152 Insects and Disease 



together and form a groove in which the piercing 

 organs lie. When the flea is feeding, the epi- 

 pharynx and mandibles are thrust into the skin 

 of the victim, the labium serving as a guide. As 

 the sharp cutting organs are thrust deeper and 

 deeper the labium doubles back like a bow and 

 does not enter the skin. Saliva is then poured into 

 the wound through minute grooves in the mandi- 

 bles, and the blood is sucked up into the mouth by 

 the sucking organ which lies in the head at the 

 base of the mouth-parts. Just above this piercing 

 proboscis is a pair of flat, obtuse, somewhat tri- 

 angular pieces, the maxillary blades or maxillae. 

 When the proboscis is fully inserted into the skin 

 the tips of these maxillae may also be embedded in 

 the tissue and perhaps help to make the wound 

 larger. Attached to these maxillae is a pair of 

 rather stout, four- jointed appendages, the palpi. 

 They probably act as feelers. 



If the flea chances to be feeding on a plague- 

 infected rat or person many of the plague bacilli will 

 get on the mouth-parts and myriads of them are of 

 course sucked up into the stomach with the blood. 

 Those on the proboscis may be transferred directly 

 to the next victim that it is thrust into, and those in 

 the stomach may be carried for some time and 

 finally liberated when the flea is feeding again or 



