PROF. CHARLES A. KOFOID AND
MRS. PRUDENCE W. KOFOID
Parts I, II. Ill,
Price, 50 cents a Part,
THE STUDY OF INSECTS,
AND A TREATISE ON THOSE
INJURIOUS AND BENEFICIAL TO CROPS.
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GUIDE TO THE STUDY OF INSECTS.
THE CLASS OF INSECTS.
THAT branch of the Animal Kingdom known as the ARTICU-
LATA, includes all animals having the body composed of rings
or segments, like short cylinders, which are placed successively
one behind the other. Cuvier selected this term because he
saw that the plan of their entire organization, the essential
features which separate them from all other animals, lay in the
idea of articulation, the apparent joining together of distinct
segments along the line of the body. If we observe carefully
the body of a Worm, we shall see that it consists of a long
cylindrical sac, which at regular intervals is folded in upon
itself, thus giving a ringed (annulated, or articulated) appear-
ance to the body. In Crustaceans (crabs, lobsters, etc.)
and in Insects, from the deposition of a peculiar chemical
sul (stance called chitine, the walls of the body become so
hardened, that when the animal is dead and dry, it
ivadily breaks into numerous very perfect rings.
Though this branch contains a far greater number of
species than any other of the animal kingdom, its myr-
iad forms can all be reduced to a simple, ideal, typical
iigure ; that of a long slender cylinder divided into
numerous segments, as in Fig. 1, representing the larva
of a Fly. It is by the unequal development and the
various modes of grouping them, as well as the differ-
ences in the number of the rings themselves, and also in Fi #- *
the changes of form of their appendages, i.e. the feet, jaws,
antennae, and wings, that the various forms of Articulates are
FIG. 1. Worm-like larva of a Fly, Scenopinus. Original.
THE CLASS OF INSECTS.
Articulated animals are also very distinctly bilateral, i. e. the
body is symmetrically divided into two lateral halves, and
not only the trunk but the limbs also
show this bilateral symmetry. In a less
marked degree there is also an antero-
posterior symmetry, i. e. each end of
the body is opposed, just as each
side of the body is, to the other.*
The line separating the two ends is,
however, imaginary and vague. The
antennae, on the anterior pole, or head,
are represented by the caudal, or anal,
stylets (Fig. 2), and the single parts
on the median line of the body corre-
spond. Thus the labrum and clypeus
are represented by the tergite of the
eleventh segment of the abdomen.
Flg .2. In all Articulates (Fig. 3) the long,
tubular, alimentary canal occupies the centre of the body ; above
it lies the "heart," or dorsal vessel, and below, upon the under
side, rests the nervous system. b
The breathing apparatus, or
"lungs," in Worms consists of /|
simple filaments, placed on the
front of the head ; or of gill-like
processes, as in the Crustaceans,
which are formed by membran-
ous expansions of the legs ; or,
as in the Insects (Fig. 4), of delicate tubes (tracheae), which
* Professor Wyman (On Symmetry and Homology in Limbs, Proceedings of the
Boston Society of Natural History, 1867) has shown that antero-posterior symmetry
is very marked in Articulates. In the adjoining figure of Jcera (Fig. 2) the longi-
tudinal lines illustrate what is meant by bilateral symmetry, and the transverse
lines " fore and aft" symmetry. The two antero-posterior halves of the body are
very symmetrical in the Crustacean genera Jcera, Oniscus, Porcellio, and other
Crustacea, and also among the Myriapods, Scutigera, Polydesmus, " in which the
limbs are repeated oppositely, though with different degrees of inequality, from the
centre of the body backwards and forwards." "Leuckart and Van Beneden have
shown that MjrtSis has an ear in the last segment, and Schmidt has described an eye
in the same part in a worm, Amphicora." From Wyman.
FIG. 3 represents an ideal section of a Worm. / indicates the skin, or mus-
cular body-wall, which on each side is produced into one or more fleshy tubercles,
usually tipped with bristles or hairs, which serve as organs of locomotion, and
THE CLASS OF INSECTS. 3
ramify throughout the whole interior of the animal, and con-
nect with breathing pores (stigmata) in the sides of the body.
They do not breathe through the mouth as do the higher ani-
mals. The tracheae and blood-vessels, follow closely the same
a Fig. 4.
course, so that the aeration of the blood goes on, apparently,
over the whole interior of the body, not being confined to a
single region, as in the lungs of the vertebrate animals.
