AN EXPERIMENTAL ANALYSIS OF THE DISCONTINUOUS RESPIRATION OF THE CECROPIA SILKWORM

¹ Department of Zoology, Cornell University, Ithaca, N. Y., and the Biological Laboratories, Harvard University, Cambridge 38, Mass.

1. The respiration of diapausing Cecropia pupae is remarkable in that metabolic carbon dioxide is retained within the insect and released during brief periods as "bursts." This finding confirms Punt's prior observations on several species of diapausing insects.

2. At 25° C. in 14 pupae with an average weight of 4.7 grams, bursts of carbon dioxide occurred on the average of once every 7.3 hours and the burst volumes averaged 271 mm³/animal. The average interburst rate of carbon dioxide output was 2.7 mm³/gm. live wt./hr., i.e., about one-sixth the average rate of oxygen uptake.

3. The utilization of oxygen, unlike the release of carbon dioxide, shows no discontinuities and "direct" Warburg manometric procedures reveal an apparently continuous uptake of oxygen.

4. Several independent lines of experimentation confirm the manometric observations that oxygen uptake by the pupa is continuous and non-cyclic and direct attention to the surprising fact that, in the interburst period, oxygen may enter the insect at many times the rate at which carbon dioxide makes exit.

5. Virtually all respiratory exchange ceases after the spiracles are sealed with wax; the tracheal system and spiracles are therefore the site of both the discontinuous release of carbon dioxide and the simultaneous continuous uptake of oxygen.

6. The cycle of carbon dioxide release is a function of metabolic rate and therefore of temperature. At low metabolic rates the bursts are accentuated. Thus, at 10° C. carbon dioxide is given off only once in several days and the interbrust rate of carbon dioxide output may be but 1/100th the rate of oxygen uptake. If the insect's metabolism is increased by integumentary injury, or development, or increase in environmental temperature, the bursts become more frequent and the continuous release of carbon dioxide more pronounced. The burst phenomenon usually disappears when the oxygen uptake rises beyond 160 mm³/gm. live wt./hour.

7. The bursts also vanish when the external oxygen tension is decreased below 15 per cent or the external carbon dioxide increased above 10 per cent. The interburst rate of carbon dioxide output is especially sensitive to oxygen tension and in pure oxygen the interburst carbon dioxide output may become undetectable.

8. For bursts of equal volume, the rate of release of carbon dioxide during the burst is substantially the same at 10° and 25° C., signifying that the rate-limiting processes in the rapid release of carbon dioxide are of a physical character with low temperature coefficients.

9. The discontinuous release of carbon dioxide is apparently a widespread phenomenon in diapausing pupae. This fact complicates determinations of respiratory quotients and is evidently responsible for the extremely low and apparently erroneous values reported for diapausing pupae.

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