or metamorphosis. After they hatch,
they begin life in a form called a larva. It
is completely unlike the adult form. For
instance, a larva may look more like a
worm than an insect. The larva eats a
great deal and molts several times. It
then enters a resting stage in a form
Scientists who
study insects
are called
entomologists.
As an insect grows, it sheds its
exoskeleton from time to time.
The insect then forms a new,
larger exoskeleton.
BRITANNICA STUDENT ENCYCLOPEDIA Insect 139
called a pupa. The pupa’s body changes
greatly during this stage. It develops all
of the features of the adult, including
wings and legs. When this stage is complete,
the adult emerges from the pupa.
Bees, butterflies, and moths are some
insects that develop this way.
Helpful and Harmful Insects
Many insects are helpful to humans and
other animals. Some kinds eat other
insects that are pests. Insects are also
food for other animals. Some insects
produce valuable products such as
honey, silk, and dyes.
Many flowering plants depend on
insects such as bees to help them reproduce.
As insects feed on flowers, they
spread a substance called pollen from
the male parts of flowers to the female
parts. This allows the female parts to
make seeds.
Insects can also be pests. Some insects
sting or bite people or other animals.
Many insects can spread diseases to
people, other animals, and plants. And
some harmful insects eat crops, wood,
clothing, and carpet.
#More to explore
Animal • Metamorphosis • Parasite
• Pollen • Protective Coloration
Internal-
Combustion
Engine
Engines powered by internal combustion
run cars, airplanes, lawn mowers,
and other machines. Combustion means
“burning.” Fuel, usually gasoline, burns
inside an internal-combustion engine to
make it work. Compared to earlier
steam engines, internal-combustion
An internal-combustion engine goes through four strokes: intake, compression, combustion
(power), and exhaust. As the piston moves during each stroke, it turns the crankshaft.
140 Internal-Combustion Engine BRITANNICA STUDENT ENCYCLOPEDIA
engines are lighter and more powerful.
They made automobiles possible.
How Internal-Combustion
EnginesWork
In many internal-combustion engines a
moving part called a piston slides up and
down in a cylinder. Most car engines
have four to eight cylinders. Valves at
the top of a cylinder let in fuel and air
and allow burned fuel to escape. Also at
the top of the cylinder is a spark plug.
The spark plug ignites, or sets fire to,
the fuel.
As the engine runs, it goes through a
cycle of four strokes: intake, compression,
combustion, and exhaust. During
the intake stroke, a valve opens and the
cylinder fills with fresh fuel and air.
Next, during the compression stroke,
the piston moves toward the top of the
cylinder. The piston creates pressure by
squeezing the fuel and air into a smaller
and smaller space with both valves
closed. When the piston can go no
higher, a spark from the spark plug
ignites the fuel. Then, in the combustion
stroke (also called the power
stroke), an explosion of burning fuel
forces the piston back down. During the
exhaust stroke, a second valve opens and
the piston rises up. It pushes the hot
gases from the burned fuel out of the
cylinder.
The piston is connected to an engine
part called the crankshaft, which is outside
of the cylinder. As the piston moves,
it turns the crankshaft. The crankshaft is
connected to other parts that run the
machine.
Other types of internal-combustion
engines are two-stroke, diesel, rotary,
and turbine. Two-stroke engines go
through two strokes instead of four. In
diesel engines hot air, not a spark, causes
the fuel to burn. Rotary engines have a
spinning part instead of a piston. Turbine
engines, used in jet airplanes, continually
pull air into the front of the
engine. The exhaust leaves out the back
of the engine.
Although internal-combustion engines
are powerful, most of the energy from
the burning fuel is lost as heat. The
engines must be cooled by water or air
so they do not overheat.
History
In 1860 the French inventor Etienne
Lenoir built the first internalcombustion
engine, which he used in a
car. In 1876 the German engineer
Nikolaus Otto built a four-stroke
engine. Otto’s invention was the first
successful internal-combustion engine.
Rudolf Diesel, another German engineer,
developed the diesel engine in
about 1890.
#More to explore
Airplane • Automobile • Combustion
Internet
The Internet is a network, or system,
that connects millions of computers
worldwide. It was one of the greatest
inventions of the 1900s. Today the
Internet helps many people communicate,
work, learn, and have fun.
Two-stroke
engines are
popular for
lawn mowers
and small
motorcycle
engines.
BRITANNICA STUDENT ENCYCLOPEDIA Internet 141
In the 1960s the U.S. government, businesses,
and colleges worked together to
make a system that would let computers
across the United States share information.
They created an early form of the
Internet called ARPANET in 1969. In
1971 electronic mail, or e-mail, was
invented as a way to send a message
from one computer to another.
By the mid-1970s many groups of computers
