The relation between the uptake of atmospheric CO2 and insect's production of silk fiber has not yet been reported. Here, we provide the first quantitative demonstrations that four species of silkworms (Bombyx mori, Samia cynthia ricini, Antheraea pernyi, and Antheraea yamamai) and a silk-producing spider (Nephila clavata) incorporate atmospheric CO2 into their silk fibers. The abundance of C-13 incorporated from the environment was determined by mass spectrometry and C-13 NMR measurements. Atmospheric CO2 was incorporated into the silk fibers in the carboryl groups of alanine, aspartic acid, serine, and glycine and the C-gamma of aspartic acid. We show a simple model for the uptake of atmospheric CO2 by silkworms. These results will demonstrate that silkworm has incorporated atmospheric CO2 into silk fiber via the TCA cycle; however, the magnitude of uptake into the silk fibers is smaller than that consumed by the photosynthesis in trees and coral reefs.