3 o THOMPSON YATES AND JOHNSTON LABORATORIES REPORT 
of the ' sternum ' being, in fact, the episternum. In Pulex irritans, however, the 
epimerum is not separated from the sternum, the suture referred to being absent 
(Fig. B). There is also no internal thickening of the skeleton corresponding to the 
suture. A similar fusion of the plates is found in all Sarcopsyllidae (Fig- C). All 
the other Syphonaptera have the ' meral ' suture separating the mesosternum of the 
mesosternite from the mesomerum. 
The hinder edge of the mesosternite is produced into a rounded lobe similar to 
that of the prosternite, but usually less heavily chitinized. This lobe is present in 
all species of the genus Hectopsylla, but is not so well developed in Derma tophilus and 
Echidnophaga. A similar lobe is found on the metasternite of Hectopsylla, here the 
episternum being produced. In the $ of H. psittaci, however, this is not the case ; 
while in % $ of Hectopsylla coniger and broscus also the epimerum bears a lobe (PI. 
I> Fig. 5)- 
From the great similarity in the development of the mesosternites of Pulex 
irritans and the Sarcopsyllids, an equally great similarity might be expected to be 
present in the structure of the metasternites of these insects. This is not the case. 
In the true Pulex, namely irritans and lepotis as well as in Pulex pallidas, cleopatrae, 
etc., there is at least one very striking peculiarity which is not found in the Sarco- 
psyllidae. In these members of the genus Pulex the episternum and the sternum of 
the metathorax are not completely separated from one another by a suture as is the 
case in nearly all other Siphonaptera, but are fused together anteriorly, the suture not 
extending to the anterior edge of the plate. The only other Siphonapteron in which 
a similar fusion has taken place is, as far as we know, the American Ceratophyllus 
charlottensis. In the American species of the genus Pulex, as well as in Malacopsylla, 
Lycopsy/la, and V ermipsylla, the episterum of the metasternite if separate. There is, 
however, a point of resemblance between the genus Pulex and the Sarcopsyllids in the 
size of the episterum and sternum of the metathorax ; the former being comparatively 
large and the latter small. The metathoracic epimerum of those Sarcopsyllids which 
are the least specialized in respect to this plate has essentially the same shape as that 
of the genus Pulex. It is long in a dorso-ventral direction, being hardly broader in 
Hectopsylla than in Pulex. It has become much enlarged in Dermatophilus. This 
enlargement has probably taken place in connexion with the acquirement of the 
distended abdomen of the pregnant female. The metathoracic epimerum serves as a 
basal side-plate to the abdomen covering the proximal tergites and the proximal 
sternites laterally where they become separated when the abdomen of the individual 
gets distended. 
The bristles on the metasternite do not offer a character distinctive of the whole 
family of Sat copsyllidae, but they are nevertheless of interest, inasmuch as they 
represent various degrees of reduction. The metathoracic sternum bears in the 
Pulicidae one bristle, seldom more, standing near the upper edge rather close to the 
