r animal that may be convenient.

COMMON SPECIES OF FLEAS

Throughout India and in all the warm climates where plague frequently
occurs the most common flea found on rats has come to be known as the
plague flea (_Laemopsylla cheopus_) (Figs. 105, 106), and is doubtless
the principal species that is concerned in carrying the disease in those
climates. It now occurs quite commonly on the rats in the San Francisco
Bay region and is occasionally found there on man also. In the United
States, Great Britain and other temperate regions another larger
species, _Ceratophyllus fasciatus_ is by far the most common flea found
on rats, and is commonly known as the rat flea. It occurs on both the
brown and the black rats _Mus norvegicus_ and _M. rattus_, on the house
mouse and frequently on man. It has also been taken in California on
pocket gophers and on a skunk.

The common human flea (_Pulex irritans_) (Figs. 108, 109), is found in
all parts of the inhabited world. Although we regard it primarily as a
pest of human beings it often occurs very abundantly on cats, dogs, mice
and rats as well as on some wild mammals such as badgers, foxes and
others and has occasionally been found on birds.

Most entomologists regard the fleas commonly found on cats and dogs as
belonging to one species _Ctenocephalus canis_. Others believe them to
be distinct species and call the cat flea _Ctenocephalus felis_. So far
as our personal comfort and safety is concerned it makes but little
difference to us whether the flea that bites us is called _canis_ or
_felis_ for they both look very much alike, and act alike and the bite
of one hurts just as much as the bite of the other. Although cats and
dogs are their normal hosts they are very often troublesome household
pests, sometimes making a house almost uninhabitable. They are
frequently found on rats, and therefore may carry the plague bacillus
from rat to rat or from rat to man.

GROUND-SQUIRRELS AND PLAGUE

As early as 1903 Dr. Blue, in charge of the plague suppressive measures
in San Francisco, became impressed with the possibility of the common
California ground-squirrels (_Otospermophilus beecheyi_), acting as an
agent in the transmission of plague. It was rumored at that time that
some epidemic disease was killing the squirrels in some of the counties
surrounding San Francisco Bay, notably in Contra Costa County. None of
the squirrels were examined at that time, but since then many thousa