344 THE MITES 



male and female mites. . (^he duration of the two nymphal periods 

 is from three and one-half to six days, the entire development of 

 the mites therefore requiring from nine to sixteen days. The 

 mites are not necessarily nocturnal as was formerly supposed, 

 but wander about on the surface of the skin when the latter is 

 warm, which is most frequently th case when the host is in bed. 

 Copulation takes place on the surface of the skin, and apparently 

 the males do not burrow or enter the burrows made by the fe- 

 males, but merely hide under superficial dead cells of the 

 epidermis. Since they die very soon after copulation, they are 

 seldom found. The young impregnated females soon begin 

 fresh excavations, and produce more eggs. Fifteen or twenty 

 eggs each generation, of which approximately two-thirds are 

 females, and a new generation about every four weeks, results 

 in an enormous rate of increase. In less than six months the 

 progeny of one pair of itch mites theoretically would number 

 several millions! 



The Disease. The " itch " is a disease which in the past has 

 swept over armies and populations in great epidemics, but it has 

 decreased with civilization and cleanliness, and is fortunately 

 less common now, at least in civilized communities. 



As its name implies, the disease is characterized by itching of 

 the most intense kind where the mites burrow in the skin. The 

 itching is probably due only to a very slight extent to the me- 

 chanical irritation in the skin, but is induced rather v by poisonous 

 substances secreted or excreted by the mites. Injection of fluid 

 containing crushed mites produces an eruption and irritation 

 similar to that caused by the burrowing of the living mites. 



The excretions of the mites as they feed in their burrows form 

 little hard pimples, about the size of a pinhead or a little larger, 

 containing yellow fluid. When these are scratched, as they are 

 almost certain to be on account of the unbearable itching, they 

 frequently become secondarily infected and may give rise to 

 larger sores. Ultimately scabs form over them. 



Since the entire life history of the parasites is passed on a 

 single host, generation after generation may develop from a 

 single infection, and although the infection apparently may dis- 

 appear temporarily, it persists recurrently for many years. Since 

 the mites are sensitive to cold the infected areas of skin not only 

 do not spread but may become restricted during the winter, to 



