THE LIFE HISTORY OF HUMAN LICE 309 



are preferred. However, in case of necessity, the lice can and will oviposit 

 on smooth materials such as silk and sateen. It has been suggested that 

 infestation could be greatly reduced and even remedied entirely by wear- 

 ing for one to twenty) -four hours a broad band of felt or rough wool under 

 the clothes, with the idea that lice would collect on this, and they and 

 their eggs could then be destroyed by burning. But the preference of 

 lice for such material and the difference between this and the uniform is 

 not marked enough to make it really effective. 



In practical control work the question is likely to arise as to how 

 long discarded but untreated clothing will remain infective. The an- 

 swer to this, of course, depends on how long lice can live without food 

 and how long it takes for all the eggs to hatch. Experiments show 

 that lice can live without food from two to three days at 35° C, three 

 days at 30° C, three to five da^^s at 22° C, and about seven days at 

 10° C. The lice cannot live long without food unless at ineffective tem- 

 peratures, the longest period recorded being ten days at 5° C. 

 (-11° F.). The longest record of fed adults is 46 days for a female 

 recorded by Bacot. One male lived 32 days and fertilized eighteen females. 



As stated above, eggs will hatch in sixteen days at 25° C, but 

 below 22° C. they usually do not hatch. How long a period of low tem- 

 peratures they can endure, and still hatch when the temperature is again 

 raised, is not known beyond a statement by Nuttall that he delayed hatch- 

 ing to 35 days by low temperatures. Certainly the safest plan would be 

 to allow 30 to 40 days of cool weather or«two weeks of hot weather for 

 all the eggs in discarded clothing to hatch. 



Tliere are three larval stages, or possibly we may call the last the 

 nvniphal stage. The larvae suck blood from their human host. The first 

 molt occurs on the third to eighth day, and the other stages are corre- 

 spondingly long. 



In molting, the skin splits longitudinally from base to apex of 

 thorax and along the base of the head to near the base of the palpi. 



The entire life cycle of corporis on the human body may be as short 

 as sixteen days, eight for the egg, two each for the first and second larval 

 stages, three for the third stage, and one day preovipositional period. 

 The head louse has been carried through its entire life cycle in seventeen 

 days. 



The frequency with which lice feed is dependent upon the rate of diges- 

 tion, which is dependent upon climatic conditions. They feed more fre- 

 quently at body temperatures than when kept cool. They feed at all 

 times of the day. Lice which liavc not had a feed for some time become 

 ravenous and often feed to excess, rupturing tlie intestines and causing 

 death. 



