Insects
Introduction
The insects are the largest group of animals. In fact, about 75 percent of all animals
are insects. Insects developed on Earth long before humans did. Today there are
about 1 million known species, or types, of insect. And scientists are constantly
discovering new species. Butterflies, beetles, ants, flies, grasshoppers, silverfish,
and bees are all insects.
Where Insects Live
Insects live all over the world. They can survive in almost any place where food is
available. For instance, insects can be found in cold regions, hot rain forests,
deserts, mountains, caves, and freshwater. A few kinds even live in salt water.
Physical Features
Insects are members of a larger group called the arthropods. This group also
includes spiders, ticks, centipedes, lobsters, and crabs. Like all arthropods, insects
have a body that is divided into segments, or sections. They also lack a skeleton
inside the body. Instead, insects and other arthropods have a covering on the
outside of the body called an exoskeleton. This exoskeleton protects the body.
Unlike other arthropods, insects have three major body segments. Insects also have
six legs. This is one way that insects differ from spiders, which have eight legs.
Insects also have at least one pair of antennas, or feelers.
The three major segments of an insect body are the head, the thorax, and the
abdomen. The head contains the insect’s antennas, mouthparts, and eyes. The
adults of many species have two kinds of eyes, simple and compound. Compound
eyes have many lenslike parts. Each of these receives a separate image. The images
are combined into a single picture in the insect’s brain.
The thorax has three pairs of legs and usually two pairs of wings. But some insects
have only one pair of wings or no wings at all.
The abdomen is made up of as many as 11 segments. It contains organs that digest
food, push out wastes, and help the insect reproduce.
Insects vary greatly in size. Most insects are small, usually less than 0.2 inch (6
millimeters) long. However, some insects called walking sticks can grow to more
than 12 inches (30 centimeters) long.
Behavior
Insects have a variety of ways to protect themselves from their enemies. For
example, some insects hide by blending in to their surroundings. Many moths have
a similar coloring to the bark of the trees on which they rest. Beetles have a very
hard body armor to protect them. Some insects bite or sting their enemies. Some
kinds produce poison.
Some kinds of insects live alone, while others live in groups. Termites, ants, and
some bees and wasps live in very organized groups. Each individual has a
particular job to do for the group. For example, some termites are born to be
soldiers or workers. Other termites will be the king and queen.
Insects eat a great variety of plants, animals, and other living things. Certain
insects, such as fleas and lice, live on the body of another animal. They get all their
food from that animal’s body. Such insects are called parasites.
Life Cycle
Most insects hatch from eggs. A few kinds are born live. An insect’s exoskeleton
cannot grow. Instead, as the insect grows, this covering splits apart and falls off
from time to time. The insect grows a new covering. This process is called molting.
There are two basic life cycles found in insects. Some insects are born in nearly the
same form they will have as an adult. The insect grows and molts, eventually
becoming an adult. Grasshoppers and cockroaches go through this kind of growth
process.
Other types undergo a complete change, or metamorphosis. After they hatch, they
begin life in a form called a larva. It is completely unlike the adult form. For
instance, a larva may look more like a worm than an insect. The larva eats a great
deal and molts several times. It then enters a resting stage in a form called a pupa.
The pupa’s body changes greatly during this stage. It develops all of the features of
the adult, including wings and legs. When this stage is complete, the adult emerges
from the pupa. Bees, butterflies, and moths are some insects that develop this way.
Helpful and Harmful Insects
Many insects are helpful to humans and other animals. Some kinds eat other
insects that are pests. Insects are also food for other animals. Some insects produce
valuable products such as honey, silk, and dyes.
Many flowering plants depend on insects such as bees to help them reproduce. As
insects feed on flowers, they spread a substance called pollen from the male parts
of flowers to the female parts. This allows the female parts to make seeds.
Insects can also be pests. Some insects sting or bite people or other animals. Many
insects can spread diseases to people, other animals, and plants. And some harmful
insects eat crops, wood, clothing, and carpet.
