Internal Anatomy of an Insect

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An Introduction to Insect Organs and Internal Structures

Internal anatomy of an insect. Illustration courtesy of Piotr Jaworski (Creative Commons license).

Have you ever wondered what an insect looks like inside? Or whether an insect has a heart or a brain?

The insect body is a lesson in simplicity. A three-part gut breaks down food and absorbs all the nutrients the insect needs. A single vessel pumps and directs the flow of blood. Nerves join together in various ganglia to control movement, vision, eating, and organ function.

This diagram represents a generic insect, and shows the essential internal organs and structures that allow an insect to live and adapt to its environment. Like all insects, this pseudo bug has three distinct body regions, the head, thorax, and abdomen, marked by the letters A, B, and C respectively.

In this article, you'll learn about each of the insect's body systems: nervous, digestive, circulatory, respiratory, reproductive, and excretory. As you read about each system, use the diagram to identify the organs and structures discussed.

This original diagram of the internal anatomy of an insect was created by Piotr Jaworski, and licensed through Creative Commons. You can see more of his excellent graphics work on his Wikimedia Commons user page. On the following pages, we have modified his diagram to highlight only the organs and structures of the body system discussed on that page.

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Diagram of the Insect Nervous System

Insect nervous system. Illustration courtesy of Piotr Jaworski (Creative Commons license), modified by Debbie Hadley

The insect nervous system consists primarily of a brain (5), located dorsally in the head, and a nerve cord (19) that runs ventrally through the thorax and abdomen.

The insect brain is a fusion of three pairs of ganglia, each supplying nerves for specific functions. The first pair, called the protocerebrum, connects to the compound eyes (4) and the ocelli (2, 3) and controls vision. The deutocerebrum innervates the antennae (1). The third pair, the tritocerebrum, controls the labrum, and also connects the brain to the rest of the nervous system.

Below the brain, another set of fused ganglia forms the subesophageal ganglion (31). Nerves from this ganglion control most of the mouthparts, the salivary glands, and the neck muscles.

The central nerve cord connects the brain and subesophageal ganglion with additional ganglion in the thorax and abdomen. Three pairs of thoracic ganglia (28) innervate the legs, wings, and muscles that control locomotion.

Abdominal ganglia innervate the muscles of the abdomen, the reproductive organs, the anus, and any sensory receptors at the posterior end of the insect.

A separate but connected nervous system called the stomodaeal nervous system innervates most of the body's vital organs. Ganglia in this system control functions of the digestive and circulatory systems. Nerves from the tritocerebrum connect to a ganglia on the esophagus; additional nerves from this ganglia attach to the gut and heart.

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Diagram of the Insect Digestive System

Insect digestive system. Illustration courtesy of Piotr Jaworski (Creative Commons license), modified by Debbie Hadley

The insect digestive system is a closed system, with one long enclosed tube (alimentary canal) running lengthwise through the body. The alimentary canal is a one-way street – food enters the mouth and gets processed as it travels toward the anus. Each of the three sections of the alimentary canal performs a different process of digestion.

The salivary glands (30) produce saliva, which travels through salivary tubes into the mouth. Saliva mixes with food and begins the process of breaking it down.

The first section of the alimentary canal is the foregut (27) or stomodaeum. In the foregut, initial breakdown of large food particles occurs, mostly by saliva. The foregut includes the Buccal cavity, the esophagus, and the crop, which stores food before it passes to the midgut.

Once food leaves the crop, it passes to the midgut (13) or mesenteron. The midgut is where digestion really happens, through enzymatic action. Microscopic projections from the midgut wall, called microvilli, increase surface area and allow for maximum absorption of nutrients.

In the hindgut (16) or proctodaeum, undigested food particles join uric acid from Malphigian tubules to form fecal pellets. The rectum absorbs most of the water in this waste matter, and the dry pellet is then eliminated through the anus (17).

