24 THOMPSON YATES AND JOHNSTON LABORATORIES REPORT 
themselves on the skin of the host by means of the mouthparts, it is doubtless a 
great support to these organs that the head is firmly pressed against the skin of the 
host, lessening the strain on the mandibles and labrum when the host is trying to get 
rid of the parasite by scratching. It is easy to conjecture the line of evolution on 
which the head developed from a help to the organs of fixation into an organ of fixa- 
tion itself. This is well illustrated by the species of the genus ~Dermatophilus. In 
these insects the sharp angle of the dorsal frontal edge stands close to the frontal oral 
corner. This portion of the head is, we think, employed either for widening the hole 
made by the piercing organs or for penetrating direct into the skin, the sharp angJe 
acting in either case as a hook. In a similar way the tubercles of Echidnophaga 
(PI. I, Fig. i, 2) may help to fix the parasite more firmly when the skin of the host 
becomes irritated and swollen, and therefore more easily penetrated. The swollen 
skin of the host partly envelops the frons, as we know from E. galtinaceus. 
In the genus Hectopsylla the dorsal horseshoe-shaped depression of the frons is 
absent. The heads of the species of this genus, moreover, have another characteristic 
not represented in the species of the genera Echidnophaga and Dermatophilus. This 
is a dorsal internal incrassation between the frontal oral corner and the antennal groove 
(PI. I, Fig. 4, 5). This thickening is not met with in the two other genera. The 
same kind of incrassation — which serves as a point of insertion for muscles — is also 
found in the family Pulicidae, but here also in a few of the species only, such as Pulex 
cleophontis, australis, bohlsi, and a few others. It is worthy of remark that these species 
are American, like Hectopsylla. We must, however, add that some small American 
species of the genus Pulex (for instance simonsi and cocyti) are devoid of this incrassa- 
tion, as are the members of the genus Pulex from the Old World. Now, does the 
similarity in this character between the genus Hectopsylla and these American species 
of the genus Pulex indicate relationship or convergent development ? It is obvious 
that a curious instance of non-mimetic similarity between species of the same country 
is here represented. The incrassation is not present in the American genus Malaco- 
psylla, and the old-world genera Vermipsylla and Lycopsylla, which are also probably 
derivations from Pulex. 
The eye is present in all the Sarcopsyllidae, though its pigment is much reduced 
(or absent ?) in Dermatophilus caecata. It is placed about midway between the dorsal 
and ventral edges of the head. This seems to be the normal position of the eye in 
Siphonaptera, though it is sometimes placed more dorsal and sometimes more ventral. 
In the Sarcopsyllidae as in the Pulicidae there is a marked difference among the various 
genera and species in the relative distance of the eye from the antennal groove. When 
viewed in optical section the eye is always placed immediately at the groove, but 
externally the eye is often separated from the groove by a strip of chitin. In the 
genus Hectopsylla this is well demonstrated (PI. I, Fig. 3, 4, and 5), and among the 
Pulicidae in Pulex irritaus, Ctenocephalus canis and felts, and some other species. In 
