Introduction
Insects, with overa milliondescribed species, arethe mostdiversegroupof animalson Earth.
They may be found in nearly all environments on the planet. That would explain that no
matter where you travel, you are bound to see representatives from this group and probably
lots of different kinds as well. Even if you were not partial to bees, wasps, and ants perhaps,
it would be difficult to not admire the beauty of a butterfly, moth, or even a dragonfly!
As you learn about the amazing diversity within this group and some of the fascinating
behaviors, you may begin to look upon some of the insects you come upon with a bit more
interest! Perhaps you will even learn to appreciate some of the species you may dislike now,
such as bees and wasps, when you realize how beneficial they are to humans and especially
necessary for the continued presence of some of the beautiful flowers or delicious fruits that
may grace your yard or nearby park.
What Are Insects?
Insects are a major group of arthropods and the most diverse group of animals on the planet,
with over a million described species and more than half of all known living organisms. They
are found in nearly all environments on Earth, although only a few species occur in the
oceans. Adults range in size from a minuscule fairy fly to a 21.9 in (55.5 cm) long stick
insect (Figure12.28).
Figure 12.28: A stick insect, showing how well it blends in to its environment. ( 29 )Insects have segmented bodies with an exoskeleton. The outer layer of the exoskeleton,
the cuticle,is made up of two layers, a thin and waxy water resistant outer layer (the
exocuticle), and an inner, much thicker layer. The exocuticle is greatly reduced in many
soft-bodied insects and especially in larval stages, such as caterpillars (Figure12.29).
The segments of the body are organized into three distinctive but joined units: a head, a
thorax, and an abdomen (Figure12.30).