REVISION OF THE SARCOPSYLLIDAE 23 
This forward movement of the mouth in the Sarcopsyllidae may, we think, be 
connected with the assumption of a stationary life on the part of the It is 
probable that the Siphonaptera which fasten themselves permanently to the skin of their 
host do it in a manner similar to that employed by ticks and mites, with the mouth- 
parts more or less in a line with the longitudinal axis of the body. This attitude 
appears to be assumed more or less by the of all the Sarcopsyllids, the of Derma- 
tophilus going right into the skin of their host. Another morphological peculiarity is 
connected with this tick-like position of the parasite. The maxillary palpi are inserted 
close to the corner of the oral edge of the frons in the Pulicidae, the piercing organs 
projecting downward behind or in between the palpi. In the Sarcopsyllids, however, 
the oral slit of the caputal capsule is more or less extended upwards beyond the in- 
sertion of the maxillary palpi, the piercing organs (mandibles and upperlip) protruding, 
therefore, in this family in front of the palpi in the more specialized species, Echidno- 
phaga bradyta being the most generalized species, in which this development has 
hardly begun. The frontal corner of the mouth does not project downwards as in 
the Pulicidae, except in the most generalised Echidnophaga, this corner being more 
obtuse than in the Pulicidae, whereby greater freedom is given to the mandibles 
and labrum. 
There is another modification which has been evolved, we think, in connexion 
with the acquirement of a tick-like terminal fixation to the host ; that is the angulated 
dorsal edge of the frons prevalent among the Sarcopsyllids. The dorsal edge of the 
frons of the Siphonaptera is normally rounded to a more or less high degree, the 
curvature being sometimes so strong that the mouth is thrown backwards (as in the 
genus Ctenopsyllus). The stronger the convexity of the dorsal edge of the frons is, 
the less easy would be a fixation to the host in the manner of mites, the projecting 
supra-oral portion of the frons being in the way. Where the dorsal edge of the frons 
is arched in side-view among the Sarcopsyllids the curvature is slight {Hectopsylla pules, 
PI. I, Fig. 4). This simple slight curvature, which we meet also among the Pulicidae, 
appears to us to be original, the angulated dorsal edge of the frons representing a 
higher specialization. The angulation is accompanied by a semicircular depression 
posterior of the angle, and becomes the more marked the deeper the impression is, 
Such a horseshoe-shaped depression occurs apparently nowhere else among Siphonaptera. 
The edge of the depression is sometimes divided anteriorly in the middle 
appearing in a side-view as two tubercles, for instance in Echidnophaga liopus (PI. I, 
Fig. 2). These tubercles are not homologous to the frontal tubercles so commonly 
found in Pulicidae. They are doubtless acquired subsequent to the acquirement of 
the frontal depression. 
The obtusity of the supra-oral portion of the frons, which gives the head the 
appearance of being truncate, has doubtless yet another meaning besides allowing the 
piercing organs to be thrown out almost horizontally. As the Sarcopsyllid fix 
