Arthropoda – Insecta Ashley Libby
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Insects are the largest class in the kingdom Animalia and outnumber all other life forms combined. The class Insecta is very diverse and divided into approximately twenty-six orders. Adult insects are terrestrial creatures, with open circulatory systems, that have developed the ability of flight. The field of study dedicated to the class Insecta is called Entomoly.
As part of the phylum Arthropoda, insects have exoskeletons. Exoskeletons are hard external skeletons made of multiple layers of protein and chitin, a structural polymer of a simple sugar or carbohydrate, which cover the arthropod. The arthropods are invertebrates, which means they lack a vertebral column. Because they lack this column, arthropods use the exoskeleton for support and a place for muscle attachments. The exoskeleton is segmented and contains jointed appendages. These joints result in flexibility that would be impossible with a continuous, un-jointed exoskeleton. Bendable appendages have, through evolution, led to a division of labor where certain appendages are modified for walking, feeding, sensory reception, copulation, and defense. Because an insect’s exoskeleton cannot develop and grow along with the insect, the insect must occasionally shed its exoskeleton. Molting takes much energy and temporarily leaves an insect vulnerable to predators.
Insect bodies can be divded into three parts: the head, thorax, which has 6 legs (and often supports wings), and the abdomen. In their development, most insects go through metamorphosis. Insects display two types of metamorphosis; incomplete and complete. In incomplete metamorphosis, such as what grasshoppers go through, the offspring resemble miniature adults with slightly different body proportions. Throughout the young insect’s life it goes through a series of molts, each time looking more like the adult insect. When the insect finally reaches its full size, it is considered an adult. Insects that go through incomplete metamorphosis start out in a larval stage where there is little to no resemblance to its adult form. The larval stage is designed especially for eating and growing. Then during a pupal stage the larva dons its adult form, which is specialized for dispersal and reproduction. A well known example of an insect that goes through complete metamorphosis is the butterfly.
Insects display extensive cephalization, an evolutionary trend toward the concentration of sensory equipment on the anterior end of the body. Such sensory organs are highly relied upon, when it comes to finding a suitable mate. Insects usually reproduce sexually, with male and female individuals advertising themselves with bright colors, sound, or odors.
Insects are normally thought of as harmful to humans, yet they possess many qualities that prove to be helpful in the Earth's ecosystem. A few of these qualities include: pollinating many of Earth's plants, decomposing organic materials, and helping to recycle carbon and nitrogen.

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(Complete metamorphosis)

Drawing showing incomplete metaporphosis © Paul Billiet
Drawing showing incomplete metaporphosis © Paul Billiet

Drawing showing incomplete metaporphosis

Acquiring and Digesting Food

Flight is very important in the acquiring of food. Many insects feed on gymnosperms and other carboniferous plants, which are not always located within walking distance, making flight necessary to obtain food.
In insects digestive track is regionally specialized with distinct organs working together in order to break down food and absorb nutrients. A grasshopper for instance has several chambers that can be clustered into three main regions. A foregut: with an esophagus and a crop, where food is moistened and stored. A midgut: with gastric ceca, pouches that extend from to midgut, where most digestion takes place. And finally the hindgut: which includes the rectum and anus where waste is expelled from the insect’s body. In the hindgut, undigested food particles are joined by uric acid to form fecal pellet, which is eliminated through the anus.

Insects also have paired salivary glands and salivary reservoirs. These structures are found in the thorax. Salivary ducts lead from the glands to the reservoirs and then forward, through the head, to an opening (the salivarium) behind the hypopharynx, an insect’s version of a tongue. Movements of the mouthparts help mix saliva with food in the buccal cavity, found on the side of the mouth. This saliva helps the breakdown of foods.

Sensing the Environment

Insects have compound eyes and antennae that are used with the help of their nervous system in order to sense different stimuli in their environment. Mating rituals between insects are very dependent upon how well an insect can sense a suitable mate, whether through their smell, coloration, or a visually stimulating routine performed for a mate.
The nervous system consists of a pair of ventral nerve cords (along the back of the insect) with several segmental ganglia, clusters of nerve cell bodies. The two nerve cords meet in the head, where the ganglia of several anterior segments are fused together. This is the cerebral ganglion, which is an insect’s version of the human brain. The cerebral ganglion is close to the eyes, antennae, and other sense organs in the head.

Insects hear through unique parts of their bodies.


Flight is most insects’ main way of locomotion. It is used to find food, mates, and escape predators. Although insects have legs and can walk, flight is usually the fastest and most effective way to travel. Even though the wing shapes of insects can vary, all insects have an anterior and posterior set of wings. Dragonflies, who have two similar pairs of wings, were one of the first insects to fly and several other insect orders have modified versions of dragonfly wings, with simple anterior and posterior wings. The anterior and posterior wing sets can be hooked together and used as one, as displayed in bees and wasps, or be over lapping, like butterfly wings. Beetles have posterior wings that are used for flight, while the anterior ones have evolved to be used as covers to protect the flight wings when beetles are burrowing.


