Guide to the study of insects, and a treatise on those injurious and beneficial to crops: for the use of colleges, farm-schools, and agriculturists online

size. The wings of insects, then, are
simple expansions of the crust, spread
over a framework of horny tubes.
These tubes are really double, consist-
ing of a central trachea, or air tube,
inclosed within a larger tube filled with

blood, and which performs the functions of the veins. Hence
the aeration of the blood is carried on in the wings, and thus
they serve the double purpose of lungs and organs of flight.

The number and situation of these veins and their branches
(veinlets) are of great use in separating genera and species.
The t} T pical number of primary veins is five. They diverge
outward at a slight angle from the insertion of the wing, and
are soon divided into veinlets, from which cross veins are
thrown out connecting with others to form a net-work of veins
and veinlets, called the venation of the wing (Figs. 28, 29).
The interspaces between the veins and veinlets are called cells.
At a casual glance the venation seems very irregular, but in
many ingects is simple enough to enable us to trace and name
the veinlets. The five main veins, most usually present, are

FIG. 27. The semipupa of Bombus, the larva skin having been removed, show-
ing the two pairs of rudimentary wings growing out from the mesothorax (&), and
metathorax (?ra). n and the seven succeeding dots represent the eight abdominal
stigmata, the first one (n) being in the pupa situated on the thorax, since the first
ring of the abdomen is in this stage joined to the thorax. Original.



THE WINGS.



23



called, beginning at the costa, or front edge, the costal, subcostal,
tii tuibmedian, and internal, and sometimes the median




divides into two, making six
veins. The costal vein is un-
divided ; the subcostal and me-
dian are divided into several
1 (ranches, while the submedian
and internal are usually simple.

The venation of the fore-
wings affords excellent marks
in separating genera, but that
of the hind wings varies less,
and is consequently of less use.

The wings of many insects
are divided by the veins into
three well-marked areas; the
costal, median, and internal.
The costal area (Fig. 31 b) forms
the front edge of the wing and
is the strongest,
since the veins are
nearer together than
elsewhere, and thus
afford the greatest
resistance to the air Fig. 20.

FIG. 28. Fore and hind wings of a Butterfly, showing the venation. I. fore wing:
a, costal vein; 6, subco-t.il vein; 61, 62, 63, 64, 65, five subcostal veinlets; c, inde-
pendent vein (it is sometimes a branch of the subcostal, and sometimes of the me-
dian vein) ; rf, median vein ; d i, </-', r/3, d4, four median veinlets ; e, submedian vein ;
/, internal vein ; h, interno-median veinlet (rarely found, according to Doubleday,
except in Tapilio and Morpho) ; b and d are situated in the " diseal cell ; ' gi, g2, ft*,
the upper, middle', and lower discul veinlets. In the Bombycidie and many other
moths f/i and </-' are thrown off from the subcostal and median veins respectively,
meeting in the middle of the cell at (j~*. They are sometimes wholly absent.

II. The hind wing; the lettering and names of the veins and veinlets the same
as in the fore wing. Slit/litlif <'lt<ingedfr<>m Donh/cif/n/.

FKJ. 29. Fore wing of a Ilymenopterous insect. <*, costal vein; sc, subcostal
vein; ///, median vein; .sm, submedian vein; i, internal vein; c, 1,2,3, the first,
MToml, and third costal cells; the second frequently opaque and then called the
pt<-n>xiiijinn. .sv, 1, 2, 3, 4, the four subcostal cells; ?., 1, 2, .'$, 4, the median cells;
KIII, 1, -2, :5, the three submedian cells; /i,the internal cell; this is sometimes divided
into two cells, and the numbers of all but the costal cells is inconstant, the outer
row of celN I. t. :; brin.ir the lir.-t t<> disappear.

The r.mtui t ;i ; ic extends from c to c; the outer c,\\\capex; the outer edge extends
fi-om the apex (r) to it, and the hiiii-r edf/c extends from a, the inner angle, to the
insertion of the wing at i. Original. Figs. 30-32 from Scudder.




