Insect tissue is able to maintain its activities in very abnormal surrounding media. However, there is an obvious need to determine the exact tolerance of insects to different ions. This information may lead to the formation of an adequate insect physiological solution. It is the purpose of this investigation to determine the tolerance of isolated insect hearts to different concentrations of sodium, potassium, calcium, and magnesium ions, as well as to changes in the osmotic pressure of the surrounding medium. These experiments were performed on a representative insect from each of the two groups described by Florkin, Duch~teau, and Leclercq (1949). Those selected were the adult grasshopper, Ckortopkaga viridifasciata, which typifies insects with a high sodium and a low potassium index (the percentage of a cation in relation to the sum of the cations is its index), and the pupae of the moth, Saraia walkeri, which typify those with a low sodium and high potassium index.Florkin, Duch~teau, and Leelercq (1949) divided insects into two groups, distinguished by differences in the concentration of sodium and potassium in the blood: those in which the sodium index is high and that of potassium low, and those in which the reverse relationship exists. They postulated that insects with a high potassium and low sodium index, such as the moth, Samia walkeri, have developed in the Tertiary in connection with the development of higher plants. Insects having a high sodium index were found to be either the ancestral forms or those whose development was independent of the evolution of plants. They also stated that the blood of insects with a low sodium and high potassium index has a higher magnesium contant than that of the more primitive insects.Most saline solutions used in insect work are variations of frog Ringer solution, and completely fail to duplicate the concentration of ions found in insect blood. There have been two principal methods used in the formulation of insect saline solutions. Lewis and Robertson (1916) adapted the classical Ringer-Locke solution to insect material. They tried to formulate a medium which would have nutrient components 79

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