111

FIG. 102. SALT-MARSH MOSQUITO (_O. lativittatus_) STANDING ON
A TABLE 118

FIG. 103. ANOPHELES HANGING FROM THE CEILING 118

FIG. 104. YELLOW FEVER MOSQUITO (_Stegomyia calopus_) 122

FIG. 105. RAT-FLEA (_Laemopsylla cheopis_); MALE 152

FIG. 106. RAT-FLEA (_L. cheopis_); FEMALE 152

FIG. 107. HEAD OF RAT-FLEA SHOWING MOUTH-PARTS 153

FIG. 108. HUMAN-FLEA (_Pulex irritans_); MALE 153

FIG. 109. HUMAN-FLEA (_P. irritans_); FEMALE 156

FIG. 110. MOUSE-FLEA (_Ctenopsyllus musculi_); FEMALE 156

FIG. 111. TRYPANOSOMA GAMBIENSE 164

FIG. 112. TSETSE-FLY 164

INSECTS AND DISEASE

CHAPTER I

PARASITISM AND DISEASE

PARASITES

The dictionary says that a parasite is a living organism, either animal
or plant, that lives in or on some other organism from which it derives
its nourishment for a whole or part of its existence. This definition
will serve as well as any, as it seems to include all the forms that
might be classed as parasites. As a general thing, however, we are
accustomed to think of a parasite as working more or less injury to its
host, or perhaps we had better say that if it does not cause any
irritation or ill effects its presence is not noted and we do not think
of it at all.

As a matter of fact the number of parasitic organisms that are actually
detrimental to the welfare of their hosts is comparatively small while
the number of forms both large and small that lead parasitic lives in
or on various hosts, usually doing no appreciable harm, often perhaps
without the host being aware of their presence, is very great indeed.

Few of the higher animals live parasitic lives. The nearest approach to
a true parasite among the vertebrates is the lamprey-eel (Fig. 1) which
attaches itself to the body of a fish and sucks the blood or eats the
flesh. Among the Crustaceans, the group that includes the lobsters and
crabs, we find many examples of parasites, the most extraordinary of
which is the curious crab known as _Sacculina_ (Fig. 2). In its early
stages this creature is free-swimming and looks not unlike other young
crabs. But it soon attaches itsel