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Lesson Objectives

  • Describe the characteristics of insects.
  • Explain how insects obtain food.
  • Describe reproduction and the life cycle of insects.
  • Explain how insects are important.
  • Describe how insect pests are controlled.

Check Your Understanding

  • What is an arthropod?
  • Is a spider an insect? Why or why not?

Vocabulary

  • exocuticle
  • homing
  • larvae
  • metamorphosis
  • nymphs
  • pheromones
  • pupa
  • spiracles
  • sponging

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, showing how well it blends in to 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, a thin and waxy water-resistant outer layer (the exocuticle), and an inner, much thicker layer. The exocuticle is thinner in many soft-bodied insects and especially in caterpillars (Figure below).

Caterpillar feeding on a host plant

Caterpillar feeding on a host plant.

  • The segments of the body are organized into three distinct but joined units: a head, a thorax, and an abdomen (see Figure below and Table below).

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

Head Thorax Abdomen
A pair antennae, a pair of compound eyes (one to three simple eyes) and three sets of appendages that form the mouthparts Six segmented legs and two or four wings Has most of the digestive, respiratory, excretory, and reproductive structures
  • The nervous system is divided into a brain and a ventral nerve cord.
  • Respiration occurs without lungs. Insects do 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.
  • The circulatory system is simple and consists of only a single tube with openings. The tube pulses and circulates blood-like fluids inside the body cavity.
  • Insect movement includes flight, walking, and swimming. Insects are the only invertebrates to develop the ability to fly and this has played an important role in their success. Insect flight is not very well understood. Primitive insect groups use muscles that act directly to move the wings. More advanced groups have foldable wings and their muscles act on the wall of the thorax and give power to the wings indirectly.
  • Many adult insects use six legs for walking and 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, especially water striders (Figure below). A large number of insects 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 pair of water striders mating, showing how water surface tension allows for them to stand on the water.

Insects use many different senses for both communicating and receiving information. Many insects have special sensory organs. Table below summarizes five types of communication that are used by various insects.

Types of Communication Representative Organisms Description
Visual
Ultraviolet wavelengths (light most animals cannot see) Bees Perceive ultraviolet wavelengths of light (outside of what humans can see)
Polarized light (light most animals cannot see) Bees Detect polarized light
Bioluminescence Fireflies Reproduction and Predation: Some species produce flashes to attract mates; other species to attract prey (food).
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.
Hearing Some predatory and parasitic insects Predation: Some nocturnal species can hear the ultrasonic sounds of bats, which help them avoid predators. Can detect sounds made by prey or hosts.
Chemical
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 can detect pheromones (chemicals released by animals, especially insects, that influence the behavior of others within the same species) of female moths over distances of many miles (Figure below).
Infrared Blood-sucking insects Have special sensory structures that can detect infrared light in order to find their hosts.
“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.

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

Insects are Social

Social insects, such as termites (Figure below), 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, after a long trip or after a long time.

A few insects migrate, such as the monarch butterfly, which flies from Mexico to North America every winter (Figure below).

Damage to this nest brings the workers and soldiers of this social insect, the termite, to repair it.

A wasp building its nest

A wasp building its nest.

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. How many winged orders are there?

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; Scorpios and hangingflies (including fleas); true flies; caddisflies; and butterflies, moths, and skippers.

How Insects Obtain Food

Insects have different types of appendages (arms and legs) adapted for capturing and feeding on prey. They also have special senses that help them detect prey. Insects have a wide range of mouthparts used for feeding. Examples include:

  • Insects like mosquitoes and aphids have special mouthparts that help them pierce and suck. Some are herbivorous, like aphids and leafhoppers, while others eat other insects, like assassin bugs and female mosquitoes.
  • Examples of chewing insects include dragonflies, grasshoppers, and beetles. Some larvae have chewing mouthparts, as in moths and butterflies (Figure below).
  • Some insects use siphoning, as if sucking through a straw, like moths and butterflies. You may have seen a butterfly or moth putting a long mouth-tube into at a flower while it siphons the nectar of the flower.
  • Some moths, however, have no mouthparts at all.
  • Some insects obtain food by sponging, like the housefly. Sponging means that the mouthpart can absorb liquid food and send it to the esophagus. The housefly is able to eat solid food by releasing saliva and dabbing it over the food. As the saliva dissolves the food, the sponging mouthpart absorbs the liquid food.

The mouth of a butterfly up close.

Reproduction and Life Cycle of Insects

Most insects can reproduce very quickly within a short period of time. With a short generation time, they evolve faster and can adjust to environmental changes faster. Most insects reproduce by sexual reproduction. The female produces eggs, which are fertilized by the male, and then the eggs are usually placed in a precise microhabitat at or near the required food. Most insects are oviviparous, where the young hatch after the eggs have been laid. In some insects, there is asexual reproduction. In the most common type, the offspring are almost identical to the mother. This is most often seen in aphids and scale insects.

