EXPERIMENTS ON TOLERATION OF TEMPERATURE BY DROSOPHILA.

1. Most wild stocks of Drosophila melanogaster can be bred indefinitely on banana agar at a temperature of 31 degrees C. There is no relation between the geographical origin of these stocks and their ability to tolerate this temperature. 2. A single wild stock has been found which will breed for only one generation at temperatures above 29 degrees C. The offspring hatched at 31 degrees C. will breed normally at 24 degrees C. This difference from other wild stocks is apparently genetic, but its genetic basis has not yet been worked out. 3. The mutant stocks of D. melanogaster tested by us will breed for only one generation at 31 degrees C. and their offspring at this temperature are also fertile at 24 degrees C. This condition is apparently a physiological effect of the presence of any of the mutant genes in a homozygous condition. 4. Similar tests indicate that wild stocks of D. virilis and Chymomyza procnemis will breed at 31 degrees C., while D. simulans, D. immigrans, and D. funebris will not. The last two species are northern forms not commonly found in the tropics. 5. Both male and female flies from mutant stocks hatched at 31 degrees C. produce offspring at this temperature if mated to flies hatched at 24 degrees C. Their germ cells are therefore capable of development, and the cause of their failure to develop at 31 degrees C. when inbred must lie either in the failure of the germ cells to reach each other or in the fertilization process itself.