Arthropoda - Insecta

By Michael Schanz

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Diagnostic Characteristics:

All insects are members of the Arthropoda kingdom. This Kingdom is made up of all crustaceans, spiders, and insects. Nearly one million Arthropods have been discovered. Of those one million species, the majority are classified as insects. The Arthropod kingdom is a dominate factor across the entire world. Two out of every three organisms are considered Arthropods. A key characteristic is the presents of segmentation, a hard exoskeleton, and jointed appendages. The exoskeleton, as known as a cuticle, is constructed from proteins and layers of chitin. The structure of the cuticle can transit from being a hard defense to being paper thin and flexible on different sections of the organism. The exoskeleton is strong and waterproof, allowing Arthropods to exist in the seas, the air, and on land. A disadvantage of the exoskeleton is the necessity of the organism to molt, to shed its exoskeleton when the organism out grows it. The period after molting is a dangerous time and can often lead to easier predation upon the organism. The appendages of Arthropods have differed greatly throughout the kingdom. From species to species, the appendages have adapted for a multitude of uses in all aspects of life. In the early history of Arthropods, the group diverged into four lineages: Trilobites, Chelicerates, Uniramians, and Crustaceans. Insects belong to the lineage of Uniramians. This lineage is identified through many characteristics. Such as Jaw like mandibles, a pair of antennae, compound eyes, and un-branching appendages. Along with these characteristics, an insect’s body is made up of three segments: the head, the thorax, and the abdomen. Insects often have wings, which grant a great advantage to any organism. The wings developed through the insect lineage in a unique format. One or two pairs of wings often extend from an insect’s thorax and are extensions of the cuticle. This allows insects to develop flight without sacrificing an appendage. The thorax is also the location of six appendages, normally legs. The scientific community recognizes 26 different orders of insects, each with their own slightly different characteristics. These different orders each go through some form or metamorphosis, either complete, involving a complete change from larva to adult like the butterfly, or an incomplete metamorphosis, involving a gradual shift through molting periods like with the grasshopper.

Acquiring and Digesting Food:

Insects are a diverse section of the Kingdom Arthropoda. Across the 26 orders of insects, the function of the head to obtain food has changed and transformed. The two main structures that insects use to obtain food are either jaw like mandibles to capture and chew prey or piercing or sucking mouthparts. The large variety of insects prompts many different diets. Insects have been know to feed on all varieties of crops, natural or crafted by man, or on other insects, small fish, or anything they can capture, or often certain orders of insects have been know to have a diet on blood and are ectoparasite upon other organism. Insect have a complete digestive tract with organs functioning in breaking down and absorbing nutrients.

All insects have a complete digestive system. This means that food processing occurs within a tube-like enclosure, the alimentary canal, running lengthwise through the body from mouth to anus. Ingested food usually travels in only one direction.In most insects, the alimentary canal is subdivided into three functional regions: foregut (stomodeum), midgut (mesenteron), and hindgut (proctodeum). In addition to the alimentary canal, insects also have paired salivary glands and salivary reservoirs. These structures usually reside in the thorax (adjacent to the foregut). Salivary ducts lead from the glands to the reservoirs and then forward, through the head, to an opening (the salivarium) behind the hypopharynx. Movements of the mouthparts help mix saliva with food in the buccal cavity.(RW)

A variety of insect appendages for acquiring food. (PS

Sensing the Environment:

Insects have two main methods of sensing the surrounding environment. Characteristic of all insects is the presents of compound eyes. Compound eyes are multifaceted eyes with many separate focusing elements. These eyes allow insects to view the world. The second method insects have at their disposal are the two sensory antennae present on the head of all insects. The nervous system of an insect is made up of a pair of ventral nerve cords and segmented ganglia. These nerve cords attach to fused ganglia segments forming a brain near the antennae.

