310 SANITARY ENTOMOLOGY 



The lice seem to avoid light except when hungry. They seem to 

 be quite sensitive to excessive warmth and will leave a fever patient. 



In the absence of definite humidity data we may roughly describe 

 the zones of climatic influence on the lice as follows: The zone of mini- 

 mum fatal temperatures for eggs is below 20° C. (68° F.) and for adults 

 lies below zero centigrade (32° F. ). The zone of the dormancy in adults 

 extends from about -10° to 5° C. (14° to 41° F.). The zone of slug- 

 gish movement without reproductive activity and with practically no 

 digestive processes extends from 5° to 20° C. (41 to 68° F.). Digestion 

 ceases at 12° C. The zone of optimum activity lies between 20° and 40° C. 

 (68° to 140° F.) with the optimum about 30° C. Practically all egg 

 hatching occurs within this zone, as does all oviposition, practically all 

 assimilation of food, and all normal activity. From 40° to 44° C. the 

 lice are wildly active. This zone represents one of exhaustion in which 

 death of eggs occurs. Above 44° C. (112° F.) lies the zone of maximum 

 fatal temperatures. In control work 54° C. (131° F.) for one-half 

 hour is sufficient to kill all stages, and 60° C. (140° F.) for one-quarter 

 of an hour gives a very thorough control. 



There are several other phases of the biology of lice which may be 

 mentioned briefly. For example, the locomotory powers would repay 

 study. Their inability to make any headway on clean smooth metal or 

 glass when inclined at an angle of more than 2° to 3°, and their inability 

 to crawl on smooth vertical surfaces such as rubber gloves or boots, as 

 contrasted with their gymnastic skill on threads or fibrous materials 

 and their power of clinging to anything which they can clasp with their 

 claws, explain the different protective uniforms worn by those who have 

 had to do with typhus epidemics. 



Their different reactions to light when fully fed or when hungry have 

 a bearing on the question as to how they find new hosts. 



Thert are yet many phases of the biology which need elucidation. 

 For example, the question as to the state of development of the olfactory 

 sense and whether this comes into use in finding a new host. Hase con- 

 cludes that they have a fairly keen olfactory sense because they are 

 quickly repelled by substances like tar and ethereal oils. According to 

 him, they recognize and avoid the odor of horses, — the clothing of those 

 artillery men who drive and care for the horses is saturated with the 

 horse odor and free from lice, while others in the same battery without 

 the horse odor are infested. On the other hand, a hungry louse placed 

 on a glass slide near a freshly drawn drop of blood is apparently entirely 

 unaware of the proximity of food. Likewise a hungry louse on a piece 

 of cloth is apparently unaware of the presence of a human hand and a 

 chance to feed, until a finger has been pushed within one-half inch or 



