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

column, which is nearest to the exterior of the body, is that in
which the ganglia, or enlargements, are situated. The upper
one, or that which is internal and nearest to the viscera, is
entirely without ganglia, and passes directly over the ganglia
of the under column without forming part of them, but in very

* Bennet on the Anatomy of the Thorax in Insects, and its Function during
Flight. Zoological Journal, vol. i, p. :',!U.

tThe brain of insects is formed of several pairs of ganglia, corresponding,
probably, to the number of primitive segments composing the head. The nervous
conl is thus, in the head, massed together and compacted to form a brain.

\ '!<;. 43. Nervous System of Corydaltts cornutus. a, "cerebrum ;" b, "cere-
brellum;" c, thoracic ganglia, which distribute a nerve to each leg; d, eight pairs
of abdominal ganglia. The dotted lines represent the wings. From Leidy.




34 THE CLASS OF INSECTS.

close approximation to them." Newport also believes that the
ganglionless upper, or internal, column of fibres is analogous
to the motor column of Vertebrata, while the external, or under
one, corresponds to the sensitive column, thus representing the
cerebro-spinal system of the Vertebrata.

From each pair of ganglia are distributed special nerves to
the various organs. In the larva of Sphinx the normal num-
ber of double ganglia is thirteen, and the nervous cord of the
Neuroptera and other lowly organized and attenuated forms of
insects corresponds in the main to this number. In the adult
insect, especially in the Coleoptera, Diptera, Lcpidoptera, and
Hymenoptera, the three thoracic ganglia are fused together,
following the fusion and general headwise development of the
segments of the tegument. Besides the central nervous cord,
corresponding to the spinal cord of the Vertebrates, there is a
vagus, or visceral nerve, representing the sympathetic nerve of
higher animals. This nerve "arises, in the larva, from the
anterior part of the cerebrum, and, forming a ganglion on the
upper surface of the pharynx, always passes backward beneath
the brain, along the middle line of the oesophagus." In its
microscopic structure the nervous cord, like that of Vertebrata,
consists of a central "white" substance, and an outer or peri-
pheral part, the "gray" substance.

In the embryo the ganglia are very large and close together,
the commissures, or connecting filaments being very short, and
small in proportion.

ORGANS OF NUTRITION. These consist of the alimentary canal
and its appendages, or accessory glands (Fig. 44). We have
already treated of the external appendages (mouth-parts)
which prepare the food for digestion. The simplest form of
the alimentary canal is that of a straight tube. In the larva
of Stylops and the sedentary young of Bees, it ends in a blind
sac, as they live on liquid food and expel no solid excretions.
When well developed, as in the adult insect, it becomes a long
convoluted thick muscular tube, subdivided into different parts
which perform different functions- and have distinct names,
taken from analogous organs in the vertebrate animals. This
digestive tube is composed of three coats, the outer, or peri-



ORGANS OF NUTRITION.



35



toneal; the middle, or muscular; and the inner, or mucous. The
mucous coat is variously modified, being plaited or folded ; or,

c d e h f h

^




as in the Orthoptera and carnivorous Coleoptera, it is solidified
and covered with rows of strong horny teeth, forming a soil of
gizzard. The alimentary canal is held in place by retractor
muscles, but principally by exceedingly numerous branches of
the main tracheae.

This canal (Fig. 45) is subdivided into the mouth and pha-
rynx, the oesophagus, supplementary to which is the crop, or
" sucking stomach" of Diptera, Lepidoptera, and Hymenoptera ;
theproventriculus, or gizzard ; thevcntriculus, or true stomach,
and the intestine, which consists of the ileum, or short intes-



FlG. 44. Anatomy of Sphinx ligustri. m, ?', q, the nervous cord resting on
the floor of the body ; at c, the ganglia form a brain-like organ, much larger than
the ganglia of the thorax (m) and abdomen (q). From the brain is sent off the
subo2sophageal nerve which surrounds the gullet into which the food is conveyed
by the maxilla?, or spiral tongue (), which, when at rest, is rolled up between the
labial palpi (6).

From the nervous cord is also thrown off a pair of nerves to each pair of legs
(as at n, o,p) and a branch, rf, is sent off from above, distributing nerves to the
muscles of flight.

The heart, or dorsal vessel (e,/), lies just beneath the median line of the body,
and is retained in place by muscular bands (as at /) as well as by small tracheal
branches.