Three Types of Metamorphosis

An insect can have one of three types of metamorphosis and life cycle (Table below). Metamorphosis describes how insects transform from an immature or young insect into an adult insect (in at least two stages).

Type of Metamorphosis None Incomplete Complete
Characteristics Only difference between adult and larvae (young or non-adult insects) is size
  • Young, called nymphs (Figure below), usually similar to adult
  • Wings then appear as buds on nymphs or early forms
  • When last molt is completed, wings expand to full adult size
  • Insects have different forms in immature and adult stages, have different behaviors, and live in different habitats
  • Immature form is called larvae and remains similar in form but increases in size
  • Larvae usually have chewing mouthparts even if adult mouthparts are sucking ones
  • At last larval stage of development, insect forms into pupa (Figure below), and does not eat or move
  • During pupa stage, wing development begins, after which the adult emerges
Example Silverfish Dragonflies Butterflies and Moths

Nymphs and egg cases.

The chrysalis (pupal stage) of a monarch butterfly.

Importance of Insects

Many insects are considered to be pests by humans. At the same time, insects are very important.

Ecological Importance

In the environment, some insects pollinate flowering plants, as in wasps, bees, butterflies, and ants. Many insects, especially beetles, are scavengers, feeding on dead animals and fallen trees. As decomposers, insects help create top soil, the nutrient-rich layer of soil that helps plants grow.

Economic Importance

Insects also produce useful substances, such as honey, wax, lacquer, and silk. Honeybees have been raised by humans for thousands of years for honey. The silkworm has greatly affected human history. When the Chinese used worms to develop silk, the silk trade connected China to the rest of the world. Adult insects, such as crickets and insect larvae, are also commonly used as fishing bait.

Insects as Food

In some parts of the world, insects are used for human food. Some people support this idea to provide a source of protein in human nutrition. From South America to Japan, people eat roasted insects, like grasshoppers or beetles (Figure below).

Grasshoppers have been added to this taco.

Insects in Medicine

In the past, fly larvae (maggots) were used to treat wounds to prevent or stop gangrene. Gangrene is caused by infection of dead flesh. Maggots only eat dead flesh, so when they are placed on the dead flesh of humans, they actually clean the wound and can prevent infection. Some hospitals still use this type of treatment.

Controlling Insect Pests

Though insects can be important, some are also considered pests. Common insect pests include:

  1. Parasitic insects (mosquitoes, lice, bed bugs).
  2. Insects that transmit diseases (mosquitoes, flies).
  3. Insects that damage structures (termites).
  4. Insects that destroy crops (locusts, weevils).

Many scientists who study insects are involved in various forms of pest control, often using insect-killing chemicals, but more and more rely on other methods. Ways to control insect pests are described below.

  • Biological control of pests in farming is a method of controlling pests by using other insects. Insect predators, such as lady beetles and lacewings, consume a large number of other insects during their lifetime. If you add ladybugs to your farm or garden, then they will act like a pesticide, or insect-killing chemical.
  • Insecticides are most often used to kill insects. Insecticides are chemicals that kill insects. The U.S. spends $9 billion each year on pesticides. Disadvantages to using pesticides include human poisonings, killing of fish, honeybee poisonings, and the contamination of meat and dairy products.

Lesson Summary

  • Insects are the most diverse group of animals on Earth.
  • They have segmented bodies with an exoskeleton, and relatively simple nervous, respiratory, and circulatory systems.
  • Insects are the only invertebrates to have developed flight.
  • Some insects, like termites, ants, and many bees and wasps, are social and live together in large well-organized colonies.
  • Insect movement includes flight, walking, and swimming.
  • There are two major groups of insects, the wingless and the winged, and these are subdivided into various orders.
  • Insects obtain food with the use of specialized appendages for capturing and eating.
  • Most insects can rapidly reproduce within a short period of time.
  • An insect can have one of three types of metamorphosis and life cycle.
  • Insects are beneficial both environmentally and economically.
  • Insect pests can be controlled with chemical or biological means.

Review Questions

Recall

1. What are the three main parts of the insect body?

2. Why is the insect’s circulatory system said to be "simple"?

3. What does metamorphosis mean?

Apply Concepts

4. Describe the difference between the life cycle of a silverfish and the life cycle of a butterfly.

5. What makes parasitoids especially effective at killing other insect pests?

6. Describe what it means when an insect is said to be "social."

Critical Thinking

7. Why do you think an exoskeleton allowed insects to better adapt to their environments than some other invertebrates?

Further Reading / Supplemental Links

Points to Consider

Next we begin the discussion of fishes, amphibians, and reptiles.

  • Some of the adaptations that insects have evolved for life on land are also displayed in amphibians and reptiles. What could be some of these? How are they similar and different?
  • Insects have some very specialized sensory capabilities. How do you think these compare to those found in fish, amphibians, and reptiles?

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