Although applied entomology has made enormous progress during recent years both in its scope and importance, we cannot deny that it has hitherto moved only in certain restricted paths and is therefore in danger of becoming one-sided, for the subject is taken up at present almost exclusively with a study of lifehistories, with parasites and the empirically ascertained methods of combatting injurious species.† Thus it has come about that up to the present time we know very little concerning the secretion of poisons by the sucking plant-parasites and the effeCts of these poisons on the plants. Most attention has been paid to this question by those who have investigated the origin of plant galls. Hence it may be of interest to record some investigations which I have made on the poisons of plant-lice.