SUMMARY 1. The songs and other vocalisations of birds are of theoretical interest to zoologists and psychologists (1) as social communication systems and as a “language”; (2) as specific and inter‐specific recognition marks; (3) as a problem in the inheritance and genetical control of elaborate behaviour patterns; and (4) as a problem in the acquistion of complex behaviour patterns by individual learning. The last two constitute the main objects of this work. 2. The Chaffinch Fringilla coelebs was chosen as the main subject of investigation because its song displays both inherited and individually learned components, the relation between which is of particular interest. It is considered that neither a chain reflex type of theory nor one of reflex conditioning in the ordinary sense will provide satisfactory explanation of song production. 3. The characteristics and normal limits of variation of song of wild F. c. coelebs and F. c. gengleri are described and illustrated. There is no certain means of distinguishing the songs of these two subspecies but fairly consistent local differences occur. A wild male Chaffinch usually has more than one song type and may have as many as six. Full song is practically unknown in the female. 4. The song of insular species and sub‐species and of artificially isolated populations tend to be simpler and less variable than examples from the Continent, possibly because, in a less dense population in an ecologically simpler environment, an individually distinctive territorial proclamation will not be so important for breeding success. 5. Experiments with F. c. gengleri were carried out in aviaries, bird rooms and cages. None of these techniques had any seriously adverse effect on the drive to sing. The onset of song could be controlled experimentally by crowding, by artificial control of daily illumination in a light‐proof room and by injection of testosterone propionate. 6. Birds caught as juveniles in their first autumn and isolated until the summer following produced nearly normal songs that are almost as elaborate as are those of wild Chaffinches. If such birds, instead of being individually isolated, are kept together in groups in such a manner that they can hear only the songs of the members of their own group, these birds (as a result of counter‐singing) copy one another and so come to produce a fairly uniform community pattern. Close matching of the final phrase of the song is particularly evident. 7. Birds which have been hand‐reared in auditory isolation from the fifth day of life produce extremely simple songs which represent the inborn component of the specific song. If such Kaspar Hauser birds are themselves grouped together in isolated communities from the third to the thirteenth month of life, each group will‐during the period February to May—build up, by mutual stimulation and imitation, complex but highly abnormal songs quite dissimilar from those of normal wild Chaffinches. From this it is clear that in the wild young Chaffinches learn some features of the song from th...

SUMMARY The closed tracheal system of the aquatic larvae of insects, together with the typical tracheal gill, can only function if the tracheal tubes themselves have sufficient resistance to compression. The work of Ege showed conclusively that the ‘air stores’ of many aquatic insects can, if they are in communication with the spiracles, function as gills for a limited time. Such bubbles will, however, since they have no resistance to compression, gradually dissolve in the water unless there is opportunity for regular renewal at the surface. The significance of the concept of invasion coefficient is considered in relation to gas‐bubble respiration. The term plastron is restricted to a ‘gas store’ communicating with the tracheal system and usually in the form of a thin film of constant and negligible volume and large surface area, retained in position by a system of hydrofuge hairs or scales in such a manner that it is not subject to the ‘Ege effect’ under the normal conditions of its environment, and is therefore not liable to loss by diffusion. Provided there is adequate oxygen in solution in the medium, such a plastron can enable the insect to remain below indefinitely, obtaining all the oxygen it requires from the surrounding water. The structure and biology of Aphelocheirus as a representative plastron insect is described. The plastron hair‐pile appears to be so perfect that an additional pressure of 4–5 atm. is required before the insect is in danger of losing its plastron and becoming wet. The spiracular adaptations which secure communication between the plastron and the tracheal system are described. The theory of plastron respiration as established by a study of Aphelocheirus is outlined. It is shown that in Aphelocheirus it is fully efficient as a respiratory structure and that this method of respiration has here nearly, if not quite, attained theoretical perfection. The resistance of the plastron hair‐pile to wetting is also discussed, and it is shown that this insect appears to have attained a very nearly perfect compromise between the conflicting requirements of a large area of gas‐water interface for diffusion and a minute scale hair‐pile for efficient resistance to water penetration. The development of the plastron hair‐pile in the individual and the first appearance of gas on its surface are described. A survey of plastron respiration in the Coleoptera is given. There are here three main groups in which the method has been elaborated, and while probably none of them presents as efficient an equipment for the purpose as Aphelocheirus there are several which can be regarded as having reached virtual perfection for the particular environments in which they live. It is shown that the plastron insects can be divided conveniently into three groups on the basis of the scale and arrangement of the hair‐pile and the consequent efficiency of the resistance to wetting. A number of the Coleoptera carry, over and above the plastron, a thicker gas layer, the macro‐plastron, held either by longer hairs or by me...

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