HUMIDITY 233 



same temperature is at 73-4 per cent., when the length of its pupal 

 instar is at its minimum ; between that percentage and 7-1 per 

 cent, humidity its developmental period showed a difference of 

 10-5 days. At per cent, humidity and 27° temperature all the 

 parasites failed to complete their development, whereas all the 

 hosts emerged. This feature does not appear to be attributable 

 to variations in metabolic activity, but rather to differential 

 powers of resistance to desiccation in the two instances, since both 

 species of insect showed their highest rate of development at 

 about 73-4 per cent, humidity. Above the latter figure, up to 

 100 per cent, humidity, the duration of the pupal period of both 

 host and parasite showed a progressive lengthening. If viability 

 be judged by the percentage emergence from the pupae, it is note- 

 worthy that host and parasite behave very differently in this 

 respect, dryness favouring the former and high humidity the 

 latter {vide Figs. 71 and 72). 



Griswold and Crowell (1936) discuss the influence of humidity 

 on the length of life-cycle in the webbing clothes moth (Tineola 

 hiselliella). At a constant temperature of 25° C. this insect 

 developed under relative humidities ranging from 20 to 93 per cent. 

 At 75 per cent, humidity development was more rapid than under 

 other moisture concentrations, mortality was lowest and adults 

 lived longest. A humidity of 93 per cent, proved more deleterious 

 to the insect than 20 or 30 per cent. In the case of the cockroach 

 (Blatta orientalis), Gunn (1934) states that the preferred tem- 

 perature range is 20° to 29° C, and that this zone is not affected 

 by changes in the humidity of the air. With tsetse flies (Glossina), 

 Buxton and Lewis (1934) found that the effects of humidity 

 are complex. Flies kept at 30° C. behaved under 44 per cent, 

 humidity in a way which suggests that this concentration is near 

 the optimum : at humidities above or below this figure the flies 

 fed less often, and at 88 per cent, humidity they nearly always 

 refused to feed and died. It was also observed that the adults 

 lived longest and bred best at 44 per cent, humidity. At higher 

 temperatures the effects of humidity are clearly shown, since the 

 insects are able to survive in drier air for a short period at 42° to 

 43° C. — temperatures which are fatal in moist air. Field observa- 