Thus it is by observing the general form of the body-walls,
and the situation of the different anatomical systems, both in
relation to themselves and the walls of the body, or crust,
which surrounds and protects the more delicate organs within,
that we are able to find satisfactory characters for isolating, in
our definitions, the articulates from all other animals.
We shall perceive more clearly the differences between the
three classes of Articulates, or jointed animals, namely,
the W o K M s , CRUSTACEANS, and INSECTS, by examining
often as lungs. The nervous cord () rests on the floor of the cylinder, sending a
filament into the oar-like feet (/), and also around the intestine or stomach (6), to a
supplementary cord (d), which is situated just over the intestine, and under the
heart or dorsal vessel (c). The circle c and e is a diagram of the circulatory sys-
tem ; c is the dorsal vessel, or heart, from the side of which, in each ring, a small
vessel is sent downwards and around to e, the ventral vessel. Original.
Vic,. 4. An ideal section of a Bee. Here the crust is dense and thick, to which
strong muscles are attached. On the upper side of the ring the wings grow out,
\vhih- the legs are inserted near the under side. The trachea? (d] enter through the
atif/mn, or breathing pore, situated just under the wing, and their branches sub-
divide and are distributed to the wings, with their Jive principal veins as indicated
4 THE CLASS OF INSECTS.
their young stages, from the time of their exclusion from the egg,
until they pass into mature life. A more careful study of this
period than we are now able to enter upon would show us how
much alike the young of all articulates are at first, and how
soon they begin to differ, and assume the shape characteristic
of their class.
Most Worms, after leaving the egg, are at first like some
infusoria, being little sac-like animalcules, often ciliated over
nearly the entire surface of the infinitesimal body.
Soon this sac-like body grows longer, and con-
tracts at intervals ; the intervening parts become
unequally enlarged, some segments, or rings,
formed by the contraction of the body- walls,
greatly exceeding in size those next to them ; and it thus
assumes the appearance of being more or less equally ringed,
a as in the young Terebella (Fig. 5), where the
ciliae are restricted to a single circle surrounding
the body. Gradually (Fig. 6) the cilise disap-
e pear and regular locomotive organs, consisting
of minute paddles, grow out from each side ;
feelers (antennae), jaws, and eyes (simple rudi-
mentary eyes) appear on the few front rings
of the body, which are grouped by themselves
j C into a sort of head, though it is difficult, in a
large proportion of the lower worms, for un-
skilled observers to distinguish the head from
Thus we see throughout the growth of the
worm, no attempt at subdividing the body
into regions, each endowed with its peculiar
^functions ; but only a more perfect system of
Fi 6 rings, each relatively very equally developed,
in the figure, also to the dorsal vessel (c), the intestine (6), and the nervous cord (a).
The tracheae and a nervous filament are also sent into the legs and to the wings.
The tracheae are also distributed to the dorsal vessel and intestine by numerous
branches which serve to hold them in place. Original.
FIG. 5. Young Terebella, soon after leaving the egg. From A. Agassiz.
FIG. (5 represents the embryo of a worm (Autolytus cornutus) at a later stage
of growth, a is the middle tentacle of the head ; e, one of the posterior tentacles ;
6, the two eye-spots at the base of the hinder pair of feelers ; c is one of a row of
oar-like organs (cirri) at the base of which are inserted the locomotive bristles,
THE CLASS OF INSECTS. 5
but all becoming respectively more complicated. For example,
in the Earth-worm (Lumbricus) , each ring is distinguishable into
an upper and under side, and in addition to these a well-
marked side-area, to which, as for example in marine worms (e. g.
Nereis) , oar-like organs are attached. In most worms eye-spots
appear on the front rings, and slender tentacles grow out, and
a pair of nerve-knots (ganglia) are apportioned to each ring.
In the Crustaceans, such as the fresh-water Crawfish (Asto-
cus), as shown by the German naturalist Rathke ; and also in
the earliest stages of the Insect, the body at once assumes a
worm-like form, thus beginning its embryonic life from the goal
reached by the adult worm.
The young of all Crustaceans (Fig. 7) first begin life in the
egg as oblong flattened worm-like bodies, each end of the body
being alike. The young of the lower Crustaceans, such as the
Barnacles, and some marine forms like the Jcera and some
lowly organized parasitic species inhabiting the gills of
fishes, are hatched as microscopic embryos which would readily
be mistaken for young mites (Acarina). In the higher Crus-
taceans, such as the fresh-water Crawfish, the
young, when hatched, does not greatly differ
from the parent, as it has passed through the
worm-like stage within the egg.