The alimentary canal (h,j, #), forms a straight tube in the head and thorax; h,
the crop, or sucking stomach, which opens into the (esophagus; j, the true, chyle-
forming stomach, which contracts posteriorly, and thon dilates near its anal outlet
into a cloaca (indicated at g, but not distinctly, as it is concealed by the numerous
urinary vessels). The urinary vessels also indicated at g, form long tubes (which
correspond to the kidneys of Vertebrates), opening into the pyloric end of the
stomach. The position of the testes (fc) is the same as that of the ovary, and the
dotted line I shows the course of the efferent duct (vas deferens) and also of the
oviduct of the female.

The numerals indicate the number of segments of the body, which in the Lepi-
doptera, consists of twenty, the 21st, or llth abdominal, being absent. From
Newport.



36 THE CLASS OF INSECTS.

tine, and the colon and rectum. The latter part, as well as the
crop and proventriculus, is sometimes absent.

Of the appendages of the canal, the first
are the salivary glands, which are usually
long simple tubes, which in the larva, ac-
cording to Newport, form the silk vessels.
They " empty themselves by a single duct
through the spinneret on the floor (labium)
of the mouth." In the Ant-lion (Myrmeleon)
the silk is spun from "a slender telescopic-
like spinneret, placed at the extremity of
its body," and Westwood also states that the
larva of Clirysopa spins a cocoon "from the
spinneret, at the extremity of the body."

These silk glands when taken out of the
larva, just as it is about ready to transform,
are readily prepared as "gut" for fish-lines,
etc., by drying on a board.

In the Bees these glands are largely de-
veloped to produce a sufficient amount of
salivary fluid to moisten the dry pollen of
Fig. 45. flowers, before it enters the oesophagus.
"Bee-bread" consists of pollen thus moistened and kneaded
by the insect. The Honey-bee also dissolves, by the aid of the
salivary fluid, the wax used in making its cells. Newport
believes this fluid is alkaline, and forms a solvent for the other-
wise brittle wax, as he has seen this insect "reduce the per-
fectly transparent thin white scales of newly secreted wax to
a pasty or soapy consistence, by kneading it between its man-
dibles, and mixing it with a fluid from its mouth, before apply-
ing it to assist in the formation of part of a new cell."

Insects have no true liver; its functions being performed
"by the walls of the stomach, the internal tunic of which is
composed of closely-aggregated hepatic cells." (Siebold.) In
the Spiders and Scorpions, however, there is a liver distinct
from the digestive canal. In the Spiders it is very large,
enveloping most of the other viscera.

FIG. 45. Alimentary tube of Corydalus cornutus. a, oesophagus; 6, proven-
triculus; c, ventriculus; d, large intestine; e, urinary tubes; /, coacum; g, testis or
ovary. From Leidy.




THE CIRCULATORY SYSTEM. 37

Siebold states that in some insects the ileum has glandular
appendages whose product is perhaps analogous to the pancre-
atic fluid. In the larva of insects is found the corpus adiposum,
or fat-body, in the form of large lobes of fat-cells which spread
through the intervals of the viscera in the general cavity of
the body. It is interpenetrated and retained in place by
numerous tracheae.

THE CIRCULATORY SYSTEM. The vascular, or circulatory,
system is not a closed sac as in the Worms and Vertebrates.
The organs of circulation consist of a contractile, articulated
dorsal vessel, or so-called "heart," which terminates in a
cephalic aorta. The dorsal vessel receives the venous current
through the lateral valvular openings and pumps the blood into
its prolongation or cephalic aorta, whence it escapes, traversing
the body in all directions, in regular currents, which do not
have, however, vascular walls. "In this way, it penetrates the
antennae, the extremities, the wings, and the other appendages
of the body, by arterial currents, and is returned by those of a
venous nature. All the venous currents empty into two
lateral ones, running towards the posterior extremity of the
body, and which enter, through lateral orifices, the dorsal
vessel." (Siebold.)

"The blood of the Insecta is usually a colorless liquid,
though sometimes yellowish, but rarely red. In this liquid are
suspended a few very small, oval, or spheroidal corpuscles,
which are always colorless, have a granular aspect, and are
sometimes nucleated.

