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The Oldest Insects
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The oldest insect is known from only a single fossil of
Eopterum devonicum from Russia. This winged insect
lived more than 350 million years ago in the Devonian period,
which is more than 100 million years before the first dinosaurs.
By comparison, Homo sapiens (human beings) first appeared about
100,000 years ago.
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Many different kinds of winged insects arose during the
Carboniferous period about 300 million years ago. This was the
age of amphibians, ferns, and coal-forming swamps. Most of
these early insects are now extinct, and only cockroaches have
survived to the present time.
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Most of the orders of insects were present by the time of the
Jurassic period, about 180 million years ago. Plants were being
eaten by Orthoptera, Hemiptera, Coleoptera as well as by some
Dinosaurs. Another 60 million years or more passed until the
earliest Lepidoptera appeared during the Cretaceous period.
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Number of Species
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Scientists have described and given scientific names to about
920,000 species of insects in the world, which represents
almost 85% of all known animal species. For comparison,
only about 4,000 of the known animal species are mammals, man
being one of these. There are more species of dragonflies than
mammals, and almost as many species of cockroaches (3,500
species). There are about 9,000 species of birds, but almost
twice as many species of butterflies.
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Most insects have never been given scientific names. It is
estimated that there are 20-30 million species of insects on
the earth at present. In a good year, taxonomists throughout
the world describe and name about 2,000 species of insects.
At this rate of naming new species, it will take 10,000 years
to describe and name 20 million species. Unfortunately, many
of these 20-30 million species of insects will become extinct
before they are named because of habitat loss and other
environmental problems.
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Thirty orders of insects are listed in
Basic Facts: Insect Orders.
More than one-third of the named species of insects are in one
order, the Coleoptera (beetles), which includes about 350,000
species. The next largest orders are Lepidoptera (butterflies
and moths), with 165,000 species; and Hymenoptera (wasps and
bees), with 103,000 species. The order Diptera (flies)
includes about 120,000 species. These four orders, Coleoptera,
Lepidoptera, Hymenoptera, and Diptera, include more than 80
per cent of the named species of insects.
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In the United States and Canada, about 92,000 species of
insects have been named. It is not known how many of these
species occur in any one state or in even a small areas within
a state. A list of insects in New York, published in 1928,
included more than 15,000 species, but hundreds of species have
been discovered there since that time. States such as
California, Arizona, Texas, and Florida probably far exceed
New York in the total number of species of insects.
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Abundance of Insects
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No one knows for sure how many individuals of insects are
present in a large area. Estimates have been made by counting
the number of individuals in small areas, such as a square
foot or cubic foot, and multiplying this number times the total
square or cubic feet in a larger area. Based on such samples,
it is well known that people are surrounded by millions of
individuals of insects and other arthropods, usually without
being aware of their presence.
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A person steps upon thousands of insects whenever walking
outdoors. In an oak forest in Pennsylvania, researchers
counted the number of arthropods in leaf litter and soil in
samples that were one foot square and three inches deep.
They found an average of 9,759 arthropods per square foot.
Based on these counts, the researchers estimated that there
were more than 425 million soil and litter arthropods per acre
(43,560 square feet) of forest. How many would be in a
square mile, which includes 640 acres? In this study, mites
(Class Arachnida) were the most abundant kind of animal,
averaging more than 294 million per acre. Springtails (Class
Hexapoda, Order Collembola), which are closely related to insects,
averaged 119 million. Insects and other arthropods averaged 11 million per acre.
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The number of insects and other arthropods floating and flying
through the air are phenomenal. A researcher named P. A.
Glick collected insects in special traps that were placed on
airplanes flying over Tallulah, Louisiana. He collected
30,033 specimens in the air, including wingless insects and
spiders that were air blown. He calculated that a cubic mile
of air, starting 50 feet above the ground, contained an
average of 25,000,000 insects and other arthropods.
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Some species of insects can develop large swarms when they
become adults. The Rocky Mountain locust was a major pest in
the United States in the late 1800's, but it is now considered
to be extinct. An observer in Nebraska reported that one swarm
averaged a half mile in height (some locusts were higher than a
mile) and was 100 miles wide and 300 miles long. The swarm
moved about 5 miles an hour in the air and continued to pass
for six hours. Based on an estimate of 27 locusts per cubic
yard, he calculated that the swarm had more than 124 billion
locusts.
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Rates of Reproduction and Development
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Insects are capable of high rates of reproduction. Curtis
Sabrosky provided an example in 1952 of the reproductive
potential in house flies. Beginning with one pair of house
flies in April, there would be a total of
191,000,000,000,000,000,000 flies by August if all the
descendants of this pair lived and reproduced normally.
