The truth about saturation deficiency—an historical perspective

Studies on the water relationships of arthropods originated from the needs of applied entomology early in this century. Saturation deficiency (S.D.) was hailed as a measure whereby the effects of temperature and humidity on transpiration, and thus on development and longevity, could be simply combined. Errors in curve fitting and lack of physiological knowledge led to early misconceptions, but S.D. helped to focus attention on the need for laboratory analysis of field data and played a part in the move toward a more scientific approach leading to a great expansion of the whole field. Application of the concept to arthropods has usually been controversial, and examples of this drawn from the literature on the bionomics of the plague fleas and on cuticular waterproofing are discussed. A controversy exists at present concerning S.D. and the physics of transpiration. The concept of S.D. has been most useful where it has stimulated research to find out how arthopods avoid its consequences.