Western subterranean termites cannot survive without being in contact with moist soil. Therefore, the "subs" create shelter tubes to travel out of the ground and into homes. Using saliva and fecal matter to create a tight seal, the humidity emitted from their bodies maintains a level of moisture inside the tube in which they can survive until returning to the ground.
While drywood termites are not as problematic as subterranean termites in southern Arizona, they still pose a threat to structural lumber. Their colonies are considerably smaller yet the insect itself is much larger. Most importantly, drywoods do not need soil. This means it is more difficult for an untrained eye to identify them.