18 
The Eon-Combed Eyed Siphonaptera 
in Ctenocepkalus erinacei. Now, the species with the broadest and most 
heavily serrate mandibles are stationary parasites, fixing themselves 
firmly in the skin of the host by means of the piercing organs. As the 
stationary mode of living is without doubt a secondary development, we 
may accept the broad mandibles to he the result of this change in habits, 
the mandibles (at least in these closely allied Pulicidae) having developed 
a new function, to assist the insect in retaining its hold as well as to 
pierce the skin. Pidex irritans, however, is a very active insect and by 
no means stationary. 
The width of its mandibles and their strong serration therefore 
cannot be explained in the way indicated, unless we assume that the 
ancestral P. irritans was a comparatively stationary insect when it 
adopted man as host. This assumption, however, does not appear to be 
satisfactory, since it is hardly likely that a stationary parasite should 
become active again, there being no sufficient reason for this regression 
to a former state of habits, stationary ectoparasites thriving very well on 
man. We think, on the contrary, that the strong piercing organs of 
P. irritans were acquired after man had become the host of the insect, 
the naked skin and the garment covering it rendering the claws of the 
legs insufficient for keeping the insect steady when sucking, the 
strengthening of the mandibles, moreover, preventing them from being 
easily injured. The upper lip also is strong in P. irritans and the 
stationary fleas mentioned, hearing along the anterior edge a number of 
obtuse teeth. These teeth are present in all fleas, but are often very 
few in number, being restricted to the apex of the organ. The species 
of Loemopsylla agree with Pulex irritans in the armature of the upper 
lip, except that the teeth are less prominent and fewer in number, while 
the American non-combed eyed fleas have only two or three such teeth, 
placed near the apex of the upper lip. 
The piercing organs, when at rest, are retracted and the external 
portion is encased in a tube (= rostrum) formed by the two labial palpi, 
which are situated at the apex of the short non-divided labium. The 
number of segments composing the labial palpus as far as we know varies 
in the Siphonaptera from 2 to 17. In most fleas, however, the labial 
palpus consists of five segments. This appears to be the original state 
of development, the palpus with more and the one with less segments 
being a derivation from the normal five-jointed type. The rostrum is 
not a piercing organ like that of bugs and flies. The two palpi separate, 
when the mandibles and upper lip, which are interlocked to form a tube, 
penetrate the skin of the host, lying fiat right and left on the skin, or 