24



THE CLASS OF INSECTS.





Fig. 31.



during flight. The median area (Fig. 31 a) is the largest. It is
in the grasshoppers and crickets sometimes modified to form a
musical organ, being
drum-like, as in the
(Ecanthus (Fig. 30), or
rasp-like, as in Archyp-
tera (Fig. 3 la). The
internal area (c) is the
smallest, and less dis-
tinctly marked than the
two other regions ; the musical file-like or-
gan of Orchelimum vulgare, a common grass-
hopper (Fig. 32 d) is situated on. this area.

The limits of the edges of the wing vary
in almost every genus, and their comparative length afford
excellent generic characters. The front edge (Fig. 29) is called
the costal, its termina-
tion in the outer angle
of the wing is called
the apex; the outer edge
is situated between the
apex and the inner an-
gle, between which and
the base of the wing is
the inner, or internal,
edge. These distinc-
tions are of most use
in describing the butter-
Hies and moths.

The Appendages of
Fig. 31 a. i] lG Head. These organs
are divided into two groups,
the first of which comprise the
sensory organs, i. e. the ocelli,
eyes, and antennae, which are attached to the region in front
of the mouth, or preoral region of the head. The second
group consists of the sensorio-digestive appendages, combining
the power of finding and seizing the food and preparing it for
digestion. They are inserted behind the mouth and belong
to the pastoral region of the head.




THE APPENDAGES OF THE HEAD. 25

We will first describe the ocelli, which are theoretically the
most anterior organs of the head, ending with the basal appen-
dages, the labium (second maxilla?) being the hindermost.

The simple eye, Ocellus, or Stemma, is the simplest form of
the eye. Its most elementary form (seen in the larva of the
Bot-fly and the Cecidomyian larva of Miastor) is that of a brown
spot, or group of pigment-cells lodged under the skin and
against which a nerve-filament impinges. Over this spot New-
port states that the tegument is transparent and convex,
resembling a true cornea, or eye-lens. A well-developed
ocellus consists, according to Newport, of a "very convex,
smooth, single cornea, beneath which is a spherical crystalline
lens, resting upon the plano-convex surface of the expanded
vitreous humor, the analogue of the transparent cones of the
compound eyes." Miiller believes that the function of the ocelli
is the perception of nearer objects, while that of the compound
eyes is to sec more distant objects. The ocelli constitute the
only visual organs in the Myriapods (except Cermatia), the
Arachnida, and the larvae of many Six-footed Insects ; they
are usually from one to six on a side. In adult insects
they are generally three in number, and
are generally present except in the large
majority of Coleoptera. Their normal site
is in front of the eyes, but they are usually Fig. 33.

thrown back, during the growth of the insect, behind the eyes,
on the vertex, or topmost part of the head (Fig. 33).

The Compound Eyes are a congeries of simple eyes. During
the growth of the insect the simple eyes of the larva increase
in number, and finally coalesce to form the compound
ej-e, or compound cornea, the surface of which is
Fig. 34. very convex and protuberant in the predaceous insects,
or those requiring an extended field of vision.

The number of facets, or corneas, vary from fifty (in the Ant)
to 3,650, the latter number being counted by Geoffrey in the
eye of a Butterfly. These facets are usually hexagonal, as in
the Dragon-fly (Fig. 34), or, rarely, quadrangular.

FIG. 33. Ocelli of three species of Sand- wasps, Pompilus. From Cresson.
FIG. 34. Three hexagonal facets of the compound eye of a fossil Dragon-fly,
greatly magnified. From Dawson.





