chemo-sense to collect information about their surroundings; neither the mechanism nor the organs which are responsible for this sense are known.
The Nobles lack the large mandibles which Warriors (and it is also believed, Workers) possess, but are nonetheless capable of the peculiar “chittering” language. Their heads are also much more pronounced, almost the same size as their abdomen.
The Warriors, in contrast, are huge, around 4-6 feet tall and 6-9 feet long. They use all six of their limbs to walk and do not seem capable of standing on their hind limbs, though they can easily scale steep cliffs and safely go down near-vertical drops. Their head’s are much smaller than the Nobles, but their mandibles are greatly pronounced and sharp (and, as first-hand experience has regrettably found, can be used as deadly weapons).
Anatomy: Not much is known about Pipra anatomy, only a few traders in the world deal with the Pipra and fewer still have seen more than two castes of the colony. After a great deal of negotiation (and several tons of pyrite) a number of Pipra corpses were bought for dissection. They included members from 3 castes: Warrior, Worker and Nurse
To the surprise of many scholars, the internal physiology between three castes varied so considerably that they could be considered separate species. The degree of specialization, believed to be induced upon birth by magic, causes several major anatomical changes from caste to caste. Creating specialized organs for certain tasks while destroying or making vestigial other organs. For instance, the Nurse caste seems to have a pouch which carries chemicals of a mysterious nature (possibly magical). Feeding these chemicals to rats causes instant death, and their purpose is not understood.
However, there are several things which these castes have in common. All seem to have stomachs and a circulatory system, as well as a nervous system, though the latter two vary considerably in development and complexity from caste to caste. They also all have an auditory system far more sensitive than humans.
The biggest mystery is perhaps that of the Pipra’s diet. They have no teeth; instead their mouths are simple empty holes with large pouches at the sides. The pouches seem to contain an extremely toxic liquid which can burn holes even in metal. It has been theorized by some, that Pipra feed upon stone itself; however, these scholars have been defamed and ostracized from the general community and should not be taken in earnest.
The Pipra stomach and digestive system is the only thing that is truly consistent from caste to caste, yet, as with everything else, their workings remain a mystery.
Language: The Pipra do not have tongues, as such their primary method of communication seems to be chemo-sensitive as well as auditory. The mandibles which all castes are believed to have, can “click” at varying speeds and intervals and serve as a language. Humans have been able to figure out the language more or less through trial and error (quite a bit of error), but the Pipra, despite their looks are a patient and well-meaning race (if a bit introverted and isolationist).
Society: Pipra society is divided into castes. Each caste is differentiated at a physical level to a particular specialized task within the colony. Everything from mandible size to intelligence is controlled from the egg. The prevailing “Hexa-caste theory” claims that there are 6 castes: Queen, Nurse, Noble, Warrior, Worker, and Child. The first two are responsible for hatching and replenishing and caring for, the stock of eggs in the colony. The Noble caste dictates day to day business as an administrative force (they are also diplomats and merchants who trade with other races). The Warrior caste defends the colony from harm. The Worker caste expands the colony, digging deep into the earth and uncovering space and rare metals. The Child caste is hypothetical, but is believed to be a caste of undeveloped Queens, which mature as soon as the current Queen dies.
While contact has been established between humans and Pipra, the Pipra remain an enigmatic and secretive race. The fact that even their diets are unknown is a testament to their rigid isolationist society. What little is known about Pipra come from accounts from traders and merchants who have communed with Nobles during trade negotiations. The Pipra do not seem to any culture or "civilized society" below the Noble caste. Indeed, it has been observed by several merchants on many separate occasions, that the Warriors which usually accompany Nobles seem to be vacant and vapid. Following orders with a machine like precision, they are more automaton than organism. The Nobles, however, do seem to have some customs and habits (and some believe, even religion) though the extent of it is debatable (and debated).
However, the Pipra seem to be a pacifistic race. Neither interested in war, nor conquest of new lands. One merchant recounts an exchange between himself and a Noble, when he had asked the Noble whether he was aware of the existence of Mount Orod. The Noble replied in the negative. When asked whether the Pipra had any intention of future expansion, the Noble replied with indifference, stating "The Pipra do not run to and fro as you humans, for our [life]source lies within the depths. Bring us your stone from this other mountain and we shall trade. Or do not bring it. It matters not, we shall remain regardless."
Some scholars believe such passive indifference toward the world (not unlike the Theastan religion) is a result of racial age (i.e how long the Pipra have been around, which some estimates put around 5-6 million years) as well their general isolation. Nevertheless, there are few that would dare to aggravate the Pipra, peaceful though they are, for while their numbers and their abilities are unknown, they are almost certainly superior to anything that exists on the surface.
One final note about the Pipra, they have no formal writing system (understandable, as they are blind) nor any system of recording sensory input. Some theorize that the Nurse caste acts as recorders for Pipra society, transmitting information through chemical means or through oral tradition to the Nobles, whose superior intelligence coupled with this knowledge allow them to make informed decisions.
1. Pipra stomachs contain lithotrophic bacterium which allow them to indeed, digest rocks (pyrite, FeS2, in particular is traded only for the sake of food)
2. Pipra mouth-pouches contain concentrated acid to dissolve minerals, how does the mouth and digestive tract tissues remain intact you ask? They don't, like human stomach's they are constantly eroded and regenerated.