HUMIDITY 231 



cycle as a whole, development was most rapid at about 90 per 

 cent, humidity, the shortening apparently occurring at the stages 

 when feeding was most active ; with a humidity of 25 per cent, 

 comparatively few of the insects attained maturity, and even 

 fewer at lower moisture conditions. The same author's earlier, 

 and well-known, experiments with the aphid Toxojptera graminum 

 show a much greater tolerance of variations of atmospheric 

 humidity. At a constant temperature of 26-6° the same develop- 

 mental rate was maintained at relative humidities varying from 

 37 to 100 per cent. It has been suggested that, with sap-sucking 



7-1 25-9 73'4- 100 



Fig. 71. Relation between humidity percentage and length of pupal 

 instar in days for Protoparce quinqueniaciilatus and its parasite 

 Winthemia A^-pustulata (broken line). (Adapted from Hefley.) 



insects, this range of tolerance is explainable on account of the 

 relative unimportance of atmospheric humidity, since the insect 

 is rendered largely independent of such conditions on account of 

 the moisture obtainable through its food. This conclusion is 

 supported in a general way by more recent observations by 

 Wadley(1931). 



The experiments of Hefley (1928), with the pupae of the hawk 

 moth Protoparce quinquemaculatus, and its Tachinid parasite 

 Winthemia quadripustulata, will serve to illustrate the differential 

 effects of humidity upon two separate species, living in close 

 association. The host does not, it appears, exhibit a definite 

 optimum reaction to humidity, since the duration of its pupal 



