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So far we've learned a little bit about Arthropods and seen how all of the major groups of insects are related to each other. By now you may be wondering, "Just what is an insect, anyway?".

In spite of the enormous amount of variation within insects, they all have two things in common:

1. Insects have three, and only three pairs of legs.
2. Insects have three major body sections;
   • head,
   • thorax,
   • abdomen.

These features distinguish insects from the other arthropod groups. All other arthropods have more than three pairs of legs, and have either more or less than three body sections.

Insects seem to be put together inside-out and upside-down. Their skeletons are on the outside of their bodies, their heart runs in a thin line down their backs, and their nerve cords are found along their bellies. Some even have more than one "brain"!

The insect's external anatomy is only slightly less bizarre than its internal anatomy. Moving from front to back:

• head – has eyes and antennae for sensing the environment, and an assortment of mouthparts for consuming food.
• thorax – a three-segmented section that is the center for locomotion. Legs and wings (for those insects which have wings) are attached to the thorax.
• abdomen – this many-segmented section is the center for digestion and reproduction. Breathing holes called spiracles are found in a line along the sides of the thorax and abdomen. These lead to a branching network of tiny tubes throughout the insect's body that supply oxygen to every cell.

World of Insects

Life without | So many of them | Secrets of their success | Family Tree | Insect orders | Anatomy