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 
