First appeared in Astonishing Stories. Reprinted--in a few major genre retrospectives--by Alden H. Norton, Mary Gnaedinger, Chester Whitehorn, Robert Arthur, Terry Carr, Bill Pronzini, Barry N. Malzberg, Isaac Asimov, Martin H. Greenberg, Stefan R. Dziemianowicz, Robert Weinberg, Brian Thomsen, Steven Utley, Michael Bishop, Jeff VanderMeer, and Ann VanderMeer. Made into a movie directed by Guillermo del Toro. Read online.
Fascinating use of plot structure and societal prejudices. The plot relies on an unusual structure for its effectiveness. It begins with nonfiction, then delivers the speculative content in a narrative manner although the narrator isn't heavily involved in the tale except as an observer. Nonetheless, it delivers quite a creepiness punch as the narrative closely parallels the nonfiction.
First, it tallies off all of humanity's recent discoveries to say how little we know about the world we live in (raise credibility for the incredible that's to come). Next, it describes mimicry in nature, how various insects have used it, how it's used to protect itself by looking like something dangerous. Third, we are introduced to a solitary man who clothes himself mysteriously in slouch hat and coat, keeps to himself, draws himself up short when women pass, and makes strange noises in his apartment. Finally, the narrator and police let themselves into his apartment after a time when he doesn't show. He's passed away....
As you may have guessed, the single man/recluse is an "it." Various parts are made to look like normal human parts, i.e. wings are his coat, etc. If that's not creepy enough, the best is saved for last:
The creature has had babies. When they open a box, out fly a bunch of little men creatures into the night. And the narrator--no one else--spies a creature in the shape of a chimney, peel off a roof and pursue the little men. Cool.
I haven't investigated Wollheim's initial claims for the example creatures of mimicry in nature, but his point remains the same.
What's most fascinating is a hidden assumption within the text. As a single guy who's lived in tiny towns, where residents are so bored they make guesses about your sexuality (decorated on desks, whispered to you by preschoolers, newly placed chairs propped next to your bedroom window), how intriguing it is that the single man is a mysterious alien creature whose motives are impossible to fathom. He was and apparently still is--at least in some areas of the US.