I. The oxygen content of the haemolymph of mature larvae of Ephestia kühniella has been measured in healthy individuals and in those parasitized by the ichneumonid Nemeritis canescens. In the latter the oxygen content decreases with the progress of parasitism.

2. The respiratory rate of first, second- and third-instar larvae of Nemeritis has been measured and found to increase rapidly at the end of the first instar, at about the time at which the phenomenon of physiological suppression appears.

3. The partial pressure of oxygen in the gas mixture surrounding the Ephestia larva affects the oxygen content of its haemolymph proportionally and so was used to alter the availability of oxygen to the parasite in vivo.

4. The survival of Nemritis larvae increases with the availability of oxygen in the haemolymph of its host.

5. The capacity for survival under conditions of low oxygen tension is minimal for eggs and newly hatched larvae of Nemeritis, but rapidly increases with age.

6. The hypothesis of physiological suppression of supernumerary parasitoid larvae by asphyxiation is discussed with respect to these observations and is held to be supported by them.

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