110 VICTOR E. SHELFORD. 



In the case of bark beetles Hennings ('07) found that dry 

 air increased metabolism and some of their activities. This is 

 probably true with reference to an optimum, as moist seasons 

 usually favor insects. The problem is a complex one and much 

 data must be accumulated before a solution can be reached. 

 Headlee ('13) found that the rate of metabolism of bugs feeding 

 on succulent plants was not increased or modified by variations 

 in moisture. 



(b) Period of Over-stimulation. 



This probably results from the loss of skin action through 

 drying, in the vertebrates. It took place in the amphibians 

 when the skin became quite dry; it did not occur in the arthro- 

 pods. Skin respiration is important in most of the amphibians, 

 birds and mammals. Sheffer's pigeons passed through a period 

 of unrest preceding inactivity. 



(c) Period of Depression. 



The period of depression came on gradually in the arthropods. 

 In the case of Plethodon glutinosus when the experiments were 

 continued for more than 20 minutes, the animals sometimes came 

 to rest in an apparently fatigued state, in the medium or dry 

 air and died in that position. Durig found that the irritability 

 of the muscles ('01) was decreased (latent period increased) and 

 that rate of conduction of nerves ('02) was decreased in the case 

 of frogs that had lost from 8 to 30 per cent, of their weight in 

 water. 



4. Importance of the Evaporation Rate. 



The work on the physiological effect of evaporation from the 

 bodies of animals, has been confined chiefly to the warm- 

 blooded domestic animals and man. The loss of water from the 

 human body was early noticed by Hippocrates and by Galen. 

 Chalmers (1776), Seguin and Lavoiser (1789-90), Abernathy 

 (1793), and Shading ('42) all appear to have noted water output 

 from the body or lungs. Weyrick ('62) studied the loss of water 

 from the body, Reinhard ('69) found that the water loss was 

 dependent upon temperature, humidity, wind, velocity and 

 pressure. These factors control evaporation (see also Falck, '72; 