"The dorsal vessel, which is constricted at regular intervals,
is always situated on the median line of the abdomen, being
attached to the dorsal wall of its segments by several trian-
gular muscles whose apices point outwards. Its walls contain
both longitudinal and transverse fibres, and, externally, are
covered by a thin peritoneal tunic. Internally, it is lined by
another very fine membrane, which, at the points of these con-
strictions, forms valvular folds, so that the organ is divided
into as many chambers as there are constrictions. Each of
these chambers has, at the anterior extremity on each side, a
valvular orifice which can be inwardly closed. The returning



38



THE CLASS OF INSECTS.



blood is accumulated about the heart and enters into it during
the diastole of each of its chambers, through the lateral
orifices (Fig. 460- It then passes, by the regularly successive





Fig. 47.



Fig. 46.



contractions of the heart, from behind forwards into the aorta,
which is only a prolongation of the anterior chamber. This
aorta consists of a simple, small vessel, situated on the dorsal
surface of the thorax (Fig. 44 e,/), and extending even to the
cephalic ganglion, where it either ends in an open extremity, or
divides into several short branches which terminate in a like
manner. The length of the dorsal vessel depends, in all the
three states of insects, upon that of the abdomen. The number
of its chambers is very variable, but is, most usually, eight.

"The blood, after leaving the aorta, traverses the body in
currents which are also extravascular, and in this way bathes
all the organs. The newly-prepared nutritive fluid passes
through the walls of the digestive canal in which it is found,
into the visceral cavity, and thence directly into the blood.
Latterly, this extravascular circulation has been called in
question, but its presence may be easily and directly observed

FIG. 46. Part of the dorsal vessel or heart of Lucanus cervus ; a, the posterior
chambers (the anterior chambers are covered by a part of the ligaments which hold
the heart in place), i, the auriculo-ventricular openings; #, g, the lateral mus-
cles fixed by the prolongations h, h, to the upper side of the abdomen. From
Straus Durckheim.

FIG. 47. Interior of the dorsal vessel; a, the inner walls with their circular
fleshy fibres; c, the auriculo-ventricular opening; with its semilunar valve (c), in
front of which is d, the interventricular valvule. From Straus Durckheim.



THE CIRCULATORY SYSTEM. 39

with very many perfect Insecta and their larvoe. The vascular
walls, supposed to have been seen at certain points, are, un-
doubtedly, the result of some error of observation or interpre-
tation. This is also true of the pulsatile organs supposed to
have been observed in the legs of many water-bugs, and which
were thought to affect the circulation."

Blanchard and Agassiz believe in a " peritracheal circula-
tion," and other observers agree that the course of the circula-
tion is along the tracheae, i. e. that the blood circulates in the
space between the loose peritoneal envelope and the trachea
itself. Professor H. J. Clark objects to this view that the blood
disks are too large to pass through such an exceedingly minute
space as the distance between the trachea and its enveloping,
or peritoneal, wall.

Newport thinks that there are actual blood vessels distrib-
uted from the heart and "passing transversely across the
dorsal surface of each segment in the pupa of Sphinx. If
they be not vessels distributed from the heart, it is a some-
what curious circumstance that the whole of the blood should
be first sent to the head of the insect, and the viscera of the
abdominal region be nourished only by the returning blood,
which has in part passed the round of the circulation."

Newport also describes in Sphinx the supra- spinal, or great
ventral vessel which lies in the abdomen just over the nervous
cord, and which is also found in the Scorpion and Centipede.
He believes "this vessel to be the chief means of returning
the blood from the middle and inferior portion of the body to
the posterior extremity of the dorsal vessel or heart." He
strongly suspects that anteriorly this great ventral vessel is
connected with the aorta. The circulation of Insects, there-
fore, is probably as much a closed one as in the Myriapods, for
he states that the "blood certainly flows in distinct vessels, at
least in some parts of the body in perfect insects, and that
vessels exist even in the larva." Observations on the vascular
system are exceedingly difficult from the delicate structure of
the vessels, and the subject needs renewed observations to
settle these disputed points.

The blood is forced through the vessel into the body by regu-
lar pulsations. Herold counted thirty to forty in a minute in a



40 THE CLASS OF INSECTS.

full-grown caterpillar ; we have counted about sixty a minute
in the recently hatched larva of Diplax. During excitement,
the number of pulsations increases in rapidity. Newport found
the pulsations in a bee, Anthophora, when quiet, to be eighty a
minute ; but when "the insects were quite lively, and had been
exposed to the sun for an hour or two, the number of pulsa-
tions amounted to one hundred and forty."

