Insects are invertebrate animals that belong in the Phylum
Arthropoda and Class Insecta. The class Insecta is divided
into 2 subclasses: Apterygota, or wingless insects, and
Pterygota, or winged insects. Subclass Pterygota is further
divided on basis of metamorphosis. Insects that have
undergone incomplete metamorphosis are the Exopterygota.
Insects that undergo complete metamorphosis are the

Scientists have described and named about 800,000 insects
and believe there may be from 1 million to 10 million kinds
still undiscovered. Insects live almost everywhere on earth
- from steamy tropical jungles to cold polar regions. They
live high on snow-capped mountains, and in deserts below
sea level. They can be found in caves deep in the earth, or
flying many thousands of feet high in the sky. 

Most insects are small - less than a fourth of an inch
long. They have an amazing variety of shapes and special
structures. Insects have an outer bilateral exoskeleton to
which the muscles are attached and which provides
protection for internal organs. The body is divided into 3
main parts: the head, which include mouth parts, eyes, and
antennae; thorax, which operates the jointed legs and/or
wings; and abdomen, which has organs for digested food,
reproducing, and getting rid of waste products. 

The major systems in insects are the circulatory,
respiratory, nervous, muscular, digestive, and reproductive
systems. In the circulatory system, blood is pumped by the
heart in a tube to the aorta, the head, and to other organs
then enters the ostia openings along the sides of the tube
back to the heart. An insect's blood, like ours, carries
food and waste products to and from the cells of the body.
But unlike our blood, it has little to do with bringing
oxygen to the cells. Insect blood is greenish, yellowish,
or colorless. Few insects have red blood. 

The respiratory systems carries oxygen to cells and takes
away carbon dioxide by means of a system of tubes. An
insect breathes by means of tiny holes, called spiracles,
along the sides of its body. 

The nervous system consists of a brain receiving
information from the eyes and antennae. It controls the
insect's body activities as a whole. Another nerve center
in the head is connected to the brain and controls the
insect's mouth parts. Each of the two nerve cords contains
a cluster of nerve cells, called a ganglion, in each
segment of the thorax and abdomen. The two ganglia in each
segment are fused and form a sort of little brain that
controls the activities of that segment. The ganglia often
can work without the brain.
The muscular system is made up of a few thousand small but
strong muscles allowing the insect to carry objects heavier
than it. Many insects can lift or pull an object 20 or more
times heavier than the weight of their bodies. 

The digestive system is basically a long tube that extends
from the mouth to the anus. The food enters the mouth and
enters the forgut. The food moves along the tube until it
reaches an enlarged area called the crop, where it is
temporarily stored and partly digested. The the food passes
into the gizzard, which has thick muscular walls that
contract and grind the food into small bits. The food then
passes into the midgut, where most digestion takes place.
Nourishing parts of the food are absorbed into the blood,
and wastes and undigested parts move into the hindgut. All
wastes and extra water that enter the hindgut leave the
body through the anus. 

Most insects reproduce sexually. A new individual is
produced sexually when the female eggs produced in the
ovaries has united with male sperm produced in the testes.
During mating, the female receives the sperm and stores
them in her abdomen. Later, when she lays her eggs, the
sperm enter the eggs as they leave her body. Insect eggs
have a variety of shapes and color patterns, but most are
oval or round and are white or cream colored. The number of
eggs laid varies greatly from one species to another. A
female insect probably lays an average of 100 to 200 eggs
during her lifetime. 

Both man and insect live almost everywhere, eat all kinds
of food, and use all kinds of materials to build homes so
they constantly live in conflict. Some insects seriously
affect man's health and are parasitic on man and other
animals. Insects that feed on human or animal blood can
carry disease in their salivary juices and spread the
disease to other animals. Many insects irritate us without
disturbing our health. Some bite and sting, and some people
are allergic to them. Other insects are injurious to our
agricultural crops, food products, clothing, and wooden
buildings. So far human beings have had only partial
success in defending themselves against insects. 

Beneficial insects include bees, wasps, flies, butterflies,
moths, and others that pollinate plants. Many fruits,
including oranges, apples, strawberries and grapes depend
on insect pollinators for the production of seeds. Some
insects provide humans with products such as honey, bees
wax, shellac, and silk. Some help keep the landscape clean
by feeding on animal wastes and dead animals, or the
remains of dead plants. Insects that live in the ground
enrich the soil with their waste products and dead bodies.
Many insects are helpful because they are predators and
feed on harmful insects that destroy crops. . 

All insects form part of the great web of life that
includes human beings and all other living things. They
feed on plants and animals, but they also are food for
plants and animals.


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