INSECTS AND RELATED ORGANISMS 
Insects belong to the phylum Arthropoda, one of the major 
groups of the animal kingdom. In the hierarchy of animal phyla 
it stands near the top, far removed from the simplest, one-celled 
organisms in the phylum Protozoa. The phylum also contains 
many other well-known forms such as crawfish, shrimp, milli- 
pedes, centipedes, spiders, and mites. Members of the phylum are 
distinguished by having the body composed of a series of more or 
less similar rings or segments joined together and in having some 
of these segments bear jointed legs. In certain forms, the segmen- 
tations of the body may be obscure and not evident from cursory 
examination. In most instances this is due to a secondary modifi- 
cation of form, a result of adaptation to special modes of life 
(152). 
Insects comprise the most abundant and important group of 
arthropods, but many other members of the phylum are also com- 
mon and often important. Many of the latter are harmful, either 
to trees and other vegetation or to wildlife and man. Many are so 
small they are seldom seen, although enormously abundant; 
others, larger ones, are easily seen. Some of the latter are also 
many-legged and wormlike and bear little or no resemblance to 
insects; others are often mistakenly identified as insects. The 
arthropods discussed here belong to the Classes Hexapoda (In- 
sects) ; Crustacea (crawfish, shrimp, crabs) ; Diplopoda (milli- 
pedes) ; Chilopoda (centipedes) ; and Arachnida (spiders, mites, 
and scorpions). Mention is also made of shipworms which belong 
to the phylum Mollusca. 
Class HEXAPODA 
Insects 
An insect is an air-breathing arthropod with a distinct head, 
thorax, and abdomen. It has one pair of antennae, three pairs of 
legs, and usually one or two pairs of wings in the adult state. The 
majority of species are terrestrial and are found in an almost 
endless variety of microhabitats on the land. Many other species 
spend parts of their lives in water. 
The majority of insects hatch into wormlike larvae which grow 
by periodically shedding the outer skin, finally transforming into 
an inactive pupal stage from which the adult emerges. This sort 
of development is known as complete metamorphosis. The imma- 
ture stages of others are very similar in appearance to the adults 
and are known as nymphs. Members of the latter group are said 
to have simple or incomplete metamorphosis. 
Approximately 900,000 species of insects have been described. 
This represents 80 to 90 percent of all the known kinds of animals. 
The actual number of insect species, however, is believed to be 
much larger and to run into the millions (628). Countless numbers 
feed on plants of all kinds, attacking all parts from the roots in 
the ground to the flowers and seeds in the tops. Thousands of 
others feed on other insects or other animals, including man. 
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