Most termites are the subterranean type and are able to tunnel in the soil. The ability to tunnel allows them to find many separate pieces of wood, on which to feed. Since they are not limited to one piece of wood, their colonies can be much bigger than those of wood-inhabiting species. Usually their colonies number from hundreds of thousands to several million. The mother of the colony is usually quite grossly pregnant. These mothers are the mothers of all mothers, so to speak. Hence, they are called queens. The soil-tunnelling termites have a more advanced caste system with true workers. True workers are never able to become alates although they can become soldiers and sometimes they even transform into special reproductives called ergatoids. Soldiers of subterranean termites have a gland on the head that secretes defensive chemicals. The soldier jaws are modified in many weird and wonderful ways; they may be sword-like, serrated, toothed, hooked, rod-like, etc. Some soldiers have extraordinary snapping mandibles. In another group, the mandibles are regressed to tiny points while the defensive gland is produced forward as a nose; these are the nasute termites. Many subterranean termites have specialized diets and may eat plant litter, grass, dung or humus, instead of wood. In tropical areas many subterranean termites build nests or mounds which are among the most impressive examples of animal architecture.
Subterranean termites can be very serious structural pests of houses. They do not produce fecal pellets like the drywood termites. Instead, the entry of subterranean termites into a house is usually revealed by the presence of mud shelter tubes on walls.