CHAPTER V 



THE ORDER SIPHONAPTERA : FLEAS 



THE Siphonaptera or fleas form a well defined group of insects, sharply 

 differentiated from the other orders of insects by their structure and 

 habits. Their origin is very obscure, but it is certain, from whatever 

 form they may have originated, that they have no close connection with 

 any other order of the present day fauna. In the past they have been 

 classified as a sub-order of the Diptera, but there is little to support 

 such an -arrangement. They bear no recognizable traces of wings, and 

 the structure of the thorax makes it very doubtful if they are descended 

 from winged forms at all. From the simple type of segmentation, 

 and the condition of the nervous system, they would appear to be com- 

 paratively primitive insects. 



Fleas are laterally compressed in a characteristic manner, the only 

 exception to the rule being found in the mature females of some of 

 the Sarcopsyllidae. The mouth parts are formed for sucking and 

 piercing, the order being exclusively blood-sucking and parasitic. They 

 undergo a complete metamorphosis, the immature stages resembling 

 those of Diptera. Each species is for the most part confined to one 

 species of host, though to the general rule there are many exceptions, and 

 it is found that in the absence of the elective host most species will 

 feed on a variety of animals. 



The chief interest of the order centres round their connection with 



plague, in the transmission of which from rat to rat and from rat to man 



fleas are the active agents. It is believed by some 



Relations to Disease workers that the dog fl and poss ibl v the human flea, 



Natural Parasites . 



may play the part of invertebrate host in the trans- 

 mission of the parasite of Kala Azar on the Mediterranean littoral. 



Trypanosoma lewisi has been transmitted experimentally from rat 

 to rat by two species of flea, Ceratophylhts fasciatus and Cteuocephalus 

 canis. The early stages of tape worms have frequently been found 

 in the intestines of fleas from the cat and the dog. A considerable 

 number of protozoa have been recorded from the alimentary tract of 

 adult and larval fleas, including at least four species of the genus 

 Herpetomonas and two of Crithidia. Noller found as many as twelve 



