FLEAS 107 



each represent more and more restricted categories, are distinguished 

 by such characters as the form of the sclerites and their internal 

 supporting rods, the presence or absence of certain combs or spines, the 

 shape and structure of the genitaHa and so on. 



As we move down the scale and reach closely related species, 

 minute differences have to be taken into consideration. These 

 differences may seem trivial and even unimportant to the unpractised 

 eye, but they have been selected after a comparison of all known fleas. 

 They are the characters which specialists have found from experience 

 can be rehed upon, and which reflect the natural relationship of the 

 order as a whole. 



In written scientific descriptions no detailed account of the animal 

 in question, or group of animals, is given — only the essential points are 

 described. For practical purposes such descriptions must be as brief as 

 possible. 



The two superfamiHes into which the fleas are divided can be 

 characterised as follows : — 



L Superfamily Ceratophylloidea. (Plate XVII). This is a large 

 superfamily with the following morphological characters : A sword- 

 like ridge running down the inside of the outer wall of the mid-coxa. 

 A pointed tooth present at the apex of the outside of the hind tibia. 

 Abdominal terga II — VII with more than one row of bristles. Pygidium 

 with more than sixteen pits on each side. 



This superfamily includes the Ceratophyllidae which contain 

 thirteen out of our fourteen bird fleas. 



11. Superfamily PuHcoidea. (Plate XI). No swordlike ridge 

 running down the inside of the outer wall of the mid-coxa. No 

 pointed apical tooth present at the apex of the outside of the hind 

 tibia. Abdominal terga II — VII with at most only onerow of bristles. 

 Pygidium with fourteen or fewer pits each side (Plate XIX). 



This superfamily includes the family Pulicidae which contains the 

 most notorious of all fleas, the tropical rat flea {Xenopsylla cheopis), the 

 vector, par excellence, of bubonic plague, and the human flea {Pulex 

 irritans). The only representatives among the British bird fleas are the 

 shearwater flea {Ornithopsylla laetitae) and Hen Sticktight flea (see p. xiii). 



Although the characters separating these two families appear rather 

 obscure, they present in practice quite a striking contrast. The Puhcidae 



