AN EXPERIMENTAL ANALYSIS OF THE DISCONTINUOUS RESPIRATION OF THE CECROPIA SILKWORM 1 HOWARD A. SCHNEIDERMAN AND CARROLL M. WILLIAMS Department of Zoology, Cornell University, Ithaca, N. V ., and the Biological Laboratories, Harvard University, Cambridge 38, Mass. The breathing of insects has been an object of recurrent interest since the seventeenth century when Malpighi (1669) published his celebrated description of the tracheal system of Bomby.v mori and correctly ascribed to it the function of respiration. Though other early investigators, including Lyonet, Reaumur, Bonnet, and Spallanzani, recognized that the spiracles were the sites where air made entrance into the tracheal tubes, they believed that expiration occurred else-where, through the mouth, or anus, or invisible pores in the skin (cf. Wigglesworth, 1931). It was Treviranus (1816-20) who first suggested that the spiracles pro-vide the pathway for both inspiration and expiration a view later amplified by the British physician, George Newport (1836). Newport was especially con-cerned with the respiration of lepidopterous pupae. Though apparently aware that breathing motions are indetectable in such pupae, he nevertheless supposed that the ventilation of the tracheae was due to active respiratory movements in which "nearly all the muscles and nerves of each segment are brought into consentaneous action as the muscles of the chest and ribs in vertebrated animals" (page 546). Thomas Graham in 1833, having just discovered his law of gaseous diffusion, expressed a view contrary to that of Newport and suggested that simple physical diffusion of gases through the tracheal system might suffice for the respiration of most insects. Nearly one hundred years elapsed before experimental proof of this hypothesis was forthcoming. Then in a series of ingenious experiments of a type for which he was justly famous. Krogh (1920) showed that simple diffusion of oxygen and carbon dioxide through the spiracles and tracheae could more than suffice for the respiration of insects lacking respiratory movements. The es-sential elements of Krogh's theory have stood the test of time and have been most recently examined in detail by Thorpe and Crisp (1947). On the basis of this long evolution of present thought concerning the mechan-ism of insect respiration, there was no theoretical basis for anticipating a series of puzzling observations which we first encountered five years ago in measure-ments of the respiratory quotients of diapausing pupae of the Cecropia silkworm. The R.Q. in these studies was found to undergo large and recurrent fluctuations as a function of time, in that prolonged periods of very low quotients were punc-tuated by more transient periods of very high quotients. It soon became evident that the source of the variability was a discontinuous release of carbon dioxide from the insects' tracheal system a phenomenon which, we subsequently learned, had 1 This study was aided by a grant to C. 'M. Williams from the U. S. Public Health Service, by an Institutional grant to Harvard University from the American Cancer Society, and by the Sage and Sackett Funds of Cornell University. 123
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