26 THE CLASS OF INSECTS.

The Antennae, (Figs. 35, 36) are inserted usually in the adult
insect between, or in front of the eyes, though normally the
ft antennary is posterior to the ophthalmic ring.
It is normally a long, filiform, slender, many-
jointed appendage, undergoing great changes
in form. When it is highly specialized, as in
Coleoptera and Hymenoptera, it is divided
into three parts, the basal or scape, the middle
or pedicel, and the terminal part or flagellum, Fl - 3G -
Fig. 35. or davola, which usually comprises the greater part of
the antenna.

It is believed by some that the sense of hearing is lodged
in the antennae, though Siebold has discovered an auditory
apparatus situated at the base of the abdomen of some, and
in the fore-legs of other species of Grasshoppers.

Mr. J. B. Hicks has made the latest studies on the auditory
apparatus. According to him "it consists first of a cell, sac,
or cavity filled with fluid, closed in from the air by a mem-
brane analogous to that which closes the foramen ovale in the
higher animals ; second, that this membrane is, for the most
part, thin and delicate., but often projects above the surface, in
either a hemispherical, conical, or canoe-shaped, or even hair-
like form, or variously marked ; thirdly, that the antennal nerve
gives off branches which come in contact with the inner wall of
the sacs ; but whether the nerve enters, or, as is most probable,
ends in the small internally projecting papilla which I have
shown to exist in many of these sacs, it is very difficult, to say.
The principal part of the nerve proceeds to these organs, the
remaining portion passing to the muscles, and to the roots of
the hairs, at least to those of the larger sort." On the other
hand, Lefebvre, Ley dig, and Gerstaecker regard this so-called
"auditory apparatus" as an organ of smell.

The antennae have also the sense of touch, as may readily be
observed in Ants, Bees, and the Grasshopper and Cockroach.
"The Honey-bee, when constructing its cells, ascertains their
proper direction and size by means of the extremities of these

FIG. 35. Filiform antenna of Ampliizoa. From Horn.

FIG. 36. A, lamellate antenna of a Lamellicorn Beetle; B, antenna of a Fly,
with the bristle thrown off from the terminal joint; C, bristle-like antenna of a
Dragon-fly, Libellula. From Sanborn.



THE APPENDAGES OF THE HEAD.



27



organs; while the same insect, when evidently affected by
sounds, keeps them motionless in one direction, as if in the act
of listening." (Newport.)

After cutting off one or both antennae of the June beetle,
Lachnosterna, the insect loses its power of directing its flight
or steps, wheeling about in a senseless manner. Dr. Clemens
observed that the Cecropia moth was similarly affected after
losing its antenna 1 .

The Mandibles (Fig. 37) are inserted on each side of the
mouth-opening. They usually consist of but a single joint,




Fig. 37.

representing probably the basal part of the ideal limb. This
part, however, is often subdivided by two longitudinal furrows
into three parts, each ending in a "tooth" of unequal size for
tearing and cutting the food. This tripartite form of the man-
dibles, to which attention has been called by Mr. Scudder, is
more fully carried out in the maxilla, where each portion is
highly specialized. The mandibles vary greatly in form and
size. The two cutting edges are usually opposed to each other,
or frequently overlap in the carnivorous forms. Their base is
often concealed by the clypeus
and labrum. Their motion is
transverse, being the reverse of
the motion of the jaws of Ver-
tebrates.

The Maxillce (Figs. 38 &, 39) are Fig. 39.
much more complicated organs than the mandibles. They are

FIG. 37. Different forms of mandibles. A, mandible of Cicindelapurpurea; B,
I'Jii/llo/iteni, a green grasshopper; C, Libellula trint<-itl<if<i; I), />>/></ macidata, or
paper-making Wasp ; K, " rostrum " or jointed sucker of the Bed-bug, Cimex lectu-
hi ri IIH, roiiMsting of mandibles, maxilla*, and labinm; F, proboscis, or sucker, of a
Mosquito, Culex, in which the mandibles are long and bri-tle-like. From Sanborn.
(., mandible of Amphi~<<t ; II, mandible of Acratus, a genus of Cockchafers. l-'nnn
//>//(.

