CK-12 Life Science

by CK-12 Foundation

  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 (Figure below).

  Figure 12.28

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

  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 (Figure below).

  Figure 12.29

  Caterpillars feeding on a host plant.

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

  Figure 12.30

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

  shows the structures present in each body segment. Head Thorax Abdomen

  A pair of sensory antennae, a pair of compound eyes, one to three simple eyes, and three sets of variously modified 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. Air is taken in through the spiracles, openings on the sides of the abdomen. Insect respiration occurs without lungs, with a system of internal tubes and sacs through which oxygen is delivered directly to the adjoining body tissues. Since oxygen is delivered directly, the circulatory system is therefore greatly reduced and consists of only a single dorsal tube with openings. The tube pulses and circulates blood-like fluids inside the body cavity.

  Insect locomotion includes flight, walking, and swimming. Insects are the only invertebrates to have developed flight 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 on the wing structure. More advanced groups have foldable wings and their muscles act on the wall of the thorax and give power to the wings indirectly. These muscles are able to contract multiple times for each single nerve impulse, allowing the wings to beat faster than would ordinarily happen.

  Many adult insects use six legs for walking and have adopted a gait that uses the legs in alternate triangles touching the ground. This gait allows for rapid walking at the same time as having a stable stance. A few insects have evolved to walk on the surface of the water, especially the water striders (Figure below).

  Figure 12.31

  A pair of water striders mating, showing how water surface tension allows for them to stand on the water.

  A large number of insects live either parts of or their whole lives underwater. Water beetles and water bugs have legs adapted to paddle in the water. Dragonfly young use jet propulsion, forcibly expelling water out of the rectal chamber.

  Insects use a wide variety of senses for both communicating and receiving information. Many insects have very sensitive and/or specialized sensory organs. Table (below) summarizes five types of communication that are used by various insects and sometimes for different purposes.

  Insect Communication Types of Communication Representative Organisms Description

  Visual Bioluminescence

  Polarized light

  Ultraviolet wavelengths

  Bees Fireflies

  Bees

  Reproduction and Predation Some species produce flashes to attract mates; other species to lure prey.

  Detect polarized light

  Perceive ultraviolet wavelengths

  Hearing

  Ultrasound clicks

  Sound Production Mostly by mechanical action of appendages

  Some predatory and parasitic insects

  Moths

  Moths

  Cicadas

  Can detect sounds made by prey or hosts.

  Predation Some nocturnal species can hear the ultrasonic emissions of bats, which help them avoid predation.

  Predation Produced mostly by unpalatable moths to warn bats; other moths make similar sounds in order to mimic distasteful moths so they will be avoided by bats as well.

  Loudest sounds among insects; have special modifications of body and musculature to produce and amplify sounds.

  Chemical Wide range of insects have evolved chemical communication; chemicals often derived from plant metabolites and are used to attract, repel, or provide other kinds of information; chemicals may be targeted at individuals of same or different species; use of scents is especially well developed in social insects.

  Moths

  Antennae of males can detect pheromones (chemicals secreted by animals, especially insects, that influence the behavior or development of others within the same species) of female moths over distances of many kilometers (Figure below).

  Infrared

  Blood-sucking insects

  Have specialized sensory structures that can detect infrared emissions in order to find their hosts.

  “Dance Language” – a system of abstract symbolic communication

  Honey bees

  Thought that various species of honey bees are only invertebrates to have evolved this type of communication; angle at which bee dances represents direction relative to sun, length of dance represents distance to be flown.

  Figure 12.32

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

  Social insects, such as the 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 show any true capacity for homing. This allows an insect to return to a single hole among a mass of thousands of apparently identical holes, after a trip of up to several kilometers and as long as a year after last seeing the area, as when an insect hibernates. A few insects migrate, but this is a larger-scale form of navigation and involves only a large general region, such as the overwintering of the monarch butterfly (Figure below).

  Figure 12.33

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

  Figure 12.34

  A wasp building its nest.

  Figure 12.35

  Monarch butterflies in an overwintering cluster.

  Insects are divided into two major groups, the wingless and the winged insects. The wingless consists of two orders: the bristle tails and the silverfish. The winged orders of insects include the 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 a wide variety of appendages adapted for capturing and feeding on prey. In addition, as already discussed, they have sensory capabilities, which help them detect prey.

  Insects have a wide range of mouthparts used for feeding. Specialized part
s are mostly for piercing and sucking, as in mosquitoes and aphids. A number of insect orders have mouthparts that pierce food items to enable sucking of internal fluids. Some are herbivorous, like aphids and leafhoppers, while others are insectivorous, like assassin bugs and mosquitoes (females only).

  Examples of chewing insects include dragonflies, grasshoppers, and beetles. Some larvae have chewing mouthparts, as in moths and butterflies.

  Some insects use siphoning, as if sucking through a straw, as in moths and butterflies, where some of the mouthparts are adapted into an elongated sucking tube. You have probably seen a butterfly or moth poised at a flower while it siphons the nectar of the flower. Some moths, however, have no mouthparts at all.

  Some insects are capable of sponging, as in the housefly. One of the mouthparts is specialized for this function, where liquid food is channeled to the esophagus. The housefly is able to eat solid food by secreting saliva and dabbing it over the food item. As the saliva dissolves the food, the sponging mouthpart absorbs the liquid food.

