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PHYLUM ARTHROPODA
All forms of Iiving matter belong to either the plant or animal kingdom. The
kingdoms are divided into phyla. The phylum Arthropoda is part of the animal kingdom
and is characterized by those animals, which have an external skeleton (exoskeleton)
and jointed appendages. This group includes the following classes:
;
a. Class Insecta.
(1) This class contains the true insects and is the largest and most
medically important class of arthropods. Some medically important insects include
mosquitoes, fleas, flies, and Iice. An adult insect can be distinguished from other
arthropods since it has three distinct body regions--the head, the thorax, and the
abdomen (see figure 1-1). The head has one pair of antennae and usually one pair of
compound eyes; the thorax has three pairs of legs and up to two pairs of wings. Many
insects have only one pair of wings; others have none. The abdomen contains most of
the internal organs of the insect.
Figure 1-1. Three distinct body regions of an insect.
(2) Growth of arthropods involves shedding the cuticle, a process called
molting. In insects, the number of molts varies usually from 4 to 8. Most insects stop
molting once they become adults. Usually insects change at least somewhat in form
during development, a process called metamorphosis. There are two basic types of
metamorphosis. Gradual metamorphosis (see figure 1-11) involves some change in
form and the young, called nymphs, which are similar in habits and form to the adults
except for size and lack of sexual maturity. Insect examples include grasshoppers and
cockroaches. Complete metamorphosis (see figure 1-7) is characterized by a more-or-
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