6 MISC. PUBLICATION 5 0, U. S. DEPT. OF AGRICULTURE 



gressed so far in the family Stephanocircidae that the two parts are 

 completely separated. At the same time the anterodorsal parts of the 

 genae still retain their fusion with the front, and these three fused 

 sclerites form a conspicuous crownlike structure known as the helmet. 

 Such a profound change in the basic structure of the head sets apart 

 the family Stephanocircidae very distinctly from all the other families 

 of fleas. 



In the figure of the head and thorax of Gtenocephalides cants given 

 by Ewing (16, p. 15 If) the unpaired piercing element of the mouth 

 parts was labeled, following the practice of certain former workers, 

 the hypopharynx. This structure has been called the labrum-epiphar- 

 ynx by some workers, but it should be called the labvum (fig. 1) , since 

 there is no evidence that the epipharynx enters into its composition. 



The postantennal portion of the head of a flea (fig. 1) has long been 

 known as the occiput, or occipital region. These terms, however, should 

 be restricted in their application to the posterior part of this post- 

 antennal region. The part of the head immediately posterior to the 

 front must be the epicranium. Since the epicranium and the occiput 

 are not separated by an occipital suture, the extent of each is not 

 definitely known. The setae situated in oblique rows on the postan- 

 tennal portion of the head in some fleas ma} 7 be called the postantennal 

 setae. Those of the posterior vertical row (fig. 1) may be called the 

 occipital setae, or setae of the posterior, marginal row. 



THE THORAX 



Jordan and Rothschild (lf.7, p. 23) have given an exposition of the 

 external structure of the thorax of a flea. The description here given 

 differs from that of those authors largely in the interpretation of the 

 lateral sclerites of the metathorax. 



The body of a flea, including the thorax, has become greatly com- 

 pressed or laterally flattened. As a result the ventral sclerites have 

 assumed largely or completely a lateral position. Although much 

 modified in shape by compression, the generalized nature of the thorax 

 is easily discerned. It is divided into three distinct segments (fig. 2), 

 the prothorax, mesothorax, and metathorax. all of which are movably 

 articulated into the body "chain" of segments. The upper part of each 

 thoracic segment is covered with a single, undivided sclerite. These 

 are the pronotum. mesonotum, and metanotum. The posterior margin 

 of each of these dorsal thoracic sclerites overlaps the anterior margin 

 of the following dorsal sclerite in the same manner as do the dorsal 

 abdominal sclerites. 



The sternal and pleural sclerites of each thoracic segment have be- 

 come either fused or united (fig. 2) so as to make a single unit. Such 

 a unit or section of a thoracic segment is typically made up of a ventral 

 sternum, and on each side an anterodorsal episternum and a poster o- 

 dorsal epimeron. 



The sternal and pleural sclerites of the prothorax typically are fused 

 into a movable saddlelike mass (fig. 2) , the sternopleura. This sterno- 

 pieura extends forward and has its longest dimension in an almost 

 horizontal direction. 



The sternal and pleural sclerites of the mesothorax (fig. 2) have 

 a position and relationship more commonly found in other insects. 



