Insect

Insects are the
predominant group of
arthropods (more than
800,000 species                      Scientific classification

discovered), and of    Kingdom: Animalia
the animals in
general. They may be   Phylum:  Arthropoda
found in nearly all    Class:   Insecta
environments except
for the oceans.                               Orders
Insects have           Archaeognatha (jumping bristletails)
segmented bodies,      Monura - extinct
divided into a head,   Thysanura (common bristletails)
thorax, and abdomen.   Mantophasmatodea
The head supports a    Subclass Pterygota
pair of sensory            Palaeodictyoptera - extinct
antennae as well as        Ephemeroptera (mayflies)
the mouth, the thorax      Odonata (dragonflies and damselflies)
has six legs, and the      Infraclass Neoptera
abdomen has excetory          Blattodea (cockroaches)
and reproductive              Mantodea (mantids)
structures. A few             Isoptera (termites)
small groups with             Zoraptera
similar body plans            Grylloblattodea
are united with the           Dermaptera (earwigs)
insects as the                Plecoptera (stoneflies)
Hexapoda. The true            Orthoptera (grasshoppers, crickets, katydids)
insects are                   Phasmatodea (walking sticks, timemas)
distinguished from            Embioptera (webspinners)
other forms in part           Superorder Hemipterodea
by having                        Psocoptera (booklice, barklice)
ectognathous, or                 Phthiraptera (lice)
exposed, mouthparts.             Hemiptera (true bugs)
                                 Thysanoptera (thrips)
There are 5,000               Superorder Endopterygota
dragonfly species,               Miomoptera - extinct
20,000 orthopteran,              Megaloptera (alderflies, etc.)
170,000 lepidopteran,            Raphidioptera (snakeflies)
82,000 hemipteran,               Neuroptera (net-veined insects)
350,000 coleopteran,             Coleoptera (beetles)
and 110,000                      Strepsiptera (twisted-winged parasites)
hymenopteran.                    Mecoptera (scorpionflies, etc.)
                                 Siphonaptera (fleas)
Most insects also                Protodiptera extinct
have two pairs of                Diptera (true flies)
wings, located on the            Trichoptera (caddisflies)
second and third                 Lepidoptera (butterflies, moths)
thoracic segments.               Hymenoptera (ants, bees, wasps, etc.)
They are the only
group other than the vertebrates to have developed flight, and this has
played an important part in their success. The winged insects, and their
secondarily wingless relatives, make up the Pterygota. Insect flight is not
very well understood, relying heavily on turbulent atmospheric effects. In
primitive insects it tends to rely heavily on direct flight muscles, which
act upon the wing.

More advanced flyers, which make up the Neoptera, generally have wings which
can fold over their back, keeping them out of the way when not in use. In
these, the wings are powered mainly by indirect flight muscles, which move
them by stressing the thorax. These muscles have adapted to contract when
stretched without nervous impulses, allowing the wings to beat much faster
than would be otherwise possible.

Insects hatch from eggs, and undergo a series of moults as they develop. In
most groups the young, called nymphs, are basically similar in form to the
adults, though the wings are not yet developed. This is called incomplete
metamorphosis. Complete metamorphosis distinguishes the Endopterygota, which
include many of the most successful insect groups. In these, the egg hatches
to produce a larva, which is generally worm-like in form and may be fairly
helpless. This in turn becomes a pupa, which is often sealed within a cocoon
or chrysalis, and undergoes considerable change in form before emerging as an adult.

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