n] EXTERNAL STRUCTURE OF A FLEA 21 



turn their attention to the fleas. This is likely to 

 prove more directly efficacious than the mediaeval 

 custom of marking the house with a red cross and 

 inscribing the legend, " God have mercy on us." 



CHAPTER II 



THE EXTERNAL STRUCTURE OF A FLEA 



IN comparing the structure of a flea with that 

 of a man, or any other of the higher animals, it is 

 of the utmost importance to understand that the one 

 has an internal and the other an external skeleton. 

 In either case the skeleton serves as an attachment 

 for the muscles by which the animal moves itself. 

 Everyone is familiar with the external skeleton of a 

 lobster and can see for himself how the muscles are 

 attached. The structure of a flea, though so much 

 smaller, is somewhat similar, except that the skeleton 

 is composed of a horny substance known as chitin 

 instead of being calcified. The chitinous cuticle 

 entirely covers the flea, but it varies in hardness 

 and thickness on different parts of the body. The 

 epidermis, or true skin, lies immediately beneath. 

 On those parts of the body which are to the rear 

 of the head the chitin forms a series of plates or 

 shields which overlap one another somewhat like the 