Kii. 38. a, mentum and labial palpi; 6, one maxilla, with its palpus, of Acra-
tus. From Horn.

I to, :;'.>. Maxilla of Amphizoa, with the two lobes (stipes and lacinia), and the
palpifer bearing the four-jointed palpus. From Horn.





28 THE CLASS OF INSECTS.

inserted on the under side of the head and just behind the
mouth. The maxilla consists of a basal joint, or cardo,
beyond which it is subdivided into three lobes, the stipes, or
footstalk ; the palpifer, or palpus-bearer ; and the lacinia, or
blade. The stipes forms the outer and main division of the
organ. The lacinia is more membranaceous than the other
parts, and its upper surface is covered with fine hairs, jand
forms a great part of the side of the mouth. It is divided
into two lobes, the superior of which is called the galea, or
helmet, which is often a thick double-jointed organ edged with
stiff hairs, and is used as a palpus in the Orthoptera and many
Coleoptera. The inferior lobe is attached to the internal angle
of the lacinia. It terminates in a stiff minute claw, and is
densely covered with stout hairs. The maxillary palpi are
long, slender, one to four-jointed organs, very flexible and sen-
sitive.

The maxillae vary greatly in the different groups. Their office
is to seize the food and retain it within the mouth, and also to
aid the mandibles in comminuting it before it is swallowed.
This function reminds us of that of the tongue of vertebrate
animals.

The labium, or second maxillce (Fig. 40), is placed in front of
the gula, which" forms the under part of the head, and is bounded
a on each side by the gence, or cheeks, and
r\r\ posteriorly by the occiput. The genae are
I J bounded laterally by the epicranium and

1 I 'the under side of the eyes. In front are

rig. 40. situated the basal parts of the labium, or

second maxillae, which embraces the submentum and mentum
(or labium proper). The labial palpi are inserted into the
mentum, but often the latter piece is differentiated into two,
the anterior of which takes the name of palpiger, called by
Dr. Leconte (Smithsonian Miscellaneous Collections) the ligula,
and the palpi originate from them. The ligula is the front
edge of the labium, being the piece forming the under lip.
It is often a fleshy organ, its inner surface being continuous

FIG. 40. Ligula and labial palpi of Amphizoa, an aquatic beetle. It is quadrate
and without paraglossae ; a, mentnm of the same, being deeply incised, and with a
tooth at the bottom of the excavation. -From Horn.




THE APPENDAGES OF THE HEAD. 29

with the soft membrane of the mouth. In the Bees, it is enor-
mously developed and covered with soft hairs. It is often
confounded with the palpiger. In Hydrous it is divided into
two lobes. In most of the Carabidm and Bees it is divided
into three lobes, the two outer ones forming the paraglossce
(Fig. 4lw), and acting as feelers, while the middle, usually
much longer, forms the lingua, or tongue, being the continuation
of the ligula. In the bees, where
the ligula is greatly developed,
it performs the part of the tongue
in Vertebrates, and aids the max-
illae in collecting nectar and
pollen.

The roof of the mouth is
formed by the labrum and the
epipharynx (Fig. 42 c), a small
fleshy tubercle concealed beneath
the labrum. It is seen in the
bees on turning up the labrum.
It probably corresponds to the
"labellum" of Schiodte. The
labrum (Fig. 41 e) is usually
transverse and situated in front
of the clypeus (Fig. 416). The
shield-like clypeus is the broad, rig. 41.

visor-like, square piece forming usually the front of the head.
Behind it is the clypeus posterior, or supra-clyj>eus, a subdivision
of the clypeus, and especially observable in the Hymenoptera.
The epicranium forms a large part of the head ; it is bounded
posteriorly by the occiput, on the sides by the eyes, and in
front by the clypeus, and though usually described as a
single piece, is really composed of several. The ocelli often
appear to be situated upon it, though in reality they are placed
upon a distinct piece or pieces. The "epicranial suture" is the
line of junction of the two "procephalic lobes" (Huxley).

