A boy is resting in bed when he feels an insect sting. Now he is paralyzed and anaesthetized, and he watches in horrid fascination as a stream of antlike insects digs a hole into his abdomen. Presently there is one stream of insects going in empty-handed while another stream leaves carrying little pink bits. The parade stops at dawn and he can move again. There are no marks, and the boy wonders if he imagined the whole thing. But the same thing happens again on successive nights, and he feels powerless to stop it as the situation is too outrageous to be believed. Then one day the boy is astonished to see his double carrying a case. The doppelganger informs the boy that his body has been replaced by "Nergs;" his double has been built from the parts that the insects carried away. The young man will now have to do his bidding, or else. When the boy refuses, he is appalled to find his body dissolving into Nergs. The Nergs file into the case, and the doppelganger carries them off. End of story.
I read this horrifying little story in the 1960s in a magazine, possibly Boys' Life, although I have searched Boys' Life online without success. I did find a couple of references to it in the blogs of other people who found it as memorable as I did.