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9.18: Insects

Difficulty Level: At Grade Created by: CK-12
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What animals were the first to evolve wings?

When someone says the word "wing," you probably think of soaring birds. Or maybe chicken wings smothered in hot sauce. But insects were actually the first animals to evolve wings.

What are Insects

Insects, with over a million described species, are the most diverse group of animals on Earth. They may be found in nearly all environments on the planet. No matter where you travel, you will see organisms from this group. Adult insects range in size from a minuscule fairy fly to a 21.9-inch-long stick insect (Figure below).

A stick insect camouflaged against a branch

A stick insect, showing how well it blends into its environment.

Characteristics of Insects

Characteristics of Insects include:

  • Segmented bodies with an exoskeleton. The outer layer of the exoskeleton is called the cuticle. It is made up of two layers. The outer layer, or exocuticle, is thin, waxy, and water-resistant. The inner layer is much thicker. The exocuticle is extremely thin in many soft-bodied insects, such as caterpillars.
  • The segments of the body are organized into three distinct but joined units: a head, a thorax, and an abdomen (Figure below and Table below).

A diagram of a human and an insect, comparing the three main body parts: head, thorax, and abdomen

A diagram of a human and an insect, comparing the three main body parts: head, thorax, and abdomen.

Structure Description
Head A pair of antennae, a pair of compound eyes, and three sets of appendages that form the mouthparts.
Thorax Six segmented legs and two or four wings.
Abdomen Contains most of the digestive, respiratory, excretory, and reproductive structures.
  • A nervous system that is divided into a brain and a ventral nerve cord.
  • Respiration that occurs without lungs. Insects have a system of internal tubes and sacs that oxygen travels through to reach body tissues. Air is taken in through the spiracles, openings on the sides of the abdomen.
  • A closed digestive system, with one long enclosed coiled tube which runs lengthwise through the body, from the mouth to the anus.
  • A circulatory system that is simple and consists of only a single tube with openings. The tube pulses and circulates blood-like fluids inside the body cavity.
  • Various types of movement. Insect movement can include flight, walking, and swimming. Insects were the only invertebrates to develop the ability to fly, and this has played an important role in their success. Many adult insects use six legs for walking, and they walk in alternate triangles touching the ground. This allows the insect to walk quickly while staying stable. A few insects have evolved to walk on the surface of the water, like water striders (Figure below). A large number of other insects also live parts of their lives underwater. Water beetles and water bugs have legs adapted to paddle in the water. Young dragonflies use jet propulsion, sending water out of their back end to move.

A water strider utilizing water surface tension to stand on the water

A water strider utilizing water surface tension to stand on the water.

Communication in Insects

Insects use many different senses for both communicating and receiving information. The types of communication that are used by various insects are summarized below (Table below).

Types of Communication Representative Organisms Description
Bioluminescence Fireflies Reproduction and predation: Some species produce flashes to attract mates; other species to attract prey.
Sound Production
By moving appendages Cicadas Loudest sounds among insects; have special muscles to produce sounds.
Ultrasound clicks Moths Predation: Produced mostly by moths to warn bats.
Wide range of insects have evolved chemical communication; chemicals are used to attract, repel, or provide other kinds of information; use of scents is especially well developed in social insects. Moths Antennae of males (Figure below) can detect pheromones (chemicals released by animals that influence the behavior of others within the same species) of female moths over distances of many miles.
Dance Language Honey bees Honey bees are the only invertebrates to have evolved this type of communication; length of dance represents distance to be flown.

yellow-collared scape moth, showing its feathery antennae

A yellow-collared scape moth, showing its feathery antennae.

Insects are Social

Social insects, such as termites, ants, and many bees and wasps (Figure below), are the most familiar social species. They live together in large, well-organized colonies. Only those insects which live in nests or colonies can home. Homing means that an insect can return to a single hole among many other apparently identical holes, even after a long trip or after a long time.

A few insects migrate in groups. For example, the monarch butterfly flies between Mexico and North America each spring and fall (Figure below).

Termites, wasps, and monarch butterflies in social situations

(left) Damage to this nest brings the workers and soldiers of this social insect, the termite, to repair it. (center) A wasp building its nest. (right) Monarch butterflies in an overwintering cluster.

Two Major Groups of Insects

Insects are divided into two major groups:

  1. Wingless: Consists of two orders, the bristle tails and the silverfish.
  2. Winged insects: All other orders of insects. They are named below.

Mayflies; dragonflies and damselflies; stoneflies; webspinners; angel insects; earwigs; grasshoppers, crickets, and katydids; stick insects; ice-crawlers and gladiators; cockroaches and termites; mantids; lice; thrips; true bugs, aphids, and cicadas; wasps, bees, and ants; beetles; twisted-winged parasites; snakeflies; alderflies and dobsonflies; lacewings and antlions; hangingflies (including fleas); true flies; caddisflies; and butterflies, moths, and skippers.


  • Characteristics of insects include segmented bodies, a system of internal tubes and sacs through which oxygen travels, and a simple circulatory system.
  • Insects use many types of communication, including bioluminescence, sounds, and phermones.
  • Some insects are social and live in groups.

Explore More

Use the resource below to answer the questions that follow.

  1. How many times did arthropods invade the land?
  2. Describe the rapid underwater movement of the dragonfly larva.
  3. What arthropod adaptations led to the class Insecta? Which adaptation do you think was key?
  4. Where are insect wings the thickest?
  5. How have flowers taken advantage of the complex eyes of insects?
  6. How may the type of food available to arthropods when they first left the sea be connected to present day insects' role as decomposers?


  1. What are three key characteristics of insects?
  2. What are two ways in which insects communicate?
  3. What are the five components of the insect head?
  4. What are the features of the insect thorax?
  5. Name five types of winged insects.

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cuticle Protective, waxy layer covering an insect.
exocuticle Thin, waxy, and water-resistant outer layer of an insect; part of cuticle.
exoskeleton External protective covering of an animal.
homing Ability to return to a home base after traveling a distance away from it.
phermones Chemicals produced by animals that influence the behavior of others within the same species.
spiracles Pores on the body of an insect that allow oxygen to enter the body.

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Difficulty Level:
At Grade

Concept Nodes:

7 , 8
Date Created:
Nov 29, 2012
Last Modified:
Aug 30, 2016
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