n] EXTERNAL STRUCTURE OF A FLEA 23 



It is plentifully sprinkled over with spines, bristles, 

 or hairs, directed backwards so as not to impede 

 progress. The presence or absence, the arrange- 

 ment on the body, and the size of these serve, 

 along with other features, to distinguish different 

 species. They seem materially to help a flea in 

 those wriggles to escape with which we are all 

 familiar. The bristles are not always the same in 

 size and arrangement in the two sexes of the same 

 species of flea. As a rule the males are more bristly 

 than the females. 



These appendages of the flea's integument are 

 called by various writers either spines, bristles, or 

 hairs. There is, however, no real distinction in the 

 structure or nature of the appendages, and it is a 

 question of degree which name is most appropriate. 



In distinguishing species, very little reliance can 

 be placed on the colour of the flea. An insect newly 

 emerged from the pupa is always lighter in colour ; 

 and the difference between the appearance of an 

 empty stomach and a stomach gorged with red blood 

 is surprising. 



The general external appearance of a typical flea 

 belonging to the main family Pulicidce is fairly 

 familiar to most persons. This is well seen in the 

 figure of the oriental rat-flea. The body is compressed 

 or flattened from side to side, and this is a feature 

 which is extremely rare among insects. It doubtless 



