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. ( T ^ of an inch) in length, while the female is 

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

 S. scabiei crustosce, 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 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 daily excavations 

 of a mite amount to two or three millimeters. 



The eggs (Fig. 143) vary in number from 15 to 50. They are 

 laid in groups of from two to four, the mite resting after each 

 oviposit ion. After they are all laid the female dies, usually at 

 the end of a single tortuous burrow. The eggs hatch in from three 

 to six days into six-legged larvae. The latter transform in two or 

 three days into nymphs. The nymphs commonly build burrows 

 for themselves and moult twice, the second time becoming adult 



