1. The period which elapses between a blood-meal and oviposition on the part of the female bed-bug affects the duration of the egg stage. Eggs laid soon after the meal take longer to hatch than those laid later. It is supposed that embryonic development of eggs inside the female occurs at a rate relatively greater than that of oviposition.
2. Atmospheric humidity is without effect on the duration of the egg stage. Although the temperature-velocity graph appears to be fairly linear between 18 and 30° C. the thermal constants show considerable variation.
3. A daily alternation of temperature with a range of 10° C, between the threshold and optimum temperatures results in an acceleration of development; this can, however, be accounted for by the non-linearity of the temperature-velocity relationship, if the usual methods of thermal summation are used.
4. Eggs have been exposed to temperatures between 1 and 12° C. and the time taken to hatch on subsequent incubation at 23° C. has been ascertained. When these times are compared with times for hatching of control eggs kept at 23° C. from oviposition there is some evidence that a slight amount of development may occur at as low a temperature as 4° C.; i.e. 9° below the developmental-hatching threshold. Times after exposure are shorter than times for control eggs.
5. Thermal summation, however, suggests that temperatures between 13 and 11·7° C. result in a retardation of development since the times after exposure at which hatching occurs at 23° C. are longer than would be expected.
6. The method of thermal summation is criticized mainly on the grounds that it assumes that a temperature has the same accelerating effect on all stages of embryonic development. A retardation of development such as that mentioned in the preceding paragraph may be either a true retardation or due to errors resulting from the assumption that the reciprocal of the time for complete development represents the true amount of daily development at all stages of embryonic growth.
7. The lowest constant temperature at which complete development with hatching, of eggs laid at 23° C, can occur is 13° C. I have called this temperature the developmental-hatching threshold. The developmental threshold may be as low as 4° C, while the hatching threshold is at approximately 8° C.
8. Alternating temperatures such as occur in English houses are unlikely to affect the position of the developmental-hatching threshold. Atmospheric humidity does, however, affect it and 75–90% r.h. appear to be the only humidities at which development with hatching can take place at a constant temperature of 13° C.
9. Mortalities near the developmental-hatching threshold appear to depend also on the temperature at which eggs are laid (or perhaps the temperatures at which they develop within the female). Eggs laid at 15° C. and incubated at 15° C. and 7% r.h. suffer 97·1% mortality while those laid at 23° C. and incubated under identical conditions have 32·9% mortality.
10. If eggs are laid at 23° C. and are incubated at 15, 18 and 23° C. until nearly ready to hatch, the percentage hatch at temperatures near the batching threshold is higher with those eggs previously kept at 18° C. than with those from 15 or from 23° C. At 8° C, the lowest observed temperature for hatching, preliminary incubation at 15° C. is probably more favourable than one at 23° C.
11. Mortality of eggs above 13° C. is only slightly affected by atmospheric humidity over the optimal range. But the effects are more noticeable near the upper and lower temperature limits. 99–100% r.h. appears to be associated with a higher mortality than 90% r.h. The extreme temperature limits for eggs laid at 23° C. are 13 and 37° C. With eggs laid at 15° C. the range is restricted at both upper and lower temperature limits compared with the range for eggs laid at 23° C. The temperature of oviposition, whether it is 15 or 25° C, seems to make no difference to mortalities between 18 and 28° C.
12. The mortalities of eggs exposed for varying periods to temperatures between 1 and 13° C. and various humidities are discussed. By means of probits estimates of the times for 50 and 99·99% mortalities have been made for each of the temperature and humidity combinations.
13. Exposure for 50% mortality is affected slightly by humidity below 13° C, but no simple law relating survival to humidity has been found either above or below 13° C. Variations in temperature within the range 0—34° C. are likely to influence survival more than the humidity variations possible within this temperature range.
14. At constant saturation deficiencies the eggs survive longer at the higher temperatures between 1 and 13° C. The median exposure for death (i.e. exposure for 50% mortality) increases by 1·5–1·8 days per 1° C. rise of temperature.
15. The scatter of mortalities about the median exposure for death as measured by the regression coefficient of probits on exposure times is slightly influenced by humidity, but is not affected by temperatures between 1 and 13° C. Much of the variation in the slopes of the mortality curves is thought to be due to inherent variation in the eggs themselves.
16. With eggs laid at 23° C. and exposed to temperatures below 13° C. the longest exposure necessary to produce 99·99% mortality (as judged by subsequent incubation at 23° C.) was estimated at 79.8 days. This occurred at 12·1° C. and 73% r.h. The actual observed times for exposure for 100% mortality are somewhat shorter than the estimated times.
17. Eggs with embryos in an advanced state of development are more quickly killed by exposure to 7·7° C. and 90% r.h. than are newly laid eggs.
18. Ovipositional temperatures of 15, 18 and 23° C. produced no different effects on the mortality rates of eggs exposed to 10° C. and 90% r.h.
19. Future problems and the ecological significance of the experimental results are discussed.