8 



fleas such as Ctenocephalides, Spilopsyllus, and Chimaeropsylla; or 

 the two sclerites may be completely fused, without demarcation, as 

 in Moeopsylla, Pulex, and others. This character has been used in 

 separating the subfamily Pulicinae from the other subfamilies of 

 Pulicidae. 



The sternopleural sclerites of the metathorax (fig. 2) show the 

 greatest diversification of those of any thoracic segment. Though 

 typically of a similar arrangement to those of the mesothorax, in 

 many genera the following important diversifications may be noted: 



(a) The metepisternum may be divided into an upper and a lower 

 part by an internal chitinous ridge as in the genus Rectofrontia. 

 This division may be continued to such a degree that the upper part 

 of the metepisternum becomes detached from the lower part and 

 transferred to the metanotum, being fused with the latter as in the 

 subgenus Opisocrostis. 



(b) The metepimeron may be so expanded (fig. 2) as to be the 

 largest of all the laterally placed sclerites of the body. 



(c) The metepimeron usually is entirely free from the metanotum 

 and is clearly separated from the metepisternum by an internal 

 chitinous ridge, but in the genus Rectofrontia the metepimeron be- 

 comes completely fused, without demarcation, with the metanotum, 

 while in the genus Opisocrostis the metepimeron is movably articu- 

 lated with the metanotum by means of a condyle. Although usually 

 separated from the metepisternum by an inner vertical ridge, in the 

 genus Trichopsylloides the metepimeron is completely fused, without 

 demarcation, with the former sclerite along its upper anterior end. 



THE MODIFIED TERMINAL ABDOMINAL 

 SEGMENTS 



The abdomen of a flea usually has been regarded as being com- 

 posed of 10 segments, but according to Wagner (81) there are in 

 reality 12. Wagner's contention rests to a large extent upon whether 

 the "cerci" of a flea (present in females) represent the true cerci. 

 If they do, they indicate the presence of the eleventh segment. This 

 follows since embryological studies have shown that in general the 

 cerci of insects represent the appendages of the eleventh segment. 

 Behind the segment in a flea that bears the stylets, or "cerci," there 

 are always present sclerites which Wagner believes represent those 

 of the twelfth abdominal segment. There are reasons, however, for 

 not regarding the stylets as true cerci. In the first place they not 

 only are restricted to the female sex, but are wanting in this sex in 

 certain genera. Secondly, it has been shown by the recent study of 

 the embryology of the fleas by Kessel (S3) that there are 10 posterior 

 pairs of ganglia belonging to the abdominal region ; while the caudal 

 segment has no ganglion. Thus it would appear that there are 11 

 abdominal segments in the flea embryo and probably the same num- 

 ber in the adult. If this reasoning is correct, then the anal segment 

 of Wagner is the eleventh abdominal and the stylets are on the tenth, 

 while true cerci are wanting. 



The first seven segments of the abdomen usually are unmodified in 

 both sexes. 



