Animals P by: Geoffrey LaPage Published by ... - PSSurvival.com

Animals Parasitic in Man :
of parasitic animal, such as a mosquito, horse fly, sand fly, or
tick, which visits the host to suck its blood, injects the internal
parsite through the skin of the host.
The distinction often drawn between the external and internal
parasites is, however, mainly a topographical one, It
has its uses, but it does not help us much when we are considering
the effects of parasites on their hosts; for then we
have to consider, among other things, the reactions to the
parasite of the whole host’s body when it acts as a single, integrated
organism. When we consider these reactions of the
host, we learn that attacks made by the parasite on the surface
of the host often cause reactions in its internal organs as well,
so that the distinction between external and internal parasites
loses the value that it at first sight seemed to have.
There are, in addition to this distinction between internal
and external parasites, other distinctions which do not affect
any definition of the way of life called parasitism. There is,
for instance, the distinction sometimes drawn between temporary
and permanent parasites. Temporary parasites are such
species as the mosquito and the horse fly, which visit their
hosts only at times, only, in fact, when they need the blood of
the hosr which is their food. These species are non-parasitic in
every other respect, but, because they feed on their hosts, they
are undoubtedly parasites. They may be contrasted with
species that are sometimes called permanent parasites, which
are parasitic throughout the whole, or the greater part, of
their lives. Many temporary parasites are, as we shall see,
important because, in the act of sucking blood, they convey to
their hosts the causes of serious disease of the host. Such
causes may be either other parasitic animals, such as the trypanosomes
or the malarial parasites, or bacteria or viruses.
Some parasites, on the other hand, are parasitic only when
the opportunity for parasitic life becomes available. Normally
they lead non-parasitic lives. The maggots, for example, of
the blowflies and the bluebottle and copperbottle flies normally
develop in dead gesh or in decaying vegetable material;
but, if their parents. d&acted to sheep, or even to man, by the
smell of pus in septic wounds or sores, or by the odours of
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