Nerve cells are typically found grouped in bundles. Therefore, a nerve is a bundle of dendrites or axons that serve the same part of the body, whereas a ganglion, in comparison, is a dense cluster of interconnected neurons that process sensory information or control motor outputs. The segmented ganglia of an insect run along the ventral midline of the thorax and abdomen. More "advanced" insect orders have the tendency for individual ganglia to combine laterally and longitudinally into larger ganglia serving multiple body segments. (IL - 13)

The sensory appendages or antennae are on each side of the insect's head allowing them to sense and feel the environment around them. Insects also use these antennae to smell their environment because they do not have noses. (CP source 11)


Almost all insects have the ability to walk, some have the ability to swim, and others have the ability to fly. The presents of wings are a great advantage to winged insects. The wings extend from the thorax and are made up of the cuticle, chitin. Depending on the species, there are one or two sets of wings. To fly, depending on the species, the present wings can flap together as one or separately, but often, like in the case of bees, the two sets of wings are hooked together and move as if one.

Most insects have at six legs, although some have a few more and some have a lot more (like the caterpillar). Insects normally use static stability to walk, which is a type of movement that insures that insects are stable while walking. It works because in this method, at least three legs are always on the ground so the insect is always balanced. (MM)
They have central pattern generators that is a bunch of interconnected neurons that are responsible for generating the rhythmic motor pattern.(MP)


Gas exchange and respiration occurs through a complex tracheal system. The tracheal system is made up with branched, chitin-lined tubes. These tubes run throughout the body and to every cell in the organism. The tracheal system becomes open to the air through spiracles, pores that can become open or closed to limit water loss and to allow necessary airflow. Insects have an open circulatory system. In an open circulatory system, the heart, through short arteries and into cavities surrounding tissues and organs, pumps a fluid known as hemolymph. These cavities are known as sinuses and the collection of the sinuses throughout the body are known as the hemocoel. The hemolymph is then returned to the heart through pores equipped with valves. Oxygen is absorbed through the spiracles and transported through the tracheal system and the hemolymph fluid to all the cells in the body, allowing for proper gas exchange.

Metabolic Waste Removal:

For an insect metabolic waste is removed through two methods. The first method is through the digestive track. After being processed, nutrients, that are not absorbed, are excreted through the anus at the end of the abdomen. Metabolic waste is collected through unique organs known as Malpighian tubules. These tubules are out pockets on the digestive tract and remove metabolic waste from the hemolymph. The second method of metabolic waste removal is through the tracheal system. The tracheal system helps remove gas that is produced through cellular respiration, transporting the unneeded gas to the spiracles to be removed.

The Malpighian Tubules ensure that only dry material is excreted through the anus. They reabsorb water from the blood to conserve moisture. (PS Source 10)


Open Circulatory System
Open Circulatory System

The circulatory system of an insect is an open circulatory system. The system is made up of chitin-lined tubes making up a tracheal system. The tracheal system reaches every cell for proper gas exchange. The main transporting fluid is hemolymph, which is pumped from the heart through small arteries into the hemocoel system of sinuses that surround organs and tissues. The hemolymph then returns to the heart through valves.

There is only one closed organ, called the dorsal vessel, which extends from the hind end to the thorax, ending in the head. This is one long tube with two regions- the heart/pumping organ and the aorta, which conducts and extends forward through thehead, whereas the heart is the lower half. (JP)

Self Protection:

The main defense against predators and danger for all insects is the exoskeleton surrounding each of them. The exoskeleton is made of proteins and chitin, which allows a hard armor coating or a flexible layer to be developed on the body of an insect. The armor is also an access point for muscles to become attached to giving support and forms a relatively impermeable seal that prevents excess water loss. The strength of the armor is the exoskeleton’s greatest trait and weakness. Because of the rigidness of the exoskeleton to allow for growth, the exoskeleton must be shed through the molting process. This process then leaves the organism defenseless until a new exoskeleton forms. Along with armor, often insects have other lines of defenses. These defenses include but are not limited to chemicals, pincers, poisons, stingers, and spikes.
Bombardier Beetle on the Attack
Bombardier Beetle on the Attack

Osmotic Balance:

Osmotic balance is regulated throughout insects through the hemolymph fluid, which helps guarantee cytoplasm in all of the cells. Water in the entire system is also regulated through the spiracles. The spiracles regulate the loss of water vapor during respiration.