Fig. 7 represents the }*oung of the fresh-
water Lobster (Crawfish) before leaving the
egg. The body is divided into rings, ending
in lobes on the sides, which are the rudiments
of the limbs, b is the rudiment of the eye- Fig. 7.
stalk, at the end of which is the eye ; a is the fore antennae ;
c is the hind antennae ; d is one of the maxilla-feet ; e is the
first pair of true feet destined in the adult to form the large
1 'claw." Thus the eye-stalks, antennae, claws, and legs are
moulded upon a common form, and at first are scarcely distin-
with the cirri serving as swimming and locomotive organs ; rf, the caudal styles, or
tail-feelers. In this figure we see how slight are the differences between the
feelers of the head, the oar-like swimming organs, and the caudal filaments ; we
can easily see that they are but modifications of a common form, and all arise
from the common limb-bearing region of the body. The alimentary canal, with
the proventriculus, or anterior division of the stomach, occupies the middle of the
body; while the mouth opens on the under side of the head. From A. Agatsiz.
FIG. 7. Embryo of the Crawfish. From Rathke.
6 THE CLASS OF INSECTS.
guishable from each other. Here we see the embryo divided
into a head-thorax and a tail.
It is the same with Insects. Within the egg at the dawn of
life they are flattened oblong bodies curved upon the yelk-
mass. Before hatching they become more cylindrical, the
limbs bud out on the sides of the rings, the head is clearly
demarked, and the young caterpillar soon steps forth from the
egg-shell ready armed and equipped for its riotous life.
As will be seen in Fig. 8, the legs, jaws, and antennae are
first started as buds from the side of the rings, being simply
elongations of the body-wall,
which bud out, become larger,
and finally jointed, until the
| a? buds arising from the thorax or
abdomen become legs, those
from the base of the head be-
come jaws, while the antennae
and palpi sprout out from the
front rings of the head. Thus
while the bodies of all articulates
are built up from a common em-
bryonic form, their appendages, which are so diverse, when we
compare a Lobster's claw with an Insect's antenna, or a Spider's
spinneret with the hinder limbs of a Centipede, are yet but
modifications of a common form, adapted for the different uses
to which they are put by these animals.
FIG. 8. A Caddis, or Case-fly (Mystatides) in the egg, with part of the yolk
(a:) not yet inclosed within the body-walls, a, antennae ; between a and b the mandi-
bles; ft, maxilla; c, labium; rf, the separate eye-spots (ocelli), which afterwards in-
crease gi'eatly in number and unite to form the compound eye. The "neck" or
junction of the head with the thorax is seen at the front part of the yolk-mass; e,
the three pairs of legs, which are folded once on themselves ;/, the pair of anal legs
attached to the tenth ring of the abdomen, as seen in caterpillars, which form long
antenna-like filaments in the Cockroach and May-fly, etc. The rings of the body are
but partially formed; they are cylindrical, giving the body a worm-like form.
Here, as in the other two figures, though not so distinctly seen, the antennae, jaws,
and last pair of abdominal legs are modifications of but a single form, and grow
out from the side of the body. The head-appendages are dii'ected forwards, as
they are to be adapted for sensory and feeding purposes ; the legs are directed
downwards, since they are to support the insect while walking. It appears that the
two ends of the body are perfected before the middle, and the under side before the
upper, as we see the yolk-mass is not yet inclosed and the rings not yet formed
above. Thus all articulates differ from all vertebrates in having the yolk-mass
situated on the back, instead of on the belly, as in the chick, dog, or human em-
bryo. From Zaddach.
THE CLASS OF INSECTS. . 7
The Worm is long and slender, composed of an irregular
number of rings, all of very even size. Thus, while the size of
the rings is fixed, their number is indeterminate, varying from
twenty to two hundred or more. The outline of the body is a
single cylindrical figure. The organs of locomotion are fleshy
filaments and hairs (Fig. 3, /) appended to the sides.
