Basic Facts: Insect Body Structure
Body Regions and Exoskeleton

The insect's body is a series of segments that are grouped into three sections, the head, thorax, and abdomen. The six segments in the head cannot be seen because they are fused together. The thorax has three segments, each bearing a pair of legs. The abdomen of most insects has 10 segments, although primitive insects may have as many as 12.


The segments of the insect's body are hardened to form an external skeleton, or exoskeleton. The insect's exoskeleton provides the support for the body and connections for muscles, similar to the internal skeleton of humans.


The exoskeleton is divided into separate hardened plates on some segments. A segment can have a plate on top, one on each side, and one on the bottom. In the abdomen, only the top and bottom plates of each segment are large and visible. The plates of each segment are often connected by soft and flexible membranes to allow movement of the body parts. These membranes between the plates also can stretch to allow expansion of the abdomen, as occurs when a mosquito feeds on blood.

Head

The head bears the antennae, eyes, and mouthparts. The antennae and eyes are the main sensory organs. The two compound eyes each have many individual facets, or lens. In addition, there may be as many as three simple eyes, or ocelli, but these have only a single facet. Antennae are organs of touch, smell, and sometimes hearing. Many different forms of antennae are present in insects. Some are hair-like and others are like a string of beads. Butterflies and many beetles have antennae with enlarged ends that appear clubbed. Moths have antennae that are either feathery or thread-like.


The major mouthparts include the labrum, or upper lip, mandibles, maxillae, and labium, or lower lip. The maxillae and labium have small sensory appendages, termed palpi. These mouthparts are modified in various insects to form two major types: chewing and sucking. In chewing mouthparts, like the grasshopper, the mandibles are used for cutting and grinding food that the other mouthparts grab and bring towards the mouth. Bees have a combination of chewing and sucking mouthparts.


Insects with sucking mouthparts include those with a beak or proboscis. The beak in Hemiptera has four needle-like mouthparts for piercing a plant or animal and sucking the liquid food. The proboscis in mosquitoes and related biting flies has six needle-like mouthparts for piercing and sucking. In some flies, like the tse-tse fly, only one mouthpart is used for piercing and the other mouthparts form a tube for sucking. In many flies as well as moths and butterflies, the mouthparts do not pierce the host, but are used only for sucking or sponging the food.

Thorax

The thorax includes three segments: the first segment (prothorax), middle segment (mesothorax), and last segment (metathorax). All three segments have legs in adult insects, but many larvae, such as fly maggots, do not have legs. Wings may be present on the middle and last segment of the thorax. Spiracles are openings in the body for air, and these are often present on the middle and last segment of the thorax.


The human leg has a femur and tibia, and the foot has a tarsus and toes. The insect leg also includes a femur and tibia, the foot is the tarsus, and the claws are rather like the insect's toes. The large femur of insects is connected to the body by two other segments, the coxa and trochanter. The tarsus is divided into as many as five smaller units, which gives it more flexibility.


Legs are used for many functions besides walking. The front legs often are modified to catch prey, like a praying mantis, or to burrow through soil, like a mole cricket. The hind legs can be designed for jumping, like the flea, or for swimming, like the water boatman. The hind legs of some insects, such as the grasshopper, also are used for making sound.


Most adult insects have two pairs of wings, but some have only one pair. Some insects, like lice, fleas, and worker ants, do not have wings. The inside of each wing has many veins that give it support. In beetles, the front wings form protective elytra, and the hind wings are used for flying. Flies, such as the mosquito, use only the front wings for flying. The hind wings are modified to form very small halteres that help the fly to keep balance in the air. Most insects have membranous wings, but some are leathery or hard. Some wings are covered with hairs or scales.

Abdomen

Adult insects do not have walking legs on the abdomen. Some larvae, such as caterpillars, have fleshy appendages, called prolegs, that are used for walking. The abdomen usually has a spiracle for air on each segment. The end of the abdomen has appendages for mating and laying eggs. The part of the female abdomen that deposits eggs is called an ovipositor. Some insects, such as cockroaches and grasshoppers, also have horn-like sense organs at the end of their abdomens. A membranous tympanum, or hearing organ, is on the abdomen in some insects.

Internal Organs and Body Parts

The food canal of insects includes a foregut, midgut, and hindgut. Some insects shred their food in the foregut. Most digestion takes place in the midgut. The hindgut absorbs any remaining water before the food remains are excreted.


In people, blood is pumped by the heart through arteries and veins. Insects have a long, tube-like heart for pumping blood, but do not have arteries or veins. The insect heart is on the top side of the insect. The blood is pumped through the heart from the tail end to the front of the body, where it empties into the head. The blood then filters through open areas of the insect's body back to the tail, where it once again is pumped forward. It is possible to see the heart pumping in insects with a transparent skin, such as many caterpillars and other larvae.


Insects are like people and other animals in needing oxygen to live. Most insects get their oxygen from the air that enters their spiracles, tiny openings on the side of their bodies. The air passes through air tubes, called trachea, that divide into smaller and smaller tubes that finally reach each cell in the body. Many insects have areas in the air tubes that are large and can help pump the air when the insect moves.


Aquatic insects have special ways of breathing. Some aquatic insects have gills that are filled with air tubes, and they get their oxygen directly from the water like a fish. Others come to the water surface and obtain a bubble of air that they carry under their abdomen or under their wings.


Humans have about 800 muscles. An adult grasshopper has about 900 muscles, and a butterfly caterpillar has about 4,000 muscles. Humans get tired after running because of a substance, lactic acid, that builds up in the muscles. Insects can fly for a long time without getting tired because their muscles do not build up lactic acid.


The nervous system in insects includes a brain in the head and a bundle of nerves in each segment of the body. The nerve bundles, or ganglia, are connected by a nerve cord on the bottom part of the body.


Diagrams of the internal body parts and organ systems are available at The University of Nebraska Department of Entomology.

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