New methods have been applied to the dynamics of gas exchange in the tracheal system of pupae of the Cecropia silkworm permitting the direct observation of many features of O2, CO2, and N2 flux that had previously only been deduced from indirect evidence. This system functions so efficiently that water loss is reduced to a value which is approximately the same as that produced metabolically.This work would have been impossible without the stimulating environment provided by C. M. Williams and his students. Their generous supplying of pupae is only one of the many favors for which I am indebted.