Thanks
They are not air caste Tau, because as far as I know it is only a name, they don't have wings. I imagine that after capturing few dark eldar Scourges, earth caste scientists decided to experiment with this technology on fire warriors. Because it is tau empire, I guess they were volounteers
. But the technology turned out to be too expensive, only few prototype warriors were created and assigned to the Vespid forces.