He found that the number of pulsations decreased after each
moult of the larva of Sphinx ligustri, but increased in force;
when it was full grown and had ceased feeding it was thirty.
' ' After it had passed into the pupa state the number fell to
twenty-two, and afterwards to ten or twelve, and, during the
period of hibernation, it almost entirely ceases ; but in the per-
fect insect it rose from forty-one to fift3 r , and when excited by
flight around the room it was from one hundred and ten to one
hundred and thirty-nine."
*

ORGANS OF RESPIRATION. All insects breathe air, or, when
they live in the water, respire, by means of branchiae, the
air mixed mechanically with water. Respiration is carried on
by an intricate system of tubes (pul-
monary tracheae) which open by pores
(spiracles or stigmata) in the sides of
the body ; or, as in aquatic insects, by
branchiae, or gill-like flattened expan-
sions of the body-wall penetrated by
tracheae (branchial tracheae).

There are sometimes eleven spiracles,
or breathing-holes (Fig. 48), on each side
of the body ; each consisting of an oval
horny ring situated in the peritreme
Fi - 48 - and closed by a valve, which guards

the orifice (Fig. 49). Within this valve is a chamber closed
within by another valve which covers the entrance into the
tracheae. The air-tube itself (Fig. 50) consists of "an external

FIG. 48. Larva of the Humble-bee just beginning to change to a pupa, showing
eleven pairs of stigmata. In the adult bee, only the fourth pair is apparent, the
remaining pairs being concealed from view, or in part aborted. In most insects
there are usually only nine pairs of stigmata. Original. .




ORGANS OF RESPIRATION. 41

serous, and an internal mucous membrane, inclosing between
them a spirally convoluted fibre, thus giving great strength
and flexibility to the tube."

Nearly all the air enters through the thoracic and first
abdominal spiracles, so that on pinching most insects on

the thorax they can be

easily deprived of

breath and killed.
" In some aquatic

larvae such as those

of Dyticidce, Eristalis
Fig. 49. (Fig. 51, pupa), and

Epliydm, and also in some perfect insects,
as in Nepa and Ranatra, the parts sup-
porting the stigmata are prolonged into slen- Fig. 50.
der tubes, through which the insect, on rising to the surface,
breathes the atmospheric air.

Agrion (Fig. 52) affords a good instance of branchiae
or gill-like expansions of the crust, or skin. It is
supposed that these false gills, or branchiae, "absorb
the air from the water, and conve}< it by the minute






ramifications of the tracheal ves- (/
sels, with which they are abun-



dantly supplied, and which ter-
Fig. si. minate in single trunks, into the

main tracheae, to be distributed over the whole body,
as in insects which live in the open atmosphere."
(Newport.)

Of branchiae there are three kinds. The first, as in
the larvae and pupae of Gnats, consist of slender fila-
ments arranged in tufts arising from a single stem. Fi s- 53 -
In the larva of Gyrinus and the aquatic caterpillar of a moth,

FIG. 49. Chamber leading into the trachea; a, a, external valve protecting the
outer opening of the stigma,or breathing hole; 6, c, c, inner and more complicated
valve closing the entrance into the trachea (Z, k); TO, conical occlusor muscle
closing the inner orifice. From Strait* Diirc/cliciin.

FK;. 50. Portion of a trachea divested of its peritoneal envelope, a, spirally
convoluted fibre, closely wound around the trachea, as at e ; c, origin of a secondary
tracheal branch. from Strait* Durclshchn.

Fi<;. ") - 2. One of the three gill-like appendages to the abdomen of the larva and
pupa of Agrion enlarged, consisting of a broad leaf-like expansion, permeated by
tracheae which take up by endosmosis the air contained in water. Original.
4*



42 THE CLASS OF INSECTS.

Hydrocampa stratiolata, they form short stiff bristles placed
along the side of the body. Agrion and Ephemera, in their
larval stages, afford the second kind of branchiae, and Libettula
the third kind, or internal gill, situated in the colon. The
Mosquito breathes both by branchiae which form large club-
shaped organs, and by lateral filaments.

In those insects that fly, most of the tracheae are often dilated
into air-vesicles, so that by filling and emptying them of air the
insect can change its specific gravity. That their use is also
to lighten the body is shown by their presence in the heavy
mandibles and head of the male of Lucanus cervus. In the
adult Humble-bee there are two very large vesicles at the base
of the abdomen. These vesicles are not found in the larvae,
or in the adult forms of creeping insects.

