i] INTRODUCTORY 5 



can be no doubt that many undiscovered species still 

 remain, and will now, in due course, be collected, 

 described and named. 



The position which should be assigned to the 

 order Siphonaptera in the general scheme of insect 

 classification is a question on which the most learned 

 modern entomologists have disputed with considerable 

 vigour. Some see the nearest relatives among the 

 beetles, others among the flies. The majority, as we 

 shall see later on, would place them near the Diptera : 

 but since no convincing arguments have been pro- 

 duced on either side it may be wisest to regard the 

 question as still at present unsolved. 



Fleas belong to one of the groups of insects which 

 go through a complete metamorphosis. Their life- 

 history consequently falls into four divisions: egg, 

 larva, pupa and imago. If the climate permits, the 

 female flea lays her eggs all the year round, and from 

 one to five are dropped at a time. Unlike those of 

 many other parasites they are never attached to the 

 hairs of the hosts, but appear to be deposited indis- 

 criminately on the floors of houses or in the nests 

 and sleeping places of their hosts. The eggs generally 

 hatch in a few days, and a minute, white, wormlike 

 larva emerges (Fig. 1 ). The larvae of some, and possibly 

 of all, fleas are provided with a wonderful adaptation 

 in the shape of an egg-breaker or hatching-spine. 

 This is a thin plate, like the edge of a knife, where 