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Diagram of the Insect Circulatory System

Insect circulatory system. Illustration courtesy of Piotr Jaworski (Creative Commons license), modified by Debbie Hadley

Insects don't have veins or arteries, but they do have circulatory systems. When blood is moved without the aid of vessels, the organism has an open circulatory system. Insect blood, properly called hemolymph, flows freely through the body cavity and makes direct contact with organs and tissues.

A single blood vessel runs along the dorsal side of the insect, from the head to the abdomen. In the abdomen, the vessel divides into chambers and functions as the insect heart (14). Perforations in the heart wall, called ostia, allow hemolymph to enter the chambers from the body cavity. Muscle contractions push the hemolymph from one chamber to the next, moving it forward toward the thorax and head.In the thorax, the blood vessel is not chambered. Like an aorta (7), the vessel simply directs the flow of hemolymph to the head.

Insect blood is only about 10% hemocytes (blood cells); most of the hemolymph is watery plasma. The insect circulation system does not carry oxygen, so the blood does not contain red blood cells as ours does. Hemolymph is usually green or yellow in color.

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Diagram of the Insect Respiratory System

Insect respiratory system. Illustration courtesy of Piotr Jaworski (Creative Commons license), modified by Debbie Hadley

Insects require oxygen just as we do, and must "exhale" carbon dioxide, a waste product of cellular respiration. Oxygen is delivered to the cells directly through respiration, and not carried by blood as in vertebrates.

Along the sides of the thorax and abdomen, a row of small openings called spiracles (8) allow the intake of oxygen from the air. Most insects have one pair of spiracles per body segment. Small flaps or valves keep the spiracle closed until there is a need for oxygen uptake and carbon dioxide discharge. When the muscles controlling the valves relax, the valves open and the insect takes a breath.

Once entering through the spiracle, oxygen travels through the tracheal trunk (8), which divides into smaller tracheal tubes. The tubes continue to divide, creating a branching network that reaches each cell in the body. Carbon dioxide released from the cell follows the same pathway back to the spiracles and out of the body.

Most of the tracheal tubes are reinforced by taenidia, ridges that run spirally around the tubes to keep them from collapsing. In some areas, however, there are no taenidia, and the tube functions as an air sac capable of storing air.

In aquatic insects, the air sacs enable them to "hold their breath" while under water. They simply store air until they surface again. Insects in dry climates may also store air and keep their spiracles closed, to prevent water in their bodies from evaporating. Some insects forcefully blow air from the air sacs and out the spiracles when threatened, making a noise loud enough to startle a potential predator or curious person.

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Diagram of the Insect Reproductive System

Insect reproductive system. Illustration courtesy of Piotr Jaworski (Creative Commons license), modified by Debbie Hadley

This diagram shows the female reproductive system. Female insects have two ovaries (15), each comprised of numerous functional chambers called ovarioles (seen within the ovary in the diagram). Egg production takes place in the ovarioles. Egg are then released into the oviduct. The two lateral oviducts, one for each ovary, join at the common oviduct (18). The female oviposits fertilized eggs with her ovipositor (not pictured).

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Diagram of the Insect Excretory System

Insect excretory system. Illustration courtesy of Piotr Jaworski (Creative Commons license), modified by Debbie Hadley

The Malpighian tubules (20) work with the insect hindgut to excrete nitrogenous waste products. This organ empties directly into the alimentary canal, and connects at the junction between the midgut and hindgut. The tubules themselves vary in number, from just two in some insects to over 100 in others. Like arms of an octopus, the Malpighian tubules extend throughout the insect's body.

Waste products from the hemolymph diffuse into the Malpighian tubules, and are then converted to uric acid. The semi-solidified waste empties into the hindgut, and becomes part of the fecal pellet.

The hindgut (16) also plays a role in excretion. The insect rectum retains 90% of the water present in the fecal pellet, and reabsorbs it back into the body. This function allows insects to survive and thrive in even the most arid climates.