In one of the low intestinal worms, the Tape-worm (Tcenia),
each ring, behind the head and "neck," is provided with organs
of reproduction, so that when the body becomes broken up
into its constituent elements, or rings (as often occurs naturally
in these low forms for the more ready propagation of the
species, since the young are exposed to many dangers while
living in the intestines of animals), they become living inde-
pendent boings which "move freely and somewhat quickly
like Leaches," and until their real nature was known they
were thought to be worms. This and other facts prove, that,
in the Worm, the vitality of the animal is very equally dis-
tributed to each ring. If we cut off the head or tail of some
of the low worms, such as the Flat Worms (Planaria, etc.), the
pieces will become a distinct animal, but an Insect or Crab
sooner or later dies when deprived of its head or tail (abdomen).
Thus, in the Worm the vital force is very equally distributed
to each zoological element, or ring of the body ; no single
part of the body is much honored above the rest, so as to sub-
ordinate and hold the other a i>
parts in subservience to its
peculiar and higher ends in
the animal economy.
The Crustacean, of which
the Shrimp (Fig. 9) is a
typical example, is com-
posed of a determinate
number (21) of rings which Fig. n.
are gathered into two regions ; the head-thorax (cephalo-
thorax) and hind -body, or abdomen. In this class there
is a broad distinction between the anterior and posterior ends
of the body. The rings are now grouped into two regions,
and the hinder division is subordinate in its structure and
FIG. 9. A Shrimp. Pandalus annulicornia. a, cephalothorax ; 6, abdomen.
8 THE CLASS OF INSECTS.
uses to the forward portion of the body. Hence the nervous
power is transferred in some degree towards the head ; the
cephalothorax containing the nervous centres from which nerves
are distributed to the abdomen. Nearly all the organs perform-
ing the functions of locomotion and sensation reside in the front
region ; while the vegetative functions, or those concerned
in the reproduction and nourishment of the animal, are mostly
carried on in the hinder region of the body (the abdomen) .
The typical Crustacean cannot be said to have a true head,
in distinction from a thorax bearing the organs of locomotion,
but rather a group of rings, to which are appended the organs
of sensation and locomotion. Hence we find the appendages
of this region gradually changing from antennae and jaws to
foot-jaws, or limbs capable of eating and also of locomotion ;
they shade into each other as seen in Fig. 9. Sometimes the
jaws become remarkably like claws ; or the legs resemble jaws
at the base, but towards their tips become claw-like ; gill-like
bodies are sometimes attached to the foot-jaws, and thus, as
stated by Professor J. D. Dana in the introduction to his great
work on the Crustacea of the United States Exploring Expedi-
tion, the typical Crustaceans do not have a distinct head, but
rather a ' ' head-thorax " (cephalothorax) .
When we rise a third and last step into the world of Insects,
we see a completion and final developm'ent of the articu-
late plan which has been but obscurely hinted at in the two
lowest classes, the Worms and Crustaceans. Here we first meet
with a true head, separate in its structure and functions from
the thorax, which, in its turn, is clearly distinguishable from
the third region of the body, the abdomen, or hind-body.
These three regions, as seen in the Wasp (Fig. 10), are each
provided with three distinct sets of organs,
each having distinct functions, though all are
governed by and minister to the brain force,
now in a great measure gathered up from the
Fig. 10. posterior rings of the body, and in a more
concentrated form (the brain being larger than in the lower
articulates) lodged in the head.
Here, then, is a centralization of parts head wards ; they are
FIG. 10. Philanthus ventilabris Fabr. A Wood- wasp. From Say.
COMPOSITION OF THE INSECT-CRUST. 9
brought as if towards a focus, and that focus the head, which
is the meaning of the term " cephalization," proposed by Pro-
fessor Dana.* Ring distinctions have given way to regional
distinctions. The former characterize the Worm, the latter
the Insect. In other words, the division of the body into three
parts, or regions, is in the insect, on the whole, better marked
than the division of any one of those parts, except the abdo-
men, into rings.
