Insect Body Parts: Head, Thorax and Abdomen

The three body parts of an insect are the head, the thorax and the abdomen. Insects also have one pair of antennae, six jointed legs attached to the thorax, and a hard exoskeleton; most adults have one or two pairs of wings. Scientists call each body region a tagma (plural tagmata).

Insects have three body parts. That number is part of the definition of an insect, and it is the first thing to check when you meet a small animal with an exoskeleton: three parts and six legs means insect; two parts and eight legs means spider or another arachnid; many segments with many legs means a centipede or millipede.

Each body part is itself made of segments that fused during evolution. The head is a single capsule. The thorax has exactly three segments: the prothorax, mesothorax and metathorax. The abdomen has up to 11 segments in insects such as grasshoppers, though in many groups fewer are visible. On the 3D grasshopper you can see all three thoracic segments and count the abdominal segments along its back.

The insect head is a hard box that holds the brain and carries the sense organs and mouthparts. A grasshopper's head has the typical parts: one pair of antennae, two compound eyes made of thousands of tiny lenses, three small ocelli (simple eyes) and a set of chewing mouthparts.

Mouthparts vary more than any other head feature. Grasshoppers, beetles and ants have biting jaws (mandibles). Butterflies have a coiled drinking tube, the proboscis, made from part of the mouthparts. Whatever their shape, they are always on the head.

The thorax is the part of an insect's body that holds the legs. Its three segments each carry one pair of legs, which is why all adult insects have six legs. Wings grow only from the second and third thoracic segments (mesothorax and metathorax), never from the first, so an insect has at most two pairs of wings.

Each insect leg has the same segments: coxa, trochanter, femur, tibia and tarsus, ending in claws and often a sticky pad. Legs are shaped for different jobs: a grasshopper's hind legs are for jumping, and a honey bee's hind legs carry a pollen basket.

The abdomen is the back body part of an insect. It has no legs and no wings (except the fleshy prolegs of caterpillars, which are not true legs). It contains most of the gut, the heart, the organs that remove waste, and the ovaries or testes, and it has small breathing holes called spiracles along its sides.

Many insects carry useful tools at the end of the abdomen: an ovipositor for laying eggs (grasshoppers and crickets), a pair of cerci that feel air movements (grasshoppers, cockroaches), or a sting, which in bees, wasps and ants is a modified egg-laying organ. A grasshopper even hears with its abdomen: its eardrums are on the first abdominal segment.

Spiders are not insects. An insect has three body parts, six legs, one pair of antennae and usually wings; a spider has two body parts (cephalothorax and abdomen), eight legs, no antennae and no wings.

All of these animals are insects with a head, thorax, abdomen and six legs. What changes is the shape of each part.

Young children can learn insect body parts with three words and two numbers: head, thorax, abdomen; six legs; two antennae. A few ideas that work in class or at home:

• Build an insect from three balls of play dough, six pipe-cleaner legs and two pipe-cleaner antennae. Check that all the legs go on the middle ball.
• Sing "head, thorax, abdomen" to the tune of "Head, Shoulders, Knees and Toes", touching each part.
• Go on a bug hunt and count legs: six means insect, eight means spider.
• Rotate the 3D grasshopper above and switch off the head, thorax or abdomen to see each part on its own.

Print the worksheet below and write the correct word on each numbered line. The word bank is printed at the top. Answers: 1 antenna, 2 compound eye, 3 mouthparts, 4 head, 5 thorax, 6 abdomen, 7 leg, 8 wing.

Show the answer key

Every part of the 3D grasshopper, grouped by body region. Organs inside the body, such as the gut, heart and brain, are listed under "Organs inside".

Insects are the arthropods with a body in three parts (head, thorax and abdomen), one pair of antennae, six jointed legs attached to the thorax, and a hard outer covering, the exoskeleton. Most adult insects also have one or two pairs of wings. These features separate insects from other arthropods such as spiders, centipedes and crustaceans. (Australian Museum. Insects.)

The thorax is always made of three segments: the prothorax, mesothorax and metathorax. Each carries one pair of legs. Wings, when present, grow only from the mesothorax and metathorax, never from the prothorax. That is why an insect has at most two pairs of wings and why the legs and wings always attach to the middle body part. (NC State University, General Entomology: wings.)

Yes, every insect has a head, thorax and abdomen, but they are not always easy to see. In ants, bees and wasps the first abdominal segment (the propodeum) is joined to the thorax, forming a middle section called the mesosoma; the narrow 'waist' behind it, the petiole, is part of the abdomen. So the obvious waist of an ant is not the boundary between thorax and abdomen. (Wikipedia. Mesosoma.)

Larvae are another tricky case. A caterpillar has a head, a thorax with three pairs of true, jointed legs, and a long abdomen with fleshy prolegs. Prolegs are not true legs: they have no joints, work partly by fluid pressure, and grip with tiny hooks called crochets. So a caterpillar still has six true legs, even if it seems to have many more. (Wikipedia. Proleg.)

Wings vary even more. Flies have one pair of wings; their hindwings are reduced to small knobs called halteres, which act like gyroscopes for balance. Beetles' forewings are hardened covers (elytra). Many insects, such as worker ants, fleas and many stick insects, have no wings at all as adults. (Wikipedia. Halteres.)

A spider is an arachnid. Its body has only two parts: the cephalothorax, where head and thorax are fused, and the abdomen, joined by a slim waist called the pedicel. It has eight walking legs, all on the cephalothorax, usually eight simple eyes, fang-tipped jaws called chelicerae, a pair of pedipalps for touch and taste, and silk-spinning spinnerets on the abdomen. (Australian Museum. What is a spider?.)

Spiders have no antennae and no wings: the front pair of appendages are the pedipalps, which in males are also used for mating. Their simple eyes each have a single lens, unlike the compound eyes of insects. Most spiders breathe with book lungs, and many also have tracheae. (Wikipedia. Spider anatomy.)

Butterflies show the three parts clearly. The head carries large compound eyes, clubbed antennae and a coiled tube for drinking nectar, the proboscis, formed from the maxillae. The thorax has three pairs of legs, though brush-footed butterflies hold the front pair folded and look four-legged, and the second and third thoracic segments carry the scaled wings. Butterflies go through complete metamorphosis: egg, caterpillar, chrysalis, adult. (Wikipedia. Butterfly.)

Crickets are close relatives of grasshoppers but hear with eardrums on the tibiae of their front legs, have long thin antennae, and the female has a long, needle-like ovipositor. Male crickets sing by rubbing their leathery forewings together. Stick insects can be very long, up to 64 cm including the legs in one species, and many are wingless, but they still have the same three body parts. (Wikipedia. Cricket (insect).)

Ants have elbowed antennae and strong mandibles, and only the queens and males have wings. Honey bees and bumblebees carry pollen in a pollen basket (corbicula) on the outer side of each hind tibia. Ladybugs are small beetles: their red, spotted shell is a pair of hardened forewings (elytra) that cover the thin flying hindwings. (Wikipedia. Ant.)

To go from the three body parts to every organ inside, explore the grasshopper anatomy page.

For a grasshopper's internal organs, mouthparts and life cycle in detail, see grasshopper anatomy. For a step-by-step lab, see the grasshopper dissection guide.