a: any of numerous small invertebrate (without a backbone) animals (as spiders or centipedes) that are more or less obviously segmented 

b: any of a class (Insecta) of arthropods (as bugs or bees) with well-defined head, thorax, and abdomen, only three pairs of legs, and typically one or two pairs of wings.
from The Merriam Webster Online Dictionary

Insect Anatomy

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