FIG. 41. Front view of the head of a bee, Anthcphora. a, compound eyes; c,
three simple eyes, situated upon the 'epicranium; b, clypeus; e, labrum; d, an-
tenna^;/, mandibles: V, maxilla?; /, maxillary palpi; I, palpiler; j, labial palpi; m,
paraglossae; k, ligula. From Newport.

3*




30



THE CLASS OF INSECTS.



- 42 -



(These lobes will be explained farther on when speaking
of their development in the embryo.) Behind the epicra-

nium is the occiput,
or base of the head.
It belongs to the la-
bial, or second max-
illary segment, and
helps to form a com-
plete ring, articulat-
ing with the thorax.
e It is perforated by a

foramen to afford a
connection between
the interior of the
head and thorax. It
is sometimes, as in
many Coleoptera, Or-
thoptera, and Hemip-
tera, elongated be-
hind and constricted,
thus forming a "neck." It will be seen beyond, that the
labrum and ctypeus are in the embryo developed from a
"tongue-like process whose inferior part eventually becomes
the labrum, while superiorly it sends a triangular process (the
rudiment of the clypeus) into the interval between the proce-
phalic lobes." * This part (i. e. the clypeus and labrum) is the
most anterior part of the head, and in the embryo, as in the
adult, is normally situated in front of the ocelli, and may be
compared with the "anal plate," or eleventh tergite, of the
larva, f

FIG. 42. Side view of the front part of the head, together with the mouth-
parts of the Humble-bee (Bombus). , clypeus covered with hairs ; b, labrum ;
c, the fleshy epipharynx partially concealed by the base of the mandibles (r/);
e, lacinia, or blade of the maxilla?, with their two-jointed palpi (/) at the base ; j, the
labium to which is appended the ligula (,9); below are the labial palpi; h, the two
basal joints, being greatly enlarged; k, the compound eyes. Original.

* These lobes are folded back upon the top of the base of the head, and they
seem to form the tergal portion of the hypothetical, elemental ring, or rings, to which
they respectively belong, and do not seem to us to be the sternal portion, as sug-
gested by Huxley, for they are apparently developed in front of the mouth-opening,
and form the roof of the mouth.

t " Lastly, there are certain parts developed singly in the median line in the Artie-
ulata. Of this nature are the frontal spines of Crustacea, their telson, and the sting




THE MUSCULAR SYSTEM. 31

In describing Insects the vertex, or crown, of the head is the
highest part ; and the front is the part usually in front of the
insertion of the antennae.

THE MUSCULAR SYSTEM lies just beneath, and is continuous
with the integument. It consists of numerous "distinct isola-
ted straight fibres, which are not gathered into bundles united
by common tendons, or covered by aponeuroses [or tendinous
sheaths] to form distinct muscles, as in the Vertebrata, but
remain separate from each other, and only in some instances
are united at one extremity by tendons." (Newport.) These
minute fibres form layers, which Newport regards as separate
muscles. "Each fibre is composed of a great number of very
minute fibrillae, or fasciculi of fibrillee," and has been observed
by AVagner and Newport to be often striated as in Vertebrates.
The muscular system is simplest in the lower insects and the
larvae of the higher forms, and is more complex in the head
than elsewhere, and more complex in the thorax than in the
abdomen. These minute muscles are exceedingly numerous.
"Lyonnet, in. his immortal work on the anatomy of the larva
of Cossus ligniperda, found two hundred and twenty-eight dis-
tinct muscles in the head alone, and, by enumerating the fibres
in the layers of the different segments, reckoned 1,647 for the
body, and 2,118 for the internal organs, thus making together
3,993 muscles in a single larva. In the larva of Sphinx ligus-
tri we have found the muscles equally numerous with those
discovered by Lyonnet in the Cossus." (Newport.)

