大多数昆虫常被人们称为“虫子”，但是许多被人们称为“虫子”的动物却并不都是昆虫。例如在石块下、潮湿阴暗的角落里爬行的蜈蚣、马陆，拖着长刺蛰人的蝎子，在屋角拉丝结网的蜘蛛，家中凉席、地毯或花盆上出现的肉眼几乎难以看见的微小的螨虫，藏在土壤中的蚯蚓，寄生在人体中的蛔虫、蛲虫，以及在清清溪水中游动的小虾，还有海边沙滩上的寄居蟹……，它们的体形都不大，常常也被归为“虫子”一类，但它们都不是昆虫！
所谓昆虫，应该具备以下几个特征：（一）身体由若干环节组成，这些环节集合成头、胸、腹三个部分；（二）头部不分节，是感觉与取食的中心，具有口器和1对触角，通常还有复眼和单眼；（三）胸部分为3节，可能某些种类其中某一节特别发达而其他两节退化得较小。胸部是运动的中心，具有3对足，一般成虫还有2对翅，也有一些种类完全退化；（四）腹部应该分为11节，但也常常演化为8节、7节或4节。分节数目虽不相等，但都没有足或翅等附属器官着生。腹部是生殖与营养代谢的中心，其中包含着生殖器官及大部分内脏；（五）昆虫在生长发育过程中，通常要经过一系列内部及外部形态上的变化，即变态过程。
We live in an age of insects.About half of all known animals are insects.Scientists have found about 1million species (kinds)of insects so far.There are more species of insects than of any other animal in the world.Bees，flies，ants，grasshoppers，beetles，and butterflies are some of the insects that might live near you.
Insects buzz in the air.They crawl over stones.They hop through grass.They dig tunnels underground.
Some insects are helpful.They do important jobs to help plants grow.They help get rid of wastes and dead plants and animals.Some insects are harmful.They bite or sting.They carry diseases or destroy crops.
Morphology of a flying insect：vertebrate animal with segmented bodies，usually winged，that undergo metamorphosis.
Compound eye：one of two complex organs of sight of an insect.
Antenna：one of two organs of touch of an insect.
Head：foremost part of an insect.
Fore leg：one of two front limbs of an insect.
Thorax：central part of an insect.
Middle leg：one of two middle limbs of an insect.
Hind leg：one of two rear limbs of an insect.
Abdomen：rear part of an insect.
Hind wing：one of two rear appendages used for flying.
Fore wing：one of two front appendages used for flying.
The praying mantis is an insect that helps farmers and gardeners by eating other insects.The praying mantis got its name because when it puts its legs together，it looks like it’s praying.But it’s actually waiting to catch its dinner！
Water striders can do a neat trick they can walk on water！Waterproof hairs at the tips of their legs help these insects stay afloat.
As its name suggests，the caterpillar hunter beetle likes to eat caterpillars.This beetle is about to have a meal.
Most insects undergo a change of shape as they grow.This damselfly sheds its old body for a new one，a process called incomplete metamorphosis.
What Makes an Insect ？