It is not intended to investigate the anatomy of insects in this work
thoroughly; but, as we are about to treat of the habits and economy of
certain created beings, it is necessary first to explain the principal
parts of their structure, and the stages which every perfect insect or
_imago_ has undergone before arriving at that state.
We, therefore, proceed to explain, as simply as possible, the anatomy
of an insect, and the functions of its organs.
If we take an insect, and turn it over, and examine it carefully, the
first thing that strikes us is that it is divided into three parts:
the head; the thorax, or chest; and the abdomen, or stomach.
The head (Fig. 1) is a kind of box, formed of a single piece, having
here and there joints more or less strongly marked, sometimes scarcely
visible. It is furnished in front with an opening--often very
small--which is the mouth; and with some for the eyes, and with others
for the insertion of the antennæ or horns.
[Illustration: Fig. 1.--Head of an Insect]
The integuments of the head are generally harder than the other
parts of the body. It is necessary that this should be so. Insects
often live and die in the midst of substances which offer some
resistance. It is necessary, therefore, that the head should be strong
enough to overcome such resistance. The head contains the masticatory
organs, which, frequently having to attack hard substances, must be
strongly supported. The exception to this rule is among insects which
live by suction.
It would be out of place here to mention the numerous modifications
of the head which are presented in the immense class of insects.
The eyes of insects are of two kinds. There are compound eyes, or eyes
composed of many lenses, united by their margins and forming hexagonal
facettes; and there are also simple eyes, or ocelli.
The exterior of the eye is called the cornea (Fig. 2), each facette
being a cornea; and the facettes, which vary in size even in the same
eye, unite and form a common cornea, which is represented by the
entire figure.
[Illustration: Fig. 2.--A Compound Cornea.]
In order to show the immense number of the facettes possessed by many
insects, we give the following list:--
In the genus Mordella (a genus of beetles) the eye has 25,008 facettes.
In the _Libellula_ (dragon-fly) 12,544 "
In the genus _Papilio_ (a genus of butterflies) 17,355 "
In _Sphinx convolvuli_ (the convolvulus hawk-moth) 1,300 "
In _Bombyx mori_ (the common silkworm moth) 6,236 "
In the house-fly 4,000 "
In the ant 50 "
In the cockchafer 8,820 "
The facettes appear to be most numerous in insects of the genus
_Scarabæus_ (a genus of beetles). They are so minute, that they can
only be detected with a magnifying glass.
Looked at in front, a compound eye may be considered an agglomeration
of simple eyes; but internally this is hardly correct.
On the under side of each facette we find a body of a gelatinous
appearance, transparent, and usually conical; the base of this
occupies the centre of the facette in such a manner as to leave around
it a ring to receive some colouring matter. This body diminishes in
thickness towards its other extremity, and terminates in a point where
it joins a nervous filament proceeding from the optic nerve. These
cones, agreeing in number with the facettes, play the part of the
crystalline lens in the eyes of animals. They are straight and
parallel with each other. A pigment fills all the spaces between the
cones, and between the nervous filaments, and covers the under side of
each cornea, except at the centre. This pigment varies much in colour.
There are almost always two layers, of which the exterior one is the
more brilliant. In fact, these eyes often sparkle with fire, like
precious stones.
M. Lacordaire, in his "Introduction à l'Entomologie," from which we
borrow the greater part of this information, has summed up as follows,
the manner in which, according to M. Müller, the visual organs of
insects operate:--
"Each facette, with its lens and nervous filament, separated from
those surrounding them by the pigment in which they are enclosed, form
an isolated apparatus, impenetrable to all rays of light, except those
which fall perpendicularly on the centre of the facette, which alone
is devoid of pigment. All rays falling obliquely are absorbed by that
pigment which surrounds the gelatinous cone. It results partly from
this, and partly from the immobility of the eye, that the field of
vision of each facette is very limited, and that there are as many
objects reflected on the optic filaments as there are corneæ. The
extent, then, of the field of vision will be determined, not by the
diameter of these last, but by the diameter of the entire eye, and
will be in proportion to its size and convexity. But whatever may be
the size of the eyes, like their fields of vision, they are
independent of each other; there is always a space, greater or less,
between them; and the insect cannot see objects in front of this space
without turning its head. What a peculiar sensation must result from
the multiplicity of images on the optic filaments! This is not more
easily explained than that which happens with animals which, having
two eyes, see only one image; and probably the same is the case
with insects. But these eyes usually look in opposite directions,
and should see two images, as in the chameleon, whose eyes move
independently of each other. The clearness and length of vision will
depend, continues M. Müller, on the diameter of the sphere of which
the entire eye forms a segment, on the number and size of the
facettes, and the length of the cones or lenses. The larger each
facette, taken separately, and the more brilliant the pigment placed
between the lenses, the more distinct will be the image of objects at
a distance, and the less distinct that of objects near. With the
latter the luminous rays diverge considerably; while those from the
former are more parallel. In the first case, in traversing the
pigment, they impinge obliquely on the crystalline, and consequently
confuse the vision; in the second, they fall more perpendicularly on
each facette.
