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With Plate I and 5 Text-figures)A description is given of the design and physical performance of a laboratory spraying apparatus, which is an improved version of that described by the author in 1941. This apparatus is shown to be capable of giving good replication and an even distribution over a circular area 9 cm. in diameter with distilled water, a light petroleum oil and a heavy petroleum oil. It may therefore be used with a variety of media either for direct application to the organism, or for the application of residual films. With distilled water a difference in the environmental temperature of zoo F., between 60 and 80" F., produced approximately 10 % difference in the deposit and a difference of zo yo in the relative humidity of the environment between 60 and 80 % produced approximately 5 % difference in the deposit, so that it does not appear that closely controlled conditions, although desirable, are necessary for good replication.App. Biol. 39

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With Plate 8 and 5 Text-figures)T o ascertain whether any substance is likely to be useful as a contact insecticide two separate investigations have to be made: a laboratory examination of the effect of the substance on insects, and a study of it in the field under the conditions in which it is to be used in practice.In order to ascertain how a contact insecticide can be applied to the best advantage, it is necessary to make a detailed study of its effect both qualitatively and quantitatively, and this can be most satisfactorily carried out under the controlled conditions only to be obtained in the laboratory.Laboratory studies on contact insecticides in a fluid medium can be classified under three heads : (I) The discovery of new substances which possess insecticidal activity, and a general investigation into their possibilities.(2) The detailed study of any given substance to determine its physiological action and its effect on particular species of insects. (3) The study of the effect of varying the medium in which the toxic substance is applied, of the methods of application and of factors extraneous to the toxic substance.The first requisite for the study of these three problems is a method of applying the toxic liquids in equal doses to large numbers of individual insects of a wide variety of species. The method should also be such that the individual doses administered may range from small to large. The second requisite is to have available a wide range of species with representatives from all the important orders of insects. It should be possible to rear some, at any rate, of these species so that large numbers are continuously available; for this purpose, species with a short life cycle and no seasonal rhythm are most suitable.In the present work most of the time was devoted to the method of applying the dose of insecticide, but some prelimrnary work was also done on the selection and rearing of suitable test insects. Once a suitable method of application of the insecticide was found, the extent to which results obtained with any given species and instar are generally applicable, depends on the degree to which specificity of effect occurs, and this will probably vary both with the insecticide and the insect. The information on this subject is scanty, and it is desirable that more data should be gained on the subject of specificity as a preliminary to the selection of a range of species as test insects. In the meantime it has to be assumed that an insecticide which fails to kill the adult of one or two species of insect which have not so far been found highly resistant, is of little value as an insecticide. APPARATUS AND METHODS AT PRESENT AVAILABLETattersfield (1939) reviewed biological methods of testing insecticides, including a brief description and discussion of the laboratory methods of testing liquid contact insecticides so far used. The means of applying liquids to the integument of an insect can be divided into r '42 1