THE COMMON FLEA 249 



which attack these different hosts seem to be distinct, 

 each animal, as a rule, supporting its own special para- 

 site, but they resemble one another so closely that they 

 form a perfectly natural family, which is called Pulicidce, 

 from the principal genus Pulex. On the other hand, in 

 their most characteristic peculiarities, they are utterly 

 unlike any other insects, and hence have been a great 

 puzzle to systematists. It is not easy to find a suitable 

 corner for them in our schemes of classification, and 

 many have got over the difficulty by placing them in an 

 order by themselves, which, from the apparent absence 

 of wings, has been called Aphaniptera (without distinct 

 wings). Others have, however, seen in them some 

 affinities to the two-winged flies, or Diptera, and have 

 located them somewhere in that order; but here again 

 there has not been unanimity, and some have placed 

 them at the end, while others have inserted them in the 

 body of the order, following the Mycetophilidce, a family 

 of small flies which possess considerable jumping power, 

 and live gregariously amongst decaying vegetable matter, 

 or in fungi, dung, &c. That their affinities are strongest 

 with the Diptera is now generally recognised, and they 

 are therefore regarded as a sort of apterous flies, which 

 have addicted themselves to parasitic habits. The reasons 

 for this opinion will become evident as we proceed. 



Having premised thus much as to the zoological 

 position of the group, we may now endeavour to get 

 a clear notion of the structure of Pulex irritans (Fig. 

 77), the common human flea, so called, and afterwards 

 deal with its life history. In the shape of their body, 

 fleas are quite exceptional ; it is flattened from side 

 to side, so that when the insect is standing upright 

 its greatest diameter is the vertical one. This form 

 of body is called " compressed " ; it is exactly the 



