The Robinson Library
The Robinson Library >> Insects >> General
The Internal Anatomy of an Insect

The internal organs of insects, like those of other animals, are grouped into various systems. But these systems differ in many ways from those of other animals. The chief systems are the circulatory, respiratory, nervous, muscular, digestive, and reproductive.

diagram of the internal anatomy of a typical insect

Circulatory System An insect's blood does not flow through veins and arteries as ours does. Blood fills the whole cavity of the insect's body, and bathes all the organs and muscles. The blood is circulated by a long tube that lies just under the exoskeleton of the back. This tube extends almost the entire length of the body. The pumping part of the tube lies in the abdomen, and is called the heart. The front part of the tube extends into the head, and is called the aorta. Blood enters the tube through little openings, called ostia, along the sides. The openings have valves that allow blood to enter the tube, but not to flow out. As the heart contracts, the blood is forced along the tube and out through the aorta. The blood first bathes the brain, and then flows to other parts of the body. It then re-enters the tube through the ostia.

Unlike our blood, an insect's blood has little to do with bringing oxygen to the cells. Insect blood is greenish, yellowish, or colorless. Few insects have red blood.

Respiratory System An insect breathes by means of tiny holes, called spiracles, along the sides of its body. Each hole leads into a large tube called a trachea. The large tubes divide into small tubes, which, in turn, divide into still smaller tubes that branch out to all the cells of the body. This system of tubes carries oxygen to the cells and takes away carbon dioxide.

Nervous System consists of a brain, located in the head, and two nerve cords that lie side by side along the floor of the thorax and abdomen. The brain receives information from the eyes and antennae, and controls the insect's body activities as a whole. Another nerve center in the head is connected to the brain and controls the insect's mouth parts. Each of the two nerve cords contains a cluster of nerve cells, called a ganglion, in each segment of the thorax and abdomen. The two ganglia in each segment are fused and form a sort of little brain that controls the activities of that segment. The ganglia often can work without the brain. For example, many insects that have had their heads cut off can still walk, mate, and lay eggs. In some insects, the three pairs of ganglia in the thorax are fused into one. Various pairs of ganglia in the abdomen are also fused in many insects.

Muscular System is made up of several hundred to a few thousand small but very strong muscles. Grasshoppers have about 900 muscles, and caterpillars have from 2,000 to 4,000. By contrast, man has fewer than 700 muscles. Many insects can lift or pull an object 20 or more times heavier than the weight of their bodies. Few men can lift a weight heavier than the weight of their bodies.

Digestive System of an insect consists basically of a long tube that extends from the mouth to the anus. The tube has three main divisions: (1) the foregut; (2) the midgut, or stomach; (3) the hindgut, or intestine.

After food has been chewed or sucked up by the mouth parts, it enters the foregut through the mouth. The food moves along the tube until it reaches an enlarged area called the crop, where it is temporarily stored and partly digested. Then the food passes into the gizzard, which has thick muscular walls that contract and grind the food into small bits. The gizzard of some insects has teeth that help break up and grind the food. The food next passes into the midgut, where most digestion takes place. Nourishing parts of the food are absorbed into the blood, and wastes and undigested parts move into the hindgut.

Insects have a system of 2 to over 150 malpighian tubes attached to the digestive system where the midgut and hindgut join. The tubes float in the blood and absorb waste materials, which then pass through the tubes into the hindgut. All wastes and extra water that enter the hindgut leave the body through the anus.

Reproductive System Most insects reproduce sexually. That is, a new individual can be created only after a female sex cell (egg) has united with a male sex cell (sperm). The reproductive organs are in the abdomen. Females have two organs, called ovaries, in which eggs develop. A tube called an oviduct carries the eggs away from each ovary. The two oviducts join and form a single tube that opens near the tip of the abdomen. Male insects have two organs, called testes, that produce sperm. A tube carries the sperm from each testis. The two tubes unite and form a single tube that extends to the outside of the abdomen.

Questions or comments about this page?

The Robinson Library >> Insects >> General

This page was last updated on June 19, 2018.