bles, and a lower lip or labium. The
mandibles have the sides serrate like a two-edged saw. The labium is
divided close to its base so that it really consists of two slender
four-segmented organs which lie close together and form a groove in
which the piercing organs lie. When the flea is feeding, the epipharynx
and mandibles are thrust into the skin of the victim, the labium serving
as a guide. As the sharp cutting organs are thrust deeper and deeper the
labium doubles back like a bow and does not enter the skin. Saliva is
then poured into the wound through minute grooves in the mandibles, and
the blood is sucked up into the mouth by the sucking organ which lies in
the head at the base of the mouth-parts. Just above this piercing
proboscis is a pair of flat, obtuse, somewhat triangular pieces, the
maxillary blades or maxillae. When the proboscis is fully inserted into
the skin the tips of these maxillae may also be embedded in the tissue
and perhaps help to make the wound larger. Attached to these maxillae is
a pair of rather stout, four-jointed appendages, the palpi. They
probably act as feelers.

If the flea chances to be feeding on a plague-infected rat or person
many of the plague bacilli will get on the mouth-parts and myriads of
them are of course sucked up into the stomach with the blood. Those on
the proboscis may be transferred directly to the next victim that it is
thrust into, and those in the stomach may be carried for some time and
finally liberated when the flea is feeding again or when it is crushed
by the annoyed host. The latter is probably the most common method of
infection, for the bacilli that are liberated when the flea is crushed
may readily be rubbed into the wound made by the flea bite or into
abrasions of the skin due to the scratching. Kill the flea, but don't
"rub it in."

[Illustration: FIG. 105--Rat-flea (_Laemopsylla cheopis_); male.]

[Illustration: FIG. 106--Rat-flea (_L. cheopis_); female.]

[Illustration: FIG. 107--Head of rat-flea showing mouth-parts.]

[Illustration: FIG. 108--Human-flea (_Pulex irritans_); male.]

During the recent outbreak in San Francisco many thousand fleas that
were infesting man, rats, mice, cats, and dogs, squirrels and other
animals have been studied and it has been found that while each flea
species has its particular host upon which it is principally found, few
if any of them will hesitate to leave this host when it is dead and
attack man or any othe