The muscular system corresponds to the jointed structure of
insects, as do the other internal systems of organs. Of the
muscles belonging to a single ring, some stretch from the front
edge of one segment to the front edge of the next, and others

of the Scorpion, whose mode of development appears to be precisely similar to
that of a telson. In the same category \ve must rank the labrum in front of the
mouth, which in the Crustacea (at least) appears to be developed from the sternum
of the antennary, or third somite, the metastoma (or so called labium, or lingua)
of Crustacea, and the lingua of Jnxecta, behind the oral aperture.

" However much these appendages may occa>ionally simulate, or play the part
of appendages, it is important to remember, that, morphologically, they are of a
very different nature, and that the confusing them with true appendages nm>t
tend completely to obscure the beautiful relations which obtain among tin- dil-
ferent classes of the Articulata." Huxley, Linuajan Transactions, vol. xxii.
London.



32 THE CLASS OF INSECTS.

to the hinder edge ; there are also sets of dorsal and ventral
muscles going in an oblique or vertical course. The muscles
are either colorless and transparent, or yellowish white ; and
of a soft, almost gelatinous consistence. In form they are
simply flat and thin, straight, band-like, or pyramidal, barrel,
or feather-shaped. They act variously, as rotators, elevators,
depressors, retractors, protrusors, flexors, and extensors.

The muscular power of insects is enormous. The Flea will
leap two hundred times its own height. Certain beetles can
support enormous weights. Newport cites the case of Geo-
trupes stercorarius which is "able to sustain and escape from
beneath a pressure of from twenty to thirty ounces, a prodi-
gious weight when it is remembered that the insect itself does
not weigh even so many grains." Some beetles have been
known to gnaw through lead-pipes, and the Stag-beetle of
Europe, Lucanus cervus, has, as stated by Mr. Stephens,
gnawed "a hole an inch in diameter through the side of an
iron canister in which it was confined."

"The motions of the insect in walking as in flying are
dependent, in the perfect individual, entirely upon the thoracic
segments, but in the larva chiefly upon the abdominal. Al-
though the number of legs, in the former is always six, and in
the latter sometimes so many as twenty-two, progression is
simple and easy. Miiller states (Elements of Physiology, p.
970, Translation) that on watching insects that move slowly
he has distinctly perceived that three legs are always moved at
one time, being advanced and put to the ground while the
other three propel the body forwards. In perfect insects, those
moved simultaneously are the fore and hind feet on one side,"
and the intermediate foot on the opposite ; and afterwards the
fore and hind feet on that side, and the middle one on the
other, so that, he remarks, in two steps the whole of the legs
are in motion. A similar uniformity of motion takes place
in the larva, although the whole anterior part of the body is
elevated and carried forwards at regular distances, the steps of
the insect being almost entirely performed by the 'false,' or
abdominal legs."

"Inflight the motions depend upon the meso- and meta-
thoracic segments conjointly, or. entirely upon the former. The



THE NERVOUS SYSTEM. 33

sternal, episternal, and epimeral pieces, freely articulated
together, correspond in function with the sternum, the ribs,
and the clavicles of birds.* The thorax is expanded and con-
tracted at each motion of the
wings, as in birds and other ani-
mals, and becomes fixed at each
increased effort as a fulcrum or
point of resistance upon which
the great muscles of the wings
are to act, thus identifying this
part of the body in function as
in structure with that of other ani-
mals." (Newport.)

THE NERVOUS SYSTEM. In its
simplest form the nervous system
consists of two longitudinal cords,
each with a swelling (nerve-knot,
or ganglion,) corresponding to
each segment (Fig. 43). This
cord lies on the ventral side of the
bod} r , but in the head it passes
upwards, sending a filament from
each side to surround the oesoph-
agus, f As in the Vertebrates,
the nervous cord of insects is
composed of two distinct columns Fi - 43 -

of fibres placed one upon the other. "The under or external