Most Drosophila flies, can be subjected to a contact with pure CO₂ for long hours, without injury. L'Heritier and Teisser (1958), however, discovered a true breeding strain of Drosophila which was sensitive to CO₂. The sensitive flies, when exposed briefly to CO₂ for a short period, become unconscious in a characteristic way, with their legs becoming paralysed. When reciprocal crosses were made between CO₂ sensitive and normal strains, it could be shown that the trait was inherited only from female parent. In other words, while sensitive mothers always give sensitive progeny, sensitive fathers only rarely give sensitive progeny if the mother is normal. In rare transmission through male also, sensitivity is quickly lost after first generation. It has also been shown that if an extract obtained by crushing sensitive flies, is injected into the body of wild (normal) flies, sensitivity can be induced. It was also shown that this sensitivity can be attributed to a virus like particle called sigma found in cytoplasm of the cells of a sensitive fly.