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Insects and other arthropods

Insect: a three-part body

Parts of the body of a dragonfly.
Photo: Insectarium de Montréal (Gilles Murray)
Libellula julia, Québec, Canada.
  • Libellula julia, Québec, Canada.
  • Libellula julia, Québec, Canada.
  • Chironomus anthracinus, Denmark.
  • Phalacrognathus muelleri, Australia.
  • Extatosoma tiaratum.
  • Syrphidae, Québec, Canada.
  • Mantis religiosa, Québec, Canada.

The basic model of an adult insect is simple: It has a body divided into three parts (head, thorax and abdomen), three pairs of legs and two pairs of wings. Insects have adopted different shapes, colours and all kinds of adaptations, but their body is almost always composed of these common elements.

The head

An adult insect’s head has two large compound eyes made up of several different facets, which are small organs of sight that allow the insect to detect small movements. One single eye can be composed of thousands of facets. Many species of insects also have two or three simple eyes on top of their head called ocellae. Ocellae are generally the only type of eyes that larvae have. The head also has a pair of articulated antennae that vary greatly in size and shape according to species. They enable insects to perceive smells and vibrations. Mouthparts are usually on the underside of the head.

The thorax

The thorax is the insect’s "locomotive," in a way. It is composed of three segments, each with a pair of legs. The second and third segments also have wings, whose size, shape and texture vary from one group or order of insects to another. Mose insects have four wings. However, others, such as silverfish, lice and fleas, do not. Flies only have two wings.

The abdomen

The abdomen contains the digestive tract, the heart, part of the breathing apparatus and the reproductive organs. Like the other parts of the body, it is covered with an exoskeleton that protects it against shocks and dryness.

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