vi] CHIGOES AND THEIR ALLIES 77 



a student of fleas cannot possibly have any doubt 

 whether a flea belongs to this family or not. Yet 

 there is great diversity in general appearance, as 

 well as in details of structure. One very peculiar 

 character, namely the enormous swelling of the 

 abdomen in pregnant females is, moreover, shared 

 with certain other fleas. The most distinguishing 

 character of the family, however, is the rostrum. 

 This organ, which it may be well to remind the 

 reader, consists of the under-lip and the labial palpi, 

 sheaths the piercing and sucking mouth-parts. In 

 the chigoes and their allies the rostrum is reduced, 

 not in length, but in stiffness and in number of 

 segments. In this group there are never more than 

 three segments to the rostrum, whilst in the main 

 group of fleas, with one or two exceptions such as 

 the rabbit-flea (Spilopsyllus cnniculi), there are 

 never less than five. There is no indication of a 

 comb on the head, but all the family, without ex- 

 ception, have a large triangular projection, which is 

 more or less curved backwards, at each side of the 

 head. These two organs doubtless discharge the 

 same functions as the combs of other fleas, and 

 prevent the insect from slipping back as it works its 

 way through the fur or feathers of the host. 



The thorax of a chigoe is exceedingly short. Two 

 reasons for this may be suggested. In the first place, 

 the jumping power of these fleas is very small and 