The act of respiration consists in the alternate dilation and
contraction of the abdominal segments, the air entering the
body chiefly at the thoracic spiracles. As in the Vertebrates the
frequency of the acts of breathing increases after exertion.
"When an insect is preparing itself for flight, the act of res-
piration resembles that of birds under similar circumstances.
At the moment of elevating its elytra and expanding its wings,
which are, indeed, acts of respiration, the anterior pairs of
spiracles are opened, and the air rushing into them is extended
over the whole body, which, by the expansion of the air-bags, is
enlarged in bulk, and rendered of less specific gravity ; so that
when the spiracles are closed at the instant the insect endeavors
to make the first stroke with and raise itself upon its wings, it
is enabled to rise in the air, and sustain a long and powerful
flight with but little muscular exertion. In the pupa and larva
state respiration is performed more equally by all the spiracles,
and less especially by the thoracic ones."

During hibernation the act of breathing, like the circulation
of the blood, almost entirely ceases, and the heat of the body
is greatly lowered. Indeed Newport has shown that the devel-
opment of heat in Insects, just as in Vertebrates, depends on the
"quantity and activity of respiration, and the volume and
velocity of the circulation." The Humble-bee, according to
Newport, possesses the voluntary power of generating heat by
breathing faster. He says, confirming Huber's observations,



ORGANS OF SECRETION. 43

"the manner in which the bee performs her incubatory office is
by placing herself upon the cell of a nymph (pupa) that is
soon to be developed, and then beginning to respire at first
very gradually. In a short time the respirations become more
and more frequent, until at length they are increased to one
hundred and twenty, or one hundred and thirty per minute.
The body of the insect soon becomes of a high temperature,
and, on close inspection, is often found to be bathed with per-
spiration. When this is the case the temperature of the insect
soon becomes reduced, and the insect leaves the cell, and an-
other bee almost immediately takes her place. When respira-
tion is performed less violently, and consequently less heat is
evolved, the same bee will often continue on a cell for many
hours in succession. This extreme amount of heat was evolved
entirely by an act of the will in accelerating the respiratory ef-
forts, a strong indication of the relation which subsists between
the function of respiration and the development of animal heat."

ORGANS OF SECRETION. The urinary vessels, or what is
equivalent to the kidneys of the higher animals, consist in In-
sects of several long tubes which empty by one or two common
secretory ducts into the posterior or "pyloric" extremity of
the stomach. There are also odoriferous glands, analogous to
the cutaneous glands of vertebrates. The liquid poured out is
usually offensive, and it is used as a means of defence. The
Bees, Wasps, Gall-flies, etc., and Scorpions, have a poison-sac
(Fig. 54 g) developed in the tip of the abdomen. The bite of
the Mosquito, the Horse-fly, and Bed-bug is thought by New-
port to be due to the simple act of thrusting their lancet-like
jaws through the skin, and it is not known that these and
other insects which bite severely eject any poison into the
wound. But in the spiders a minute drop of poison exudes from
an orifice at the end of the mandibles, "which spreads over the
whole wound at the instant it is inflicted." This poison is
secreted by a gland lodged in the cephalo-thorax, and which
is thought by Audouin to correspond in position to the salivary
apparatus and the silk glands of the Winged Insects.

ORGANS OF GENERATION. We have already described the
external parts. The internal parts of the male insect consist,



44



THE CLASS OF INSECTS.




of a duct, the ductus ejaculatorius, which opens into the external
intromittent organ. This duct extends backwards, connecting
with the vesiculce seminales, which lead by
the vasa differentia to the testes (Fig. 53).
The latter are usually rounded glandular
bodies, sometimes, as in Melolontha and
Lucanus, numbering six on a side. These
organs lie in the abdominal cavity, usually
above and on each side of the alimentary
canal.

The sperm, or fertilizing fluid, contains
very active
spermatic par-
ticles which
are developed
in large cells
in the testes,
Fig. 53. where they are

united into bundles of various
forms.

In the female, the internal re-
productive organs (Fig. 54) are
more simple than those of the
other sex. The external open-
ing of the female is situated at
the end of the oviduct, that
leads by two tubes to the ovary,
which consists of two or more Fig. 54.

tubes (in the Queen Bee one hundred and sixty to one hundred
and eighty) in which the ova are developed. On the upper side

FIG. 53. Male organs of Athalia centifolice. h, the penis, or external portion,
in which the ductus ejaculatoriiis (/) terminates, which extends backwards, and is
connected with the vesiculce seminales (<?), and vasa differentia (<7) Avhich are con-
nected with the epididijmis (6), and the testes (a}, i and I, two pairs of homy plates,
surrounded by a horny ring (k). i, horny prehensile hooks attached to k. m, two