This seems very close to my recollection of "The Cocoon" (1946) by John B.L. Goodwin, but not exactly the same.

The story concerns a boy, Denny, who collects insects, mostly moths and butterflies (lepidoptera), and who kills them and pins them to the wallpaper in his bedroom. One day he finds a large, very strange caterpillar and brings it home. It pupates and he keeps it dormant through the winter and it emerges one day in the spring as a huge (10" wingspan) but strange butterfly. Denny waits for its wings to finish expanding, then kills it and pins it to his wall at the foot of the bed.

Previously it has been noted that his room smelled from the viscera and fluids that drained out of the insects and stained the walls; the new one drains more, and smells worse. Denny starts being awakened at night to the sound of something beating on the window, and the feeling of something brushing his face. The attacks get worse until one day he doesn't come down for breakfast. Finally entering his room, his father find him partially desiccated, stuck to the rug, with a pool of fluid soaking into the rug. The butterfly is gone.

This seems very close to my recollection of "The Cocoon" (1946) by John B.L. Goodwin, but not exactly the same.

The story concerns a boy, Denny, who collects insects, mostly moths and butterflies (lepidoptera), and who kills them and pins them to the wallpaper in his bedroom. One day he finds a large, very strange caterpillar and brings it home. It pupates and he keeps it dormant through the winter and it emerges one day in the spring as a huge (10" wingspan) but strange butterfly. Denny waits for its wings to finish expanding, then kills it and pins it to his wall at the foot of the bed.

Previously it has been noted that his room smelled from the viscera and fluids that drained out of the insects and stained the walls; the new one drains more, and smells worse. Denny starts being awakened at night to the sound of something beating on the window, and the feeling of something brushing his face. The attacks get worse until one day he doesn't come down for breakfast. Finally entering his room, his father find him partially desiccated, stuck to the rug, with a pool of fluid soaking into the rug. The butterfly is gone.

This seems very close to my recollection of "The Cocoon" (1946) by John B.L. Goodwin, but not exactly the same.

The story concerns a boy who collects insects, who is ultimately found dead in his room as though he had been killed in the same way as he killed insects, but I don't think he was pinned.

This seems very close to my recollection of "The Cocoon" (1946) by John B.L. Goodwin, but not exactly the same.

The story concerns a boy, Denny, who collects insects, mostly moths and butterflies (lepidoptera), and who kills them and pins them to the wallpaper in his bedroom. One day he finds a large, very strange caterpillar and brings it home. It pupates and he keeps it dormant through the winter and it emerges one day in the spring as a huge (10" wingspan) but strange butterfly. Denny waits for its wings to finish expanding, then kills it and pins it to his wall at the foot of the bed.

Previously it has been noted that his room smelled from the viscera and fluids that drained out of the insects and stained the walls; the new one drains more, and smells worse. Denny starts being awakened at night to the sound of something beating on the window, and the feeling of something brushing his face. The attacks get worse until one day he doesn't come down for breakfast. Finally entering his room, his father find him partially desiccated, stuck to the rug, with a pool of fluid soaking into the rug. The butterfly is gone.