National Museum of Natural Science

If mature silkworks are in a flat place, they will be unable to form normal cocoons because they cannot build a cocooning frame. They will crawl around in search of an appropriate place to form a cocoon and if they do not find one will give up on their instinct to build a cocooning frame. Silk is then spun on the surrounding plane forming a flat cocoon.

Flat silkworm cocoons