Answer:
All members of class Hexapedal have five lungs - just the system that their particular ancestral lungfish developed. In the second immigrant invasion the flying form that made it off of Vorcaic had a sticky saliva - something it used in making its nest (much like swifts and swallows do). Some members then began to use this as a noxious defense by spitting it at intruders (much like camels do.) In developing this ability the dragon preferentially employed the dorsal lung as the propellant chamber as it had the foundation of the spine and optimal constriction from the diaphragm. As the dragon has five lungs it had no problem in reserving one for this particular purpose. As this lung was not needed for respiration it began to fill-up with phlegm and became a reservoir for excreta in particular heavy-metals (which the Hexapedals have particular resistance to.) By the time of the Weir-dragons this lung was completely fluid filled and was being assisted by the other lungs in making a aerosol. As fluids are less compressible than air the projectile ability of the Weir-dragons was considerable and aiming was assisted by flapping of the wings. By the time of the Sore-dragons this fifth-lung fluid was acquiring several highly nitrated poisons. The high heavy-metal concentration of the fluid (only present when the dragon is in a environment rich enough to build them up) tended to decompose the rather unstable nitrated compounds so (by necessity) the fifth-lung started to become a heavy-metal free zone. When a dragon is exposed to a heavy-metal contaminated diet it excretes the excess from their scales and, in the case of the higher drags, throat plates. Therefore when the higher Sore-drags used their poisonous spray there was a tendency for them to begin thermal decomposition as they came in contact with the throat plates. The Langdorigan Sore-drag is described as having hot asphalt or napalm-like spittle. From here it is rather simple to see how the true fire-dragons developed their fire-breath ability. It should be noted that truly firm evidence on these animals is scanty as they are difficult to approach and study. They can be tamed but only with a kind heart or a brutal nature. They have enourmous power and can easily devour a human or animal.