THE first article on the speed and height of the flight of birds is a very brief account of the subject. In the second one, on the respiration of insects (about 80 pages), the function of the spiracles and the movements which ventilate the tracheae are discussed, and the principal methods for investigating the movements are described and illustrated. Due attention is given to the physical and chemical aspects of the problem, e.g. the technique of gas microanalysis. The third article discusses the flight of insects and of birds and the methods which have been employed in the elucidation of the movements and of their mechanics.