COMPOSITION OF THE INSECT-CRUST. Before describing the
composition of the body-wall, or crust, of the Insect, let us
briefly review the mode in which the same parts are formed in
the lower classes, the Worms and Crustaceans. We have seen
that the typical ring, or segment (called by authors zoonule,
zoonite, or somite, meaning parts of a body, though we prefer
the term artliromere, denoting the elemental part of a jointed
or articulate animal), consists of an upper (tergite), a side
(pleurite), and an under piece (sternite). This is seen in its
greatest simplicity in the AYorm (Fig. 2), where the upper and
ventral arcs are separated by the pleural region. In the Crus-
tacean the parts, hardened by the deposition of chitine and
therefore thick and unyielding, have to be farther subdivided to
secure the necessary amount of freedom of motion to the body
and legs. The upper arc not only covers the back of the ani-
mal, but extends down the sides ; the legs are jointed to the
epimera, or flanks, on the lower arc ; the episternum is situated
between the epimerum and sternum ; and the sternum, form-
ing the breast, is situated between the legs. In the adult, there-
fore, each elemental ring is composed of six pieces. It
should, however, be borne in mind that the tergum and ster-
* In two papers on the Classification of Animals, published in the American
Journal of Science and Arts, Second Series, vol. xxxv, p. 65, vol. xxxvi, July, 1863,
and also in his earlier paper on Crustaceans, "the principle of cephalization is
shown to be exhibited among animals in the following ways :
1. By a transfer of members from the locomotive to the cephalic series.
2. By the anterior of the locomotive organs participating to some extent in ce-
3. By increased abbreviation, concentration, compactness, and perfection of
structure, in the parts and organs of the anterior portion of the body.
4. By increased abbreviation, condensation, and perfection of structure in the
posterior, or gastric and caudal portion of the body.
5. By an upward rise in the cephalic end of the nervous system. This rise
reaches its extreme limit in Man."
10 THE CLASS OF INSECTS.
num each consist, in the embryo, of two lateral parts, or halves,
which, during development, unite on the median line of the
body. Typically, therefore, the crustacean ring consists pri-
marily of eight pieces. The same number is found in all insects
which are wingless, or in the larva and pupa state ; this applies
also to the Myriapods and Spiders.
In the Myriapoda, or Centipedes, the broad tergum overlaps
the small epimera, while the sternum is much larger than in
the Spiders and Insects. In this respect it is like the broad
flat under-surface of most worms. Hence the legs of the
Centipede are inserted very far apart, and the "breast," or
sternum, is not much smaller than the dorsal part of the crust.
In the Julus the dorsal piece (tergum) is greatly developed
over the sternum, but this is a departure from what is ap-
parently the more typical form of the order, i. e. the Centipede.
In the Spiders there is a still greater disproportion in size
between the tergum and the sternum, though the latter is very
large compared with that of Insects. The epimera and episterna,
or side-pieces of the Spiders, are partially concealed by the
over-arching tergum, and they are small, since the joints of the
legs are very large, Audouin's law of development in Articu-
lates showing that one part of the insect crust is always
developed at the expense of the adjoining part. In the Spider
we notice that the back of the thorax is a single solid plate
consisting originally of four rings consolidated into a single
hard piece. In like manner the broad solid sternal plate
results from the reunion of the same number of sternites cor-
responding, originally, to the number of thoracic legs. Thus
the whole upper side of the head and thorax of the Spider is
consolidated into a single hard horny immovable plate, like
the upper solid part of the cephalothorax of the Crab or
Shrimp. Hence the motions of the Spiders are very stiff com-
pared with those of many Insects, and correspond to those of
The crust of the winged insect is modified for the per-
formance of more complex motions. It is subdivided in so
different a manner from the two lower orders of the class, that
it would almost seem to have nothing in common, structurally
speaking, with the groups below them. It is only by examin-
COMPOSITION OF THE INSECT-CRUST. 11
ing the lowest wingless forms such as the Louse, Flea,
Podura, and Bark-lice, where we see a transition to the Or-
ders of Spiders and Myriapods, that we can perceive the plan
pervading all these forms, uniting them into a common
A segment of a winged six-footed insect (Hexapod) consists
typically of eight pieces which we will now examine more
leisurely. Figure 12 represents a side-view of
the thorax of the Telea Polyphemus, or Silk- pt
worm moth, with the legs and wings removed.
Each ring consists primarily of the tergum, the
two side-pieces (epimerum and episternum) and
the sternum, or breast-plate. But one of these
pieces (sternum) remains simple, as in the lower orders. The
tergum is divided into four pieces. They were named by Au-
douin going from before backwards,
the prcescutum, scutum, scutellum,
The scutum is invariably present
and forms the larger part of the
upper portion (tergum) of the tho-
rax ; the scutellum is, as its name
indicates, the little shield so promi-
nent in the beetle, which is also
uniformly present. The other two tr te c" tr c'" tr
pieces are usually minute and