484 THE INVERTEBRATA 



the wound is poured the salivary fluid, and the mouth funnel is 

 thrust in to enable the blood to be sucked up by the pharyngeal pump. 

 Embryological evidence tells us that the ist maxillae unite to form the 

 dorsal stylet, the ventral being formed by the labium. A pair of 

 mandibles also develops but these remain in a rudimentary and un- 

 chitinized condition. 



Pediculus humanus, the body louse (Fig. 335), is associated with the 

 spread of many diseases, such as typhus and relapsing fever. The 

 disease known as trench fever, prevalent in all war areas during the 

 Great War, has also been shown to be transmitted by this insect. 



Eggs are laid attached to hairs of the body or clothing, and the three 

 instars passed through before attainment of the mature state closely 

 resemble the adult. 



msth 

 mtfli. 



Fig. 334- 



Fig. 335. 



Fig. 334. Hen louse, Menopon pallidum. Dorsal view, showing biting 

 mandibles by transparency, an. antenna; md. mandible; mxp. maxillary 

 pd\ip',pth. prothorax; msth. mesothorax; mtth. metathorax. 



Fig. 335. Body louse, Pediculus humanus. After Imms. 



The louse has been found to lay about ten eggs daily, depositing 

 in all about three hundred. Temperature plays a big part in con- 

 trolling the development of these animals. Under average conditions, 

 the life cycle is completed in about three or four weeks. 



