The seven layers of the chorion covering the egg of the bug, Rhodnius prolixus, are all freely permeable to water. The egg has no active physiological mechanism preventing desiccation, and is waterproofed by a layer of wax, less than $0.5\mu $ thick, which covers the inside of the chorion. The wax is similar to that which waterproofs the cuticles of most adult forms of insect, and shows a typical transition point in its water loss/temperature curve at 42.5 degrees C. The waterproofing wax layer is secreted by the maturing oocyte, and is securely attached to the innermost layer of the chorion. The secretion of the wax takes place in the ovary, either just before or after the egg is released from its follicle, but a wax layer can be obtained by incubation of eggs with incomplete chorions. The layer of wax is complete across the inner openings of the micropylar tubes; it is supported at these points on the vitelline membrane before fertilization, and on the fertilization membrane after this has been formed.