Diabolik

In the Silmarillion it states "All those of the Quendi who came into the hands of Melkor, ere Utumno was broken, were put in prison, and by slow arts of cruelty were corrupted and enslaved; and thus did Melkor breed the hideous race of the Orcs in envy and mockery of the Elves..." Did Orcs retain the immortality of Elves or was it lost in the corruption and slavery? Or could the Orcs live off the power of their master? Since in Return of the King after The Ring is destroyed "As when death smites the swollen brooding thing that ... holds them all in sway, ants will wander witless and purposeless and then die feebly, so the creatures of Sauron ... ran hither and thither mindless; and some slew themselves, or cast themselves into pits". Also, I believe in the Silmarillion it says something about Morgoth's power being diminshed from controlling his armies. If anyone knows the answer please help me.