ITCH MITES — LIFE HISTORY 



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mite, but many of them can be transferred readily from one 

 host to another. In the common human species the male is 

 only about 0.25 mm. (^ J^ of an inch) in length, while the female is 

 about 0.4 mm. {-^ of an inch) in length. A variety of this mite, 

 S. scabiei crustosos, causing the so-called " Norwegian itch," is 

 found in northern Europe and occasionally in the United States, 

 but is always rare. The- disease caused by it differs in some re- 

 spects from ordinary itch. Still another species, Notoedres cati, 

 which causes a very persistent and often fatal disease in cats, 

 temporarily infests man, but is apparently unable to breed in 

 human skin, since the infection dies out in the course of a week 

 or two. 



The impregnated females of itch mites excavate tortuous tun- 

 nels in the epidermis (Fig. 143) especially on such portions as 



Fig. 143. Diagrammatic tunnel of itch mite in human skin, showing female 

 depositing eggs. X about 30. (Adapted from Riley and Johannsen.) 



between the fingers and toes, on the groins and external genitals, 

 and in the armpits, where the skin is delicate and thin. The 

 tunnels are anywhere from a few millimeters to over an inch in 

 length, and are usually gray in color from the eggs and excrement 

 deposited by the female as she burrows. 



The eggs (Fig. 143) vary in number from 15 to 50. After they 

 are all laid the female dies, having performed her duty in life. 

 But there is no respite on account of her death, for in less than a 

 week the eggs hatch into six-legged larvae. These live for about 

 two weeks in the old burrow built for them by their mother, and 

 during this time they indulge in three moults and undergo a 

 metamorphosis which transforms them into nymphs similar in 



