PARASITISM 



351' 



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 scabiei, that 

 which causes the " itch " or scabies. The female 

 bores tunnels in the epidermis, causing hyper- 

 emia, vesication, and pustulation associated 



6 ' 



FIG. 119. Pediculoides ventricosus (enlarged). (After Laboulbbne and Mtgrim.) 

 a, Male; 6, 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, an early stage of a mite of the 

 genus Trombidiurn, 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, 



