28 LICE AND THEIR MENACE TO MAN 



distinctly advantageous to the louse, since man 

 scratches at his skin and not at his garments, 

 and if he feels the movement of the louse he 

 instinctively feels on the skin for it and not on 

 the clothing over the spot where he felt it move.. 

 The only purchase which a louse needs for feeding 

 is the grip given by the teeth which are on the 

 haustellum, and this purchase is the fulcrum 

 against which the stabber works. The haus- 

 tellum around the mouth, being closely pressed 

 against the skin, forms a circular air-tight cushion, 

 and into the area thus enclosed the stabber is 

 forced down into the tissues. Salivary juice 

 now flows down the hollow stabber into the 

 wound, and as this acts a slight pink flush may 

 be seen around the position of the bite owing to 

 the increase in the blood at the spot caused by 

 the action of the fluid which is being injected 

 and which also retards the clotting of the blood. 



The salivary juice is thus mixed with the blood, 

 not in the mouth but in the tissues of the person 

 on whom the louse is feeding. This is a vastly 

 important fact in connection with the trans- 

 mission of disease by many blood-sucking insects, 

 since the fluid has become the conveyer of the 

 infection. The virulent organisms of malaria 

 and of sleeping sickness are thus injected into 

 man by the insects which carry them by means 

 of the salivary juice. It is possible that the 

 typhus infection is conveyed by the louse through 

 the same medium. 



