REVISION OF THE SARCOPSYLLIDAE 27 
backwards in Hectopsylla pulex (PL I, Fig. 4), while they are much more obtuse and 
curved forward in H.psittaci (PI. I, Fig. 3). The maxillae are not piercing organs. They 
merely protect the mandibles and upperlip, and push aside the hairs of the host. 
The maxillary palpus consists of four segments, as in other Sipbonaptera. The first 
segment is at least as long as the second, being often considerably longer. It is also 
usually distinctly curved near the base. The palpus is much more hairy in Derma- 
tophilus penetrans than in the other species of Sarcopsyllidae. 
The rostrum consists in Sipbonaptera of an unpaired underlip and the labial palpi. 
It is again strangely modified in the Sarcopsyllidae. The rostrum is very feebly 
chitinized, being of a more or less white colour, and has almost lost the segmentation 
(PI. 1, Fig. 1 and 4). There is always one joint in the labial palpus, not more, and the 
joint between the palpus and the underlip has either disappeared (as in Ecbidnopbaga 
and Derma/opbilus), or may be present (as in Hectopsylla). We are not sure if the 
latter joint is present in all specimens of Hectopsylla ; there may be some variability 
in this respect. The rostrum has not become reduced in length. In the other Siplion- 
aptera the labial palpus consist at least of four segments. The labial palpi form to- 
gether a kind of sheath enveloping the mandibles and the upperlip. This sheath 
is not a piercing organ, but only guides the mandibles and upperlip. This can 
be readily seen if a flea is placed on the hand and observed under a lens. 
A Pulex or Ceratophyllus if hungry will generally take readily to man, though a bird 
or rat flea appears to be rather disgusted with a human host, retracting the piercing 
organs hastily as a rule when the blood is first tasted, and not sucking for so long a 
time as do Pulex irritans and Ctenocepbalus cunisznd felis under the same circumstances. 
When the insect has selected a point where to pierce the skin, the rostrum, with the 
mandibles and upperlip inside it, is moved a little forward. The specimen then lifts 
the abdomen upwards and presses the piercing organs down into the skin, using its 
own weight and the strength of the fore and mid legs, the hind legs being lifted up. 
The head can soon be seen to come nearer the skin, and the rostrum then divides in 
the middle, the two labial palpi remaining together only at the tip. The nearer the 
head comes to the skin of the host, i.e., the deeper the upperlip and mandibles pene- 
trate into the skin, the further are the labial palpi separated from one another, till 
they are finally quite apart, lying right and left on the skin of the host, but assuming 
again their normal position close to one another as soon as the insect retracts the 
mandibles and upperlip from the host. This retraction is done forcibly, as the action 
of the legs indicates during the process ; the organs come out with a jerk. Now, 
from this short description of the method of sucking on the part of a Pulex, it is clear 
that the specimen, in the act of piercing the skin, has to overcome the rigidity of the 
rostrum as well as the resistance of the skin of the host, and further that, when sucking, 
the specimen has to use a certain amount of force to counteract the spring-like action 
of the labial palpi. Therefore it is conceivable that the rigidity of the labial palpi has 