Terrestrial arthropods like many insects are able to extract water vapor from the air. The insects do this by producing concentrated solutions, usually found in the rectum, that through osmosis, absorb water from the air. The rectum is where the arthropod removes water from the feces. Along with this ability, other insects like Ticks have tissues in the mouth that uptake water. Their salivary glands excrete concentrated KCL solutions, absorbing water from the air. (AR)

Another important factor in maintaining the osmotic balance in insects is through the gut and the Malpighian tubes, which control the antidiuretic and diuretic factors (controlling the rate urination to conserve water). The primary strategy which has evolved is reabsorption of fluid from the hindgut urine (this is optimal because it conserves the water in the body). Recently, it has also been discovered that the blood feeding insect Rhodnius Prolixus uses neurohormones to prevent the formation of urine in the hindgut. (RK)

Temperature Balance:

Insects, depending on the species, can be endothermic or exothermic. The advantage of being endothermic in insects is the advantage of being able to prosper in areas of fluctuating weather. Another advantage is that it allows for greater distances for flying insects. The heat that is produces is created through a high metabolism, which requires much greater amounts of food to survive. Exothermic insects have lower metabolisms and require a smaller intake of nutrients. The disadvantage of being exothermic is seen with the dependence on the environment to regulate internal body heat. Exothermic insects are most common in fresh water location because of the stable temperature in water environments.
Grasshopper body parts
Grasshopper body parts


Reproduction for insects is mostly sexual reproduction with a separate male and female. Mates of different insect species recognize their opposite partner through either bright colors that are present on the specific species, certain sounds, or odors or pheromones released by a possible mate. Fertilization often occurs through sex and direct depositing of sperm on the female’s vagina. In some species, instead of sex, the male deposits a sperm packet on the outside of the female to be used. In the female of most insect species there is a structure know as the sperm theca, which can store the sperm of the male to be used for multiple batches of offspring. Many insects only mate once in their lifetime. After the mating process, the female then goes on a search for an acceptable food source for the eggs and lays them on a source to be devoured once the eggs hatch.

As mentioned above, sexes are separate in insects and reproduction is almost always sexual. However, some insect groups do develop without fertilization by a sperm. This process is called parthenogenesis. For example, in aphids, all-female generations are produced by parthenogenesis. In some insects, bees for example, every unfertilized egg become a male, while every fertilized egg becomes a female. The eggs of insects are usually laid in a sheltered place and are hardly ever hatched internally. After hatching, an insect must continually molt (shed exoskeleton) as the exoskeleton of an insect does not provide much expansion. (VN)

The life cycles of insects are intricate and complex, consisting of a number of transitory, transformative periods called metamorphoses, which occur between intermediate life cycle stages. The exact complexity of an insect's life cycle is dependent upon the particular insect, with the most complicated life cycles, like those of butterflies, moths, and true flies (orders lepidoptera and diptera, respectively), involving four stages: the egg, larva, pupa, and adult stages; the life cycles of other insects, such as springtails and true bugs (orders collembola and hemiptera, respectively) are less complicated and more direct, usually including the egg, nymph, and adult stages. (MR; Source 16)

Adaptations to Environment:

More than 500 species have developed resistance to pesticides proliferated by humans. When pesticides are first used, a large portion of a population dies. A small percentage of insects with resistance to these chemicals survive. However, because insects reproduce quickly, the insects that survive pass on this resistance to their offspring, and the proportion of pesticide-resistant individuals increases dramatically. As a result of this resistance become widespread within the population, many pesticides used to kill insects are becoming less and less effective. (Matt B - Source 12)

Review Questions:

When talking about diagnostic characteristics how can we differentiate Arthropoda Insecta from Arthropoda Crustacea? (AP)

3. What is the exoskeleton made of, and what does it do for the insect? (CM)
4. In what ways are the respiratory and circulatory systems of insects unique; what are the important structures and functions of each system? (ZJ)


11. (CP)
12. (Matt B)
13. Meyer, John R. "The Nervous System." General Entomology. NC State University, 17 Feb. 2006. Web. 12 Nov. 2011. <>.
15. (RW)
16. "Insects." The Gale Encyclopedia of Science. Ed. K. Lee Lerner and Brenda Wilmoth Lerner. 4th ed. Detroit: Gale, 2008. Gale Science In Context. Web. 13 Nov. 2011. (MR)