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Glenn Herrick found that the cabbage aphid had an average of
41 offspring per female and that the aphid had 16 generations
from April to October. If all the descendants of one female
aphid lived, there would be 1,560,000,000,000,000,000,000,000
aphids by the end of the summer.
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Alfred Emerson found that some termite queens can lay 6,000 to
7,000 eggs a day and that the queen can live more than 15 years.
He found one colony of the South American Nasutitermes surinamensis
that had 3 million termites. Other termite queens have been
found laying an egg every two seconds, but it was not
determined how long the queen could maintain this rate of
reproduction. Many of the termite young become food for other
animals, and this is one reason so many eggs are laid.
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Rather than lay many eggs, some insects obtain many offspring
from a single egg. This unusual form of reproduction is
termed polyembryony, in which two or more young result from a
single egg. One egg divides into two, similar to how
identical twins originate in humans. Some parasitic wasps
have eggs that keep on dividing, resulting in as may as 1,500
to 2,500 young from a single egg.
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The life span of insects, from egg to adult, usually falls
between two weeks and eight months. Immature stages are
usually longer than the adult stages. Some aphids can go
through a whole generation (from one stage in the life cycle
to the same stage in the offspring) in less than five days.
In contrast, some periodical cicadas require 17 years to
develop. Some insects can have slow development because of
environmental conditions. Wood boring beetles in the
families Cerambycidae and Buprestidae have been known to take
40-50 years to develop in dry wood.
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Amount of Food
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The number of aphids eaten by a ladybird beetle larva depends
on the size of the larva. An average sized ladybird beetle
larva after its fourth molt will eat about 50 aphids each day.
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The European red ant, Formica polyctena, lives in
colonies that may include 2-3 million predaceous individuals.
A single colony of this ant in Germany was calculated to
gather as much as two pounds of insect food every day. The
ants are active for about 200 days, and one colony may kill and
consume 400 pounds of insects every year. In the Italian Alps,
more than one million nests of the red ant have been found.
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Swarms of the desert locust, Schistocerca gregaria, may
cover hundreds of square miles at one time. There may be 300
million locusts weighing 500 tons in the air above each square
mile. When swarms of locusts descent upon the land, all the
green vegetation in a field can be stripped in less than an
hour.
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An average honey bee hive requires about 220 pounds of nectar
and 55 pounds of pollen every year.
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The caterpillar of the polyphemus moth, Antheraea
polyphemus, can eat 86,000 times its weight at birth in
little less than two months.
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Size of Insects
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The smallest insect known is a fairyfly, Megaphragma
caribea, a parasitic wasp which occurs on the island of
Guadeloupe. This wasp measures 0.007 inch long and is small
enough to fly through the eye of a needle.
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The smallest beetle is a feather-winged beetle, Nanosella
fungi (family Ptiliidae). This beetle, which feeds on
spores of fungi, measures 0.01 inch long.
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The smallest Lepidoptera are moths in the family Nepticulidae.
These moths have larvae that mine the insides of leaves.
Several species of these moths have wingspans of less than
0.16 inches.
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The smallest flies are biting midges in the family
Ceratopogonidae. Also known as punkies or no-see-ums, these
flies range in length from 0.04 to 0.25 inches.
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The largest insects ever known to exist were the prehistoric
dragonflies that lived 200 million years ago. Fossils of this
dragonfly have a wingspan of 30 inches and body length of 18
inches.
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The heaviest insect in the world is the African goliath beetle,
Goliath goliath, which weighs 3.5 ounces. This is
about the same weight as 33 pennies.
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The longest insect in the world is the tropical stick insect
Pharnacia serratipes, which reaches a length of more
than 14 inches.
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The Hercules emperor moth, Coscinocera hercules, of
Australia and New Guinea has wings with the largest surface
area of any insect. The surface area of the extended wings
covers more than 110 square inches.
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Other Facts and Numbers
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Records for the fastest flier, the smallest eggs, fastest wing
beat, most instars, and other subjects can be found at the
University of Florida Book of Insect Records.
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Selected References
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Arnett, Ross H. Jr. American Insects, A Handbook of the
Insects of America North of Mexico. New York:
Van Nostrand Reinhold Company, 1985.
Borror, D.J., Triplehorn, C. A., and Johnson, N. F. An
Introduction to the Study of Insects. Saunders College
Publishing, 1989.
Chauvin, Remy. The World of an Insect. New York:
World University Library, McGraw-Hill Publishers, 1967.
Evans, H.E. Life on a Little-known Planet. New York:
E.P.Dutton & Company, Inc., 1966.
Stefferud, A., Ed. Insects. The Yearbook of Agriculture.
United States Department of Agriculture. Washington, D. C.:
U.S. Government. Printing Office, 1952.
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