74 



Parasitic Arthropods 



there develop after about seven days, the hexapod larvas. These 

 molt on the sixteenth day to form an octopod nymph, which molts 

 again the twenty-first day. At the end of the fourth week the 

 nymphs molt to form the sexually mature males and the so-called 

 pubescent females. These pair,, the males die, and the females again 

 cast their skin, and become the oviparous females. Thus the life 

 cycle is completed in about twenty-eight days. 



The external temperature exercises a great influence on the develop- 

 ment of the mites and thus, during the winter, the areas of infesta- 

 tion not only do not spread, but they become restricted. As soon as 

 the temperature rises, the mites increase and the infestation becomes- 

 much more extensive. 



57. Sarcoptes scabiei. Diagrammatic representation of the course in 

 the skin of man. 



In considering the possible sources of infestation, and the chances 

 of reinfestation after treatment, the question of the ability of the mite 

 to live apart from its host is a very important one. Unfortunately,, 

 there are few reliable data on this subject. Gerlach found that, 

 exposed in the dry, warm air of a room they became very inactive 

 within twenty-four hours, that after two days they showed only 

 slight movement, and that after three or four days they could not 

 be revived by moisture and warming. The important fact was 

 brought out that in moist air, in folded soiled underwear, they sur- 

 vived as long as ten days. Bourguignon found that under the most 

 favorable conditions the mites of Sarcoptes scabiei equi would live for 

 sixteen days. 



The disease designated the "itch" or "scabies," in man has been 

 known from time immemorial, but until within less than a hundred 

 years it was almost universally attributed to malnutrition, errors of 



