1. Experiments have been conducted to determine the extrinsic factors that cause facultative diapause in two parasitic chalcid wasps, Mormoniella vitripennis and Tritneptis klugii, and to analyse the mechanism of diapause termination.

2. In both species diapause occurs in the last larval instar after the feeding period has ended and just prior to defaecation. The diagnostic feature of the diapausing larva is that it does not immediately moult into a pupa.

3. In Mormoniella exposing females to low temperature during oögenesis causes their progeny to enter diapause at the end of the last larval instar. Low temperature thus causes the female to lay an egg that is qualitatively different from an ordinary egg in that the larva emerging from it eventually enters diapause. This action of low temperature on the female wears off after several days and the wasp returns to producing non-diapausing offspring.

4. In Tritneptis low temperature also produces diapause, but in this species low temperature, to be effective, must act on the larva itself between the second and final instar to produce diapause in that generation.

5. The diapause of Mormoniella was considered in relation to maternally induced diapause in other species and two possible mechanisms for the action of low temperature were suggested, namely, a direct action on the ovaries or an indirect action through the maternal production of a diapause hormone.

6. It was found that exposure to low temperatures enables larvae of both species to break diapause and complete their development when subsequently placed at 25° C.

7. The mechanism of action of low temperature in terminating diapause was examined in Mormoniella by exposing larvae to various temperature régimes in the presence and absence of oxygen.

8. Ten weeks at 5° C. enabled nearly 90% of the larvae to terminate diapause when returned to 25° C.; after 6 weeks at 5° C. less than 10% developed. Chilling at 2° C. was more effective than 5 or 1o° C., while temperatures above 15 or below -6° C. were ineffective.

9. Although diapause was never terminated by keeping larvae continually at temperatures above 15° C., a period of exposure to temperatures above 15° C. prior to chilling decreased the amount of chilling necessary to terminate diapause.

10. Larvae chilled in the absence of oxygen for as long as 28 weeks failed to break diapause but developed when subsequently rechilled in air.

11. After receiving a threshold exposure to low temperature larvae could not be returned to diapause by temperature shocks as high as 45° C.; however, the effects of subthreshold chilling were reversed by exposure to 25° C. Thus, animals chilled for a total of 20 weeks, with 1 week of warming after each week of chilling, failed to develop. Similarly, it was found that interrupting 8 weeks of chilling on the 25th day by 4 days of warming partially undid the chilling.

12. When warming was conducted in the absence of oxygen it failed to undo the effects of subthreshold chilling and the termination of diapause was markedly accelerated. Indeed, when larvae were chilled for a subthreshold period, a brief interval of anaerobic warming sufficed to enable some of the larvae to terminate diapause.

13. In addition to the above, a variety of other experiments were conducted with alternating periods of chilling and warming in the presence and absence of oxygen. These led to a hypothesis which seems to account for the action of low temperature in terminating larval diapause.

14. The hypothesis focuses attention on the neurosecretory cells of the insect's brain and suggests that low temperature slows down an aerobic breakdown reaction within the larval brain and permits the synthesis of a substance necessary for neurosecretory activity. The initial stages of the synthetic reaction are aerobic but later stages are favoured by anaerobic warming. The nature of these reactions was discussed.

This research was supported by grant H-1887 from the National Heart Institute of the U.S. Public Health Service.