When the insects have been collected, mounted or preserved, and labeled,
the next step is to identify or name them. This is no easy task, because
there are so many different kinds of insects. In the whole world there
are well over 1 million different kinds and in Illinois alone probably
20,000 different kinds.
The identification of insects is simplified somewhat by the fact that
many species are closely related and can be classified into a number of
major groups. Insects as a whole constitute what is called a _class_ of
animals, the Insecta. The crabs, shrimps, and their allies constitute a
class called the Crustacea; the snakes, turtles, lizards, and their
allies constitute another class called the Reptilia; and so on. The
entire insect class is divided into _orders_, such as the Coleoptera, or
beetles, the Diptera, or flies, and the Siphonaptera, or fleas. Each of
these orders may contain several dozen to 25,000 different kinds of
insects in North America alone. These orders are divided into
_families_, each of which may contain one species to many thousands of
species. The family names always end in _-idae_, as in Pentatomidae, the
name for the stink bugs. The families are divided into _genera_ (the
plural for genus), and the various _species_ (the word is the same for
both singular and plural) or kinds are placed in the genera.
The house fly bears the name _Musca domestica_ Linnaeus; this means that
the species name is _domestica_, that the name was first applied to the
species by Carolus Linnaeus (known as the describer of the insect or the
author of the name), and that the species _domestica_ is in the genus
_Musca_. The genus _Musca_ belongs to the family Muscidae, which, in
turn, belongs to the order Diptera of the class Insecta.
Scientists may decide that a certain species belongs in another genus.
When the species is transferred from the genus in which it was
originally described to another genus, the name of the author is placed
in parentheses. For example, the chinch bug was originally described by
Thomas Say in the genus _Lygaeus_ and had the name _Lygaeus leucopterus_
Say. Later the species _leucopterus_ was transferred to the genus
_Blissus_, and Say's name was placed in parentheses, thus: _Blissus
leucopterus_ (Say).
In the process of growth, insects go through a series of interesting
stages. When the immature insect reaches a certain size, its outside
skin covering or cuticle will not stretch further and the insect then
acquires a larger cuticle by a process called _molting_.
Molting consists of a definite sequence of steps: (1) A goodly portion
of the inside layer of the cuticle is dissolved by special glands
situated among cells immediately below the cuticle; (2) the cells under
the cuticle then exude material which forms a new cuticle beneath what
is left of the old cuticle; (3) when the new cuticle is completely
formed, the insect breaks a slit in the old cuticle, crawls out of it,
and leaves it behind in the form of a cast skin; (4) the insect goes
through many contortions, during which the soft parts of the new cuticle
are stretched to a larger size than the corresponding parts of the old
one; (5) the cuticle becomes set and unstretchable almost immediately,
and the insect resumes its normal activities. During the molting
process, the hard plates of each new cuticle are formed a size larger
than the corresponding parts of the old cuticle, and the soft parts are
stretched a size larger than the old. When the insect resumes its normal
activities immediately after a molt, the soft parts of the cuticle fall
into a large number of pleats or folds between the hard parts. As the
insect grows larger following a molt, the body can lengthen by the
unfolding of these pleated areas.
The stages of the insect between molts are called _instars_. Among the
different orders of insects the number of instars in the life history
may vary, and various instars may have different forms. These
characteristics of molting and instars are therefore important items in
the classification of insects.
[Illustration: Fig. 18.--A family tree representing current ideas of
how the orders of insects evolved. The early, primitive orders are
at the bottom of the tree and the later, more highly specialized
orders at the top. It is customary to list the orders of insects in
this sequence, from primitive to specialized.]
_Larvae and pupae evolved_
MOTHS, BUTTERFLIES
FLIES
ANTS, BEES, WASPS
BEETLES
CADDISFLIES
LACEWINGS
FLEAS
SCORPIONFLIES
ALDERFLIES
_Wing folding evolved_
BUGS
GRASSHOPPERS
THRIPS
EARWIGS
LICE
TERMITES
BARKLICE
MANTIDS
COCKROACHES
ZORAPTERANS
STONEFLIES
_Wings evolved_
DRAGONFLIES
MAYFLIES
_Primitive wingless insects_
SPRINGTAILS
PROTURANS
SILVERFISH
CAMPODEIDS
BRISTLETAILS
The insect orders are arranged in a classification based on the sequence
in which the orders are believed to have evolved, fig. 18. Measured by
geological time, insects are among the oldest of land animals, having
first evolved from an earlier, centipede-like ancestor about 400 million
years ago. The first insects had no wings and differed from the
many-legged centipede-like creatures of that time chiefly in having only
three pairs of functional legs. The legs were situated on the three
segments immediately behind the head; the three distinctive segments are
together called the _thorax_. The part of the body behind the thorax is
called the _abdomen_. In contrast to this arrangement, a centipede has a
pair of legs on each of its many segments for the whole length of the
body. The slow evolutionary change from such a many-legged ancestral
form to a typical insect undoubtedly occurred by a gradual enlargement
and strengthening of the front three pairs of legs and a reduction of
the legs posterior to these. Evidence supporting this idea is found in
insect embryos, which normally have rudimentary leg structures on the
abdominal segments, and in some of the extremely primitive insects,
which have rudimentary legs called styli on some segments of the
abdomen. The result of this evolutionary development is a body having
the front part, the thorax, specialized for locomotion and the back
part, the abdomen, serving chiefly as a container for the vital organs,
such as those of the digestive and reproductive systems.
