MITES, TICKS, AND OTHER ACARI. 



513 



stroyed before this extreme stage is reached. This complaint is chiefly 



seen in great towns. S. mutans (Fig. 13) is parasitic on birds and 



domestic fowls, appearing on the feet, 



on the comb, and about the beak, 



gradually spreading until the whole 



body is involved. Other species have 



been found on other domesticated 



and wild animals, while a large genus 



— the louse-mites, Myobia — infest the 



smaller mammals, as rats, mice, cats, 



etc. 



The Demodex folliculorum is a 



minute animal, -^^ to -^-^ of an incli in 



length, that lives in sebaceous sacs and 



hair follicles of the human skin, par- 

 ticularly about the nose (Fig. 14). Its 



habits are in some respects similar to fig. is.-SAKcopTESMUTANs-Maie, under- 



those of the itch-mites. It is not a " ^' 



normal inhabitant of the follicles or glands, but appears to enter 



them from without. If pressure be made upon any one of the seba- 

 ceous follicles that are enlarged and whitish, with a 

 terminal black spot, matter will be forced out, con- 

 sisting mostly of accumulated sebaceous secretions, 

 in which the parasite, if pi-eseut, will be imbedded to- 

 gether with its eggs and young. The secretion may 

 be softened with oil, so tliat these may be separated 

 and the animal removed with a pointed brush. They 

 do not seem to be present in all persons, occurring in 

 two or three cases out of ten, and seeming to prefer 

 thick, greasy skins. They are entirely harmless, and 

 their presence is no indication of disease. The family 

 roll of the acari closes with the gall-mites, Phytop- 

 tidcB, a curious and little-known group that causes the 

 abnormal growths known as galls on the leaves and 

 other parts of various plants. Any extended refer- 

 ence to them here would be out of place. The whole 

 of the order Acarina afford interesting objects of in- 

 vestigation, being in close relations to man, and yet 

 very imperfectly known. Some of their forms are 

 always attainable, and an examination of tliem under 

 the lower powers of the microscope is one of the most 

 amusing objects of study that can be presented to the 



Fig. 14 -Demodex ^ 



rOLLICULOKUM. yuuiig. 



S.VV? 



'-^ 



