REVISION OF THE SARCOPSYLLIDAE 
29 
be called normal for Siphonaptera, though many species bear more than two. Those 
species, however, whose tergites have but one row of bristles only, are doubtless 
derived from species whose tergites had originally two rows, the anterior row having 
disappeared. In the Sarcopsyllidae the loss of bristles may be due to the reduction 
of the segments. In other Siphonaptera, however, this is not necessarily the case, the 
the anterior row being often lost, though the tergite remains normal, as for instance 
in Pulex riggenbachi. The uniseriate bristles are the rule in the genus Pulex, a second 
row being developed only in forms which, though classified with Pulex irritans, are 
not true Pulex ; for, instance the American species cleophoutis, bohlsi, etc. The 
thoracic bristles of the Sarcopsyllidae are nearly all short, those of the metanotum 
being usually minute. The most ventral bristles of the pro- and mesothorax are the 
longest. There is no indication of a comb, nor are there any subapical hairlike 
spines on the meso- and metanotum, such as are present in most Pu/icidae. 
The prosternite is less reduced than the mesosternite. Its pleural plate is oblique 
in position, owing to the slanting genal edge of the head. This plate always ends in 
an obtuse lobe which projects backwards and a little sideways. The mesosternite, 
which is short, has a more or less vertical position, ami its ventral portion (the 
sternum) projects but slightly forward. There is always near the upper edge of the 
mesosternite one fairly long bristle, rarely accompanied by a minute hair. The 
smallest number of bristles on the mesosternite of Pu/icidae is two, as is the case in 
Pulex irritans, all the other Pulicidae having at least three long bristles in this 
position. The mesosternite of Pulex irritans has another character in which it differs 
from the other Pulicidae and in which it agrees with the Sarscopsyllidae. The 
mesosternite of Siphonaptera is divided by a suture extending from the insertion of 
Fio. A Fig. B Fin. C 
the coxa to the dorsal edge of the mesosternite, the suture ending usually at the upper 
frontal corner of the mesosternite. This suture separates the lateral portion of the 
mesosternite into an anterior plate, the sternum, and a posterior plate, the epimerum 
(Fig. A). The sternum is completely fused with the episternum, the upper portion 
