PARASITISM 321 



sleep upon straw beds by being shaken through 

 the ticking, penetrating the clothing, and work- 

 ing their way into the skin and causing severe 

 dermatitis. The female mite introduces its 

 proboscis into the skin, and distends its body 

 to an enormous extent while the surrounding 

 tissues swell until an umbilicated vesicle or 

 pustule, not unlike that of small-pox, is formed. 

 A more familiar mite is the Acarus scabei, that 

 which causes the " itch " or scabies. The female 

 bores tunnels in the epidermis, causing hyper- 

 emia, vesication, and pustulation associated 



b ' e 



Pia. 115. Pediculoides ventricosus (enlarged). (After Laboulbtne and Megrim.) 

 a, Male; b, young; c, mature female. 



with great itching. Scabies is by no means 

 confined to man, but appears in sheep as "sheep- 

 scab," and in dogs, cats, horses, and cattle as the 

 "mange." 



A familiar but no less disagreeable mite is the 

 "blackberry tick" or "harvest bug," the Lep- 

 tus autumnalis, which, frequenting long grass 

 and blackberry bushes, not infrequently finds its 

 way to the human skin into which it thrusts an 

 enormously long proboscis. Its irritating saliva 

 causes considerable irritation, weal formation, 

 and intense itching. When these mites are 



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