‘Bug’ is a name that is given to many invertebrate animals, or animals that do
not have a backbone. Mostly it is used to describe those invertebrate animals
that have a segmented, hard outer skeleton and jointed legs. These animals are
called arthropods and include insects, spiders, mites, scorpions, millipedes,
centipedes and crustaceans.
Bugs are Arthropods
Arthropods belong to the group or phylum Arthropoda, and their tough,
armour-like covering is called an exoskeleton. The bodies of arthropods
comprise several main divisions. Insects have three body divisions, spiders
have two body divisions, while centipedes and millipedes have many small body
divisions. Arthropods also have jointed legs.
The number of legs and body divisions are used to classify bugs into smaller
classes: Insects, Arachnids, Myriapods and Crustaceans.
Insects belong to the class Insecta. All adult insects have three body
divisions, six legs and a pair of feelers, or antennae. The three body
divisions of an insect are the head, thorax and abdomen. The head carries the
eyes, the mouthparts and the antennae. The central body region, or thorax, has
the legs attachedand wings, if there are any. The rear section, or
abdomen, carries the insect's digestive and reproductive organs.
Spiders are not insects. They belong to the class Arachnida, which also includes
scorpions, ticks and mites. The bodies of arachnids are divided into two
distinct regions. The head and thorax are fused together in the front section,
called the cephalothorax, which attaches to the abdomen. All arachnids have
eight legs attached to their cephalothorax, and all lack antennae and wings.
Centipedes and millipedes belong to the group Myriapoda. They have a pair of
antennae, many small body segments and many legs. Centipedes and millipedes
differ in that centipedes have one pair of legs per body segment while
millipedes have two.
Arthropods in the class Crustacea are mostly marine creatures, such as crabs and
crayfish. While they are not generally called bugs, other crustaceans are.
Slaters or woodlice are crustaceans. They have a variable number of legs (at
least ten), two pairs of antennae, three body sections and limbs that branch in
Invertebrate animals that are soft-bodied and without legs are not usually
considered to be bugs. Worms belong to a phylum called Annelida. Slugs and
snails belong to the phylum Mollusca.