While our knowledge of the effects of variation of the physical conditions of the environment upon mosquitos is, in some directions, fairly satisfactory, in the case of the effects of variation of temperature there is lamentably little known. So far as a fairly complete survey of the literature dealing with mosquitos has revealed, there is only one paper specifically dealing with this matter, by Macfie (1920), although incidental observations have been recorded by other workers. This present paper is an outcome of an attempt to fill, to a slight degree, this lacuna in our knowledge. The investigations were carried out while the writer was attached to the Department of Zoology at the University College of North Wales, Bangor, and he has to express his thanks to Dr. C. L. Walton, Adviser in Agricultural Zoology, Department of Agriculture, Bangor, for allowing him to carry out the experiments in the Agricultural Zoology Laboratory.