436 MEDICAL ENTOMOLOGY 



EXTERNAL ANATOMY 



The classification of the Siphonaptera is based upon such minute 

 anatomical points, and the differentiation of species' has been carried 

 to such a pitch within recent years, that a detailed knowledge of 

 the external characters of the group has become essential. The tedium 

 of mere anatomical nomenclature is enlivened, thanks to the labours 

 of Jordan and Rothschild, by many interesting points bearing on the 

 relations between the structure of the flea and its environment, and 

 on the phylogeny of the group. 



The body in the Siphonaptera is compressed strongly in the lateral 

 direction, with the evident advantage to the insect of enabling it to 

 work its way the more easily among the hair or feathers of its host. 

 Bugs and lice are compressed dorso-ventrally with a similar result. 

 Compression is limited mainly to the thorax and abdomen, and is retained 

 in the latter even when the ova are ripe, except in some Sarcopsyllidae. 

 The segments are distinct from one another except in the head, and 

 are arranged in a simple manner, one tergite and one sternite to each 

 segment as a rule. There are ten segments in the abdomen, of which 

 the terminal three are modified for sexual purposes. The wings, if ever 

 they existed in ancestral forms, have left no trace. 



The cat flea, Ctenocephalus felis, or the dog flea, C. canis, will serve 

 as an example for descriptive purposes (Plate LI 1 1). The former can 

 be obtained from almost any kitten. It belongs to the Pulicidae, 

 the central family of the Siphonaptera. The differences between the two 

 species are very slight ; they will be referred to later. 



The head is only slightly flattened laterally, its outline on section 

 being that of an arch, high in the posterior portion. When seen from 

 the side its outline is roughly triangular ; the dorsal 

 border is gently rounded from the dorsal and posterior 

 angle to the distal ventral one. The posterior border, which is mainly 

 vertical, though with a small tubercle projecting backwards in its upper 

 part, articulates with the prothorax without the intervention of a neck. 

 The ventral border is straight, and is armed with a row of stout and 

 heavily pigmented spines, projecting well beyond the margin, and direct- 

 ed downwards and backwards. These spines (g. c., fig. 1.) constitute 

 the genal comb ; they are often referred to as ctenidia. The lateral 

 area of the head on each side is divided into two portions, known as 

 the frons and the occiput respectively, by the antennal groove (a. g.), 

 which runs from the middle of the ventral border upwards and forwards. 



