What's the meaning of "collapse in on itself"?
I thought of the Joe who’d given me the address. He had probably done it, and he was probably dead. I imagined his body collapsing in on itself, the flesh rotting and then gone, the skin on his lips cracked until only dust remained in a thin veneer over his skull.
What's the meaning of "collapse in on itself"?
Thanks!

Question

What's the meaning of "collapse in on itself"?
I thought of the Joe who’d given me the address. He had probably done it, and he was probably dead. I imagined his body collapsing in on itself, the flesh rotting and then gone, the skin on his lips cracked until only dust remained in a thin veneer over his skull. 
What's the meaning of "collapse in on itself"?
Thanks!

Answer

I think that it's a bit gruesome. As the body rots, it rots from the inside, and becomes hollow. When that happens, the skin would drop into where the internal organs were - A bit like how some fruit rots.