78 THE FLEA [CH. 



the muscles in the thorax are consequently reduced. 

 In the second place, the value to the insect of a 

 contracted thorax is obvious ; for the abdomen of a 

 chigoe fixed on or in the skin of an animal does not 

 project so much as it would were the thorax of normal 

 length. The troublesome parasite is, therefore, less 

 likely to be rubbed off by the host. 



In most fleas the piercing organs of the mouth 

 (upper lip and mandibles) are directed obliquely 

 downwards. In the chigoes they are directed 

 obliquely forwards. It has been suggested that 

 this forward movement of the mouth is connected 

 with the stationary life which the females assume. 

 Fleas which fasten themselves permanently to the 

 skin of their host, do so in a manner similar to ticks. 

 The mouth-parts are in a line with the longitudinal 

 axis of the body. This attitude, so far as we know, 

 is assumed by the females of all the family. The 

 females of one genus, Dermatophilm, actually go head 

 foremost right into the skin of their host. The shape 

 of the head is also beautifully adapted to enable the 

 insect to fix itself firmly in a tick-like posture. The 

 fore-part is remarkably obtuse, and almost has the 

 appearance of being truncate and abruptly cut off. 

 When the piercing organs have been thrown forward 

 horizontally it must be a great support to the insect, 

 which is fixed by them, if it can press its head down 

 firmly against the skin of the host. The Avider the 



