A convenient and accurate measure of size is necessary for studies on the physiology of tsetse flies (Glossina). Residual dry weight (i.e., dry weight minus fat) is adequate as regards the larva, pupa and teneral fly, but cannot be used for flies that have taken their first blood-meals. A linear measurement provides an index of size that remains constant throughout the life of the fly, but the use of the length of the middle part of the fourth longitudinal vein in the wing has certain disadvantages, and a measure of surface was thought preferable. That finally adopted was a function of the dorsal thoracic surface; details of it are given and a close correlation was demonstrated between it and residual dry weight for females of G. morsitans Westw. bred in the laboratory from puparia maintained at 28°C.

The relation between this measure and the weight of the newly emerged fly is affected by the temperature at which the pupal stage is passed; above approximately 25°C. the surface area of flies of any given residual dry weight decreases quite sharply, so that puparia of a given weight produce smaller but relatively heavier flies at 30 than at 25°C. This holds good also for G. pallidipes Aust. and G. swynnertoni Aust. The dependence of surface area on the temperature at which development has taken place unavoidably complicates the use of this measurement in work on transpiration; it also limits the application of the theory that stresses acting on the pregnant fly may be gauged from the size of flies two months later, and suggests that where the mean temperature of pupal sites is likely to exceed 26°C, any inference should be based on puparial rather than adult size.