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28–3 Insects
Photo Credit: ©skip Moody/Dembinsky Photo Associates
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What Is an Insect?
What Is an Insect?
Class Insecta contains more species than any other group of animals.
The number of living orders varies from 26 to more than 30 because biologists cannot agree on how to classify them.
Insects have a body divided into three parts—head, thorax, and abdomen. Three pairs of legs are attached to the thorax.
Insects have a body divided into three parts—head, thorax, and abdomen. Photo Credit: ©Dorling Kindersley
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What Is an Insect?
Insects have a segmented body, an exoskeleton, and jointed appendages.
A typical insect also has:
a pair of antennae
a pair of compound eyes
two pairs of wings on the thorax
tracheal tubes that are used for respiration
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What Is an Insect?
Responses to Stimuli
Compound eyes made up of many lenses detect minute changes in color and movement.
The brain assembles information from the eye and directs the insect’s response.
Compound eyes produce an image less detailed than what we can see, however they are far better at detecting movement.
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What Is an Insect?
Insects have chemical receptors for taste and smell on their mouthparts, antennae, and legs.
Insects have sensory hairs that detect movements in the surrounding air or water.
Many insects also have well-developed ears that hear sounds far above the human range, located in odd places (behind the legs in grasshoppers).
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What Is an Insect?
Adaptations for Feeding
Insects have three pairs of appendages that are used as mouthparts, including a pair of mandibles. 
Insect mouthparts are specialized for feeding.
Many insects produce saliva containing digestive enzymes that help break down food.
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What Is an Insect?
Specialized Mouthparts
Insect mouthparts are specialized for a variety of functions. An ant’s mouthparts can saw through and then grind food into a fine pulp.
Mandibles used to saw and grind food
Tubelike mouthpart used to suck nectar
Spongelike mouthpart used to lap up food
Moth
Ant
Fly
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What Is an Insect?
Movement and Flight
Insects have three pairs of legs, which are used for walking, jumping, or capturing and holding prey. 
The legs of many insects have spines and hooks that are used for grasping and defense.
Flying insects typically have two pairs of wings made of chitin.
Flight has allowed insects to disperse long distances and to colonize a wide variety of habitats.
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What Is an Insect?
Metamorphosis
The growth and development of insects usually involve metamorphosis, which is a process of changing shape and form.
Insects undergo either incomplete metamorphosis or complete metamorphosis.
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What Is an Insect?
Immature insects that undergo incomplete metamorphosis look very much like the adults.
These immature forms are called nymphs.
Nymphs lack functional sexual organs and other adult structures, such as wings.
Nymphs gradually acquire adult structures as they molt and grow.
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What Is an Insect?
In complete metamorphosis, animals hatch into larvae that look and act nothing like their parents.
They feed voraciously, grow rapidly, and molt a few times.
They undergo a final molt and change into a pupa—the stage in which an insect changes from larva to adult.
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Insects and Humans
Insects and Humans
Many insects are known for their negative effects.
Termites destroy wood structures, bees and wasps give painful stings, moths eat through clothing, etc.
Mosquitoes can infect humans with microorganisms that cause disease such as malaria.
Insects also contribute to the richness of human life.
One third of the food you eat depends on plants pollinated by animals and insects.
Insects produce silk, wax, and honey.
They are considered a food delicacy in certain countries of Africa and Asia.
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Insect Communication
Insect Communication
Insects communicate using sound, visual, chemical, and other types of signals.
Much of their communication involves finding a mate.
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Insect Communication
Visual Cues
Male fireflies use visual cues to communicate with potential mates.
A light-producing organ in the abdomen is used to produce a distinct series of flashes.
When female fireflies see the signal, they flash back a signal of their own, inducing the males to fly to them.
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Insect Communication
Chemical Signals
Many insects communicate using chemical signals.
The chemicals are called pheromones.
Pheromones are specific chemical messengers that affect the behavior or development of other individuals of the same species.
Some pheromones function to signal alarm or alert other insects to the death of a member of the colony.
Other pheromones enable males and females to communicate during courtship and mating.
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Insect Societies
Insect Societies
Ants, bees, termites, and some of their relatives form complex associations called societies.
A society is a group of closely related animals of the same species that work together for the benefit of the whole group.
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Insect Societies
Castes
Within an insect society, individuals may be specialized to perform particular tasks, or roles.
These are performed by groups of individuals called castes.
Each caste has a body form specialized for its role.
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Insect Societies
The basic castes are:
reproductive females (queens) – lay eggs
reproductive males – fertilize eggs
workers – feeding, building, protection
Most insect societies have only one queen, which is typically the largest individual of the colony.
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Insect Societies
Communication in Societies
Each species of social insect use visual, touch, sound, and chemical signals to communicate information among members of the colony.
Honeybees communicate information about food through a series of complex movements.
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Insect Societies
The round dance indicates that food is fairly close to the hive.
Bees use dances to communicate information about food sources. The round dance indicates that food is fairly close to the hive.
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Insect Societies
The waggle dance indicates that food is farther away from the hive. It also indicates the direction of the food.
Bees use dances to communicate information about food sources. The waggle dance indicates that food is farther away. It also indicates the direction of the food.
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