[Illustration: Fig. 19.--Diagram of a typical adult winged insect.
This illustrates many of the parts that are useful in identifying
these creatures. (Drawing adapted from R. E. Snodgrass.)]
HEAD
Antenna
Ocelli
Eye
THORAX
1
Front leg
2
Front wing
Middle leg
3
Hind wing
Hind leg
ABDOMEN
Cercus
Five existing orders of insects, all found in Illinois, are
representative of the primeval wingless insects. In all five, the
individual molts at intervals, even after becoming adult and sexually
mature, and the old and young are extremely similar. Three of the
primitive orders, the campodeids, proturans, and springtails, belong to
a specialized early branch of the insect family tree, fig. 18, in which
the cheeks have grown forward to form a pouch surrounding the
mouthparts. In the other two primitive orders, the bristletails and
silverfish, the mouthparts are in a normally exposed condition, but the
legs are larger and the insects are rapid runners.
From one of these ancestral, silverfish-like insects arose a form in
which wings evolved. In birds and bats, the wings are converted front
legs with membranes or feathers attached to form planing or flying
surfaces. In insects, on the other hand, the wings are outgrowths of the
edge of the body where the sides and top come together, fig. 19. The
wings probably began as side flanges of the thorax and permitted a
certain amount of planing. Whatever their origin, two pairs of flying
wings did evolve, one pair on the second and another on the third
segment of the thorax.
In the early winged forms, the wings could not be folded back in repose
over the body, but were held out from the sides like airplane wings or
together above the body like sails. Two living orders of insects
represent this type, the mayflies and the dragonflies. In both of these,
as in other winged insects, the wings form as small pads during the
early stages of the individual; then at a final molt they are unfolded
as functional units. In the mayflies, one more molt occurs after the
wings are formed; in this molt the old outside covering of the wings is
shed along with that of the rest of the insect. In all other winged
insects no molt occurs after the wings are formed and the individual
becomes sexually mature.
Insects having erect, nonfolding wings were abundant some 300 million
years ago. Fossil remains of many of these early forms have been found
in the Mazon Creek area in Illinois, fig. 20.
From one of these early winged types a form evolved in which the wings
could be folded compactly over the body; this form gave rise to a great
many of our present-day insects. In the first insects that evolved from
this form the wing pads of the immature stages, called nymphs, grew as
external and often inconspicuous flaps held close to the body. These
primitive insects comprise three distinctive groups, characterized
mainly by differences in leg and body structure. One group contains the
cockroaches, grasshoppers, and their allies; the second includes only
the stoneflies; and the third group includes the barklice, true lice,
bugs, and their allies.
[Illustration: Fig. 20.--An insect fossil from an iron nodule or
concretion found at Mazon Creek, Illinois; hind leg of an ancestral
mayfly, _Lithoneura mirifica_ Carpenter. Actual length of wing about
one-half inch. This fossil represents an insect which lived during
the Pennsylvanian period, about 250 million years ago. (Photograph
courtesy of Illinois State Museum.)]
One of the primitive lines of wing-folding insects, possibly an offshoot
from the base of the barklouse-bug line, evolved into a distinctly
different type in which the wing pads of the immature stages developed
internally and appeared as external pads only in the stage before the
adult form. The type was characterized by a marked difference of
appearance between the various stages of the life history; these stages
have been given distinctive names. The first immature stage, which is
without external wing pads, is called a _larva_; the single stage with
the external wing pads is called the _pupa_; and the final winged,
sexually mature stage is called the _adult_. The larva is essentially a
growing stage, the pupa is a quiescent stage of internal reorganization,
and the adult is the _egg_-producing stage. This type of insect
gradually gave rise to the orders which now contain the largest number
of species, including the beetles, moths, and flies. In many lines of
this neuropteroid branch, as it is called, the larva has become adapted
to a mode of life quite different from that of the adult. Many fly
larvae, for example, live in rotting organic material or live as
parasites within the tissues of other kinds of insects, whereas the
adult flies often feed at flowers on pollen and nectar, visiting the
site of the larval habitat only to lay eggs. As a result of this type of
evolution, members of the neuropteroid orders exhibit many bizarre and
complicated life histories.
Occasionally certain groups of winged insects evolved new types which
had small wings or were wingless. The new types resulted because the
groups were subjected to environmental situations where wings were of
little survival value. Within the groups, individuals with smaller wings
were favored; they survived and reproduced in greater numbers than other
individuals. Over many generations then, the wings in some groups became
very small or were entirely lost. Sometimes this loss of wings occurred
in all the adult forms, as in the fleas. Individuals of only one sex may
be wingless, as are the females of two Illinois moths, one of which
produces bagworms and the other cankerworms. In two groups, the termites
and the ants, a wingless worker or soldier caste is produced; in these
groups the normal adults which swarm and reproduce are fully winged.
These winged forms establish new colonies.
Wings were lost in the evolutionary development of two insect groups
that became parasites of warm-blooded animals, both birds and mammals.
Each of these insect groups developed into a large, distinctive order.
One order, the true lice, evolved from the barklice, and the other, the
fleas, evolved possibly from a primeval fly group. Although these two
orders, the lice and the fleas, are without wings, the structure of
their bodies and their life histories provide adequate testimony of
their evolutionary affinities.
The following synopsis of Illinois insects treats the various kinds in
the sequence in which we believe they evolved, from the extremely
primitive bristletails to the highly specialized flies.