28 THE FLEA [CH. 



The other consists merely of a number of highly 

 chitinised bristles arranged in a row. They probably 

 both serve the same function. Apart from their use 

 as organs to assist movement onwards, they may also 

 serve as hair-tight joints and protect the flea from 

 the inconvenience of getting the tips of the host's fur 

 into the joints of its horny armour. 



The size of the head compared to the thorax and 

 abdomen varies considerably in different species. 

 Some fleas have what may be called by comparison 

 large heads and others very small ones. 



A small head is never found in a flea with powerful 

 mouth-parts. The head being the bearer of piercing 

 and sucking organs, which require strong muscles, 

 there must be room not only for the organs but for 

 their extensors and retractors. 



There are normally three rows of bristles on a 

 flea's head which divide the head into four sections. 

 It is possible that these correspond to the four 

 segments of the ancestral insect which are now fused 

 together. 



The head of a flea is closely applied by the whole 

 of its back surface to the body and that slender 

 and conspicuous neck which is characteristic of the 

 Diptera, or flies, is not to be found in any fleas. 

 For this reason a flea cannot turn its head in any 

 direction without at the same time following it round 

 with its body. 