  Reproduction and Life Cycle of Insects

  Most insects have a high reproductive rate and can rapidly reproduce within a short period of time. With a short generation time, they evolve faster and can adjust to environmental changes faster. Although there are many forms of reproductive organs in insects, there is a basic design and function for each reproductive part. These parts may vary in shape (gonads), position, and number (glands), with different insect groups.

  Most insects reproduce via sexual reproduction. The female produces eggs, which are fertilized by the male, and then the eggs are usually deposited 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 and in the most common type, the offspring are essentially identical to the mother. This is most often seen in aphids and scale insects.

  An insect can have one of three types of metamorphosis and life cycle:

  Type of Metamorphosis None Incomplete Complete

  Characteristics Only difference between adult and larvae 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; they usually have chewing mouthparts even if adult mouthparts are sucking ones; at last larval stage of development insect forms into pupa (Figure below), doesn’t feed and is inactive; here wing development is initiated, and adult emerges

  Example Silverfish Dragonflies Butterflies and Moths

  Figure 12.36

  Heteroptera nymphs and egg cases.

  Figure 12.37

  The chrysalis (pupal stage) of a monarch butterfly.

  Importance of Insects

  Many insects are considered to be pests by humans. In spite of this, insects are also very important. 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, and insects are responsible for much of the process by which topsoil is created.

  Insects also produce useful substances as honey, wax, lacquer, and silk. Honeybees have been cultured by humans for thousands of years for honey. The silkworm has greatly affected human history, as silk-driven trade established relationships between China and the rest of the world.

  Fly larvae (maggots) were formerly used to treat wounds to prevent or stop gangrene, as they would only consume dead flesh. This treatment is finding modern usage in some hospitals. Adult insects such as crickets, and insect larvae of various kinds, are also commonly used as fishing bait.

  In some parts of the world, insects are used for human food, while being a taboo in other places. Some people support this idea to provide a source of protein in human nutrition. Insects also have a role in controlling insect pests, as we will see in the next section.

  Controlling Insect Pests

  Insects commonly regarded as pests include those that are parasitic (mosquitoes, lice, bed bugs), transmit diseases (mosquitoes, flies), damage structures (termites), or destroy agricultural products (locusts, weevils). Many entomologists are involved in various forms of pest control, often using insecticides, but more and more relying on methods of biocontrol.

  Biological control of pests in agriculture is a method of controlling pests that relies on predation, parasitism, herbivory, or other natural mechanisms. Insect predators, such as lady beetles and lacewings, are mainly free-living species that consume a large number of prey during their lifetime.

  Parasitoids are species whose immature stage develops on or within a single insect host, ultimately killing the host. Most have a very narrow host range. Many species of wasps and some flies are parasitoids. Both of these types of predators and parasitoids are used to control insect pests. Pathogens are disease-causing organisms including bacteria, fungi, and viruses, which kill or debilitate their host and are specific to certain insect groups.

  Most of the insecticides now applied are long-lasting synthetic compounds that affect the nervous system of insects on contact. Agricultural pesticides prevent a monetary loss of about $9 billion each year in the U.S. These benefits, however, must be weighed against the costs to society of using pesticides, which include human poisonings, fish kills, honeybee poisonings, and the contamination of livestock products.

  Lesson Summary

  Insects are the most diverse group of animals on Earth; they are found in nearly all environments. They have segmented bodies with an exoskeleton; the nervous, respiratory, and circulatory systems are fairly simple. Insects are the only invertebrates to have developed flight. Insects have very sensitive and/or specialized organs of perception, including visual, chemical, heat-sensitive, and auditory. Some insects, like termites, ants, and many bees and wasps, are social and live together in large well-organized colonies.

  Insect locomotion includes flight, walking, and swimming. There are two major groups of insects, the wingless and the winged, and these are further subdivided into various orders. Insects obtain food with the use of specialized appendages for capturing and eating the prey. Most insects have a high reproductive rate and 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 with natural means, some of which are insects themselves; even though agricultural pesticides prevent a major monetary loss, they have major drawbacks, too.

  Review Questions

  What are the main characteristics of insects?

  Why is the insect’s circulatory system greatly reduced?

  Give an example of mimicry in insects.

  How do female accessory glands aid in the development of eggs?

  What makes parasitoids especially effective against pests?

  Further Reading / Supplemental Links

  http://homeschooling.gomilpitas.com/explore/bugs.htm

  http://rusinsects.com/links/view.php?id=20

  http://www.kidsolr.com/science/page18.html

  http://pestworldforkids.org/learninggames.html

  Vocabulary

  cuticle

  The outer layer of the exoskeleton.

  exocuticle

  The thin and waxy water resistant outer layer of the cuticle.

  nymphs

  A developmental stage of insects, where the young is usually similar to the adult.

  oviviparous

  A method of reproduction where the young hatch after the eggs have been laid.

  parasitoids

  Species whose immature stages develop on or within a single insect host, ultimately killing the host.

  pheromones

  Chemicals secreted by animals, especially
insects, that influence the behavior or development of others within the same species.

  spiracles

  Openings on the sides of the insect abdomen, through which air is taken in.

  Points to Consider

  Some of the adaptations that insects have evolved for a terrestrial existence 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?

  Chapter 13: Fishes, Amphibians, and Reptiles

  Lesson 13.1: Introduction to Vertebrates

  Lesson Objectives

  Describe the general features of chordates.