The insect respiratory system (KA)
The insect respiratory system (KA)

The Insect Respitory System

Because insects have a relatively inefficient open circulatory system, a centralized respiratory system would not be an efficient way to supply the cells of the body with oxygen. Instead, insects have evolved a tracheal system which channels oxygen to the different parts of the body via an internal network of small tubes. Insects use a tracheal system in order to perform respiration. The tracheal system is composed of chitin-lined, air tubes that branch throughout the body. Rings of chitin reinforce the largest tubes called trachea, which open to the outside through pores, called spiracles, which can open and close to regulated air flow and limit water loss. Near organs that require a large supply of oxygen, trachea are enlarged to form air sacs. The finer branches, tracheoles, infiltrate to the surface of nearly every cell, where gas diffuses through the moist epithelium, sheets of tightly packed cells, which line the ends of the tracheal system. The tips of these tracheoles are closed and contain fluid. When an insect is active and requires more oxygen, most of the fluid is withdrawn into the body, creating a larger surface area for the diffusion of oxygen.


Insects have open circulatory systems. Fluid called hemolymph is propelled through short arteries, by a heart, and then into spaces called sinuses, that surround the tissues and organs of an insect. These body sinuses are called hemocoel. Hemolymph then reenters the insect’s heart through pores that are usually equipped with valves that regulate the amount of hemolymph that flows through the heart. Although hemolymph resembles blood, the term blood is reserved for fluid in a closed circulator system. 90% of hemolymph is plasma, which accounts for its high concentrations of amino acids, proteins, sugars, and inorganic ions compared to vertebrates, and 10% is made up of hemocytes, various cell types that do not contain hemoglobin.

Metabolic Waste Removal

Metabolic wastes are removed from an insect’s hemolymph by unique organs called malpighian tubules. Malpighian tubules are out pocketings of the digestive tract that put metabolic wastes into the digestive track. When hemolymph washes over the outside of the tubules, the cells of the tubules extract nitrogenous waste from the hemolymph and into the tubule. The waste then enters the hind intestine and is dispelled through the anus.

Self Protection

Insects have developed several ways of protecting themselves from predators. When the choice between flight or fight is given, most insects would chose to use their convenient wings to fly away from dangerous situations. Some insects such as wasps and bees have developed stingers that release toxins into their opponents, and others have large pinchers that can be used for protection.
Some insects take a more indirect route to protection. Instead of developing offensive forms of self protection, they use defensive cryptic coloration and camouflage in order to avoid predators. Here are some cool examples of
camoflage at its best.
Several unusual examples involve imitation in appearance: Batesian mimicry involves a harmless species protecting itself by evolving to look like a harmful one, so that predators steer clear for fear of getting poisoned or stung; Mullerian mimicry occurs when two harmful species evolve to resemble each other--this causes the predator to learn more quickly to associate the appearance with negative results, thus benefiting both insect species.

Osmotic Balance

The spiracles present on the surface of an insects body can open and close to regulate and limit an insects water loss. One way that allows insects to extract water from the air is through the insects' water capillaries, which are located between insects' hairs

Temperature Balance

Several species of flying insects, such as bees and moths, are endothermic and are considered the smallest of all endotherms. Endotherms are animals that use metabolic energy to maintain a constant temperature. Powerful flight muscles generate large amounts of heat when operating, causing an insect’s body temperature to elevate. To warm up before taking off, many insects shiver, contracting flight muscles in synchrony, so that only slight wing movement occurs, but heat is produced. During this warming process, chemical reactions and cellular respiration speed up (the Q10 effect), enabling insects to fly on cold days or even at night.
Many endothermic insects have a counter current heat exchange that helps maintain a high temperature in the thorax, the area where an insect's wings are located. This helps insects keep a constant temperature even on cold, snowy nights. On the other hand, insects flying in warm weather have a risk of becoming overheated because of the formidable amount of heat produced by the working flight muscles. Some species are able to shut off the counter current mechanism, allowing heat to be lost from the thorax to the abdomen and then to the environment. Some insects such as honeybees use this when incubating their eggs by pressing their abdomen against their eggs.


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Review Questions

1. What are some of the benefits of insects having wings?
2. What makes the metabolic waste removal of an insect unique?
3. Many insects are fuzzy, (e.g. bumblebees and caterpillars). Is insect hair analogous or homologous to mammal hair?
4. List and analyze the three main regions that contribute to digestion?
5. What make insects different than other arthropods?
6. Why isn't it beneficial for an insect to have a centralized respiratory system?

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