"Objects do not appear of the same size to each optic filament, unless
the eye is a perfect section of a sphere, and its convexity concentric
with that of the optic nerve. Whenever it is otherwise, the image
corresponds more or less imperfectly with the size of the object, and
is more or less incorrect. Hence it follows, that elliptical or
conical eyes, which one generally finds among insects, are less
perfect than those referred to above.
"The differences which exist in the organisation of the eye among
insects are explicable, to a certain point, on the theory which we are
about to explain in a few words. Those species which live in the same
substances on which they feed, and those which are parasitical, have
small and flattened eyes; those, on the contrary, which have to seek
their food, and which need to see objects at a distance, have large or
very convex eyes. For the same reason the males, which have to seek
their females, have larger eyes than the latter. The position of the
eyes depends also on their size and shape; those which are flat, and
have consequently a short field of vision, are placed close together,
and rather in front than at the sides of the head, and often
adjoining. Spherical and convex eyes, on the contrary, are placed on
the sides, and their axes are opposite. But the greater field of
vision which they are able to take in makes up for this position."
Almost all insects are provided with a pair of compound eyes, which
are placed on the sides of the head. The size and form of these organs
are very variable, as we shall presently see. They are generally
placed behind the antennæ.
Although simple eyes (ocelli or stemmata) are common, they do not
exist in all the orders of insects. They are generally round, and more
or less convex and black, and there are three in the majority of
cases. When there is this number they are most frequently placed in a
triangle behind, and at a greater or less distance from the antennæ.
Under the cornea, which varies in convexity, is found a transparent,
rather hard, and nearly globular body, which is the true crystalline
resting on a mass, which represents the vitreous body. This vitreous
body is enclosed in an expansion of the optic nerve. Besides these,
there is a pigment, most frequently red-brown, sometimes black, or
blood-red. The organisation of these eyes is analogous to the eyes of
fishes, and their refractive power is very great.
With these eyes insects can only see such objects as are at a short
distance. Of what use then can stemmata be to insects also provided
with compound eyes? It has been remarked that most insects having
this arrangement of eyes feed on the pollen of plants, and it has
been surmised that the stemmata enable them to distinguish the parts
of the flowers.
The antennæ, commonly called horns, are two flexible appendages,
of very variable form, which are joined to different parts of the
head, and are always two in number. The joints of which they are made
up have the power of motion, which enables the insect to move them in
any direction.
The antennæ consist of three parts: the basal joint, commonly
distinguished by its form, length, and colour; the club, formed by a
gradual or sudden thickening of the terminal joints, of which the
number, form, and size present great variations; lastly, the stalk,
formed by all the joints of the antennæ, except the basal, when no
club exists, and in case of the existence of a club, of all those
between it and the basal one.
We give as examples the antennæ of two beetles, one of the genus
_Asida_, the other of the genus _Zygia_ (Figs. 3 and 4).
[Illustration: Fig. 3.--Antenna of a species of Asida.]
[Illustration: Fig. 4.--Antenna of Zygia oblonga.]
Insects, for the most part, while in repose, place their antennæ on
their backs, or along the sides of the head, or even on the thorax.
Others are provided with cavities in which the antennæ repose either
wholly or in part.
During their different movements, insects move their antennæ more or
less, sometimes slowly and with regularity, at other times in all
directions. Some insects impart to their antennæ a perpetual
vibration. During flight they are directed in front, perpendicular to
the axis of the body, or else they repose on the back.
What is the use of the antennæ, resembling as they do, feathers, saws,
clubs, &c.? Everything indicates that these organs play a very
important part in the life of insects, but their functions are
imperfectly understood. Experience has shown that they only play a
subordinate part as feelers, and have nothing to do with the senses of
taste or smell. There is no other function for them to fulfil, except
that of hearing.
On this hypothesis the antennæ will be the principal instruments for
the transmission of sound-waves. The membrane at their base represents
a trace of the tympanum which exists among the higher animals. This
membrane then will have some connection with an auditory nerve.
The mouth of insects is formed after two general types, which
correspond to two kinds of requirements. It is suited in the one case
to break solid substances, in the other to imbibe liquids.
At first sight there seems no similarity between the mouth of a biting
insect and of one living by suction. But on examination it is found
that the parts of the mouth in the one are exactly analogous to the
same parts in the other, and that they have only modifications suiting
them to the different purposes which they have to fulfil.
The mouth of a biting insect is composed of an upper lip, a pair of
mandibles, a pair of jaws, and a lower lip (Fig. 5).
[Illustration: Fig. 5.--Mouth of a masticating insect.]
[Illustration: Fig. 6.--Thorax of Acrocinus longimanus (a beetle).]
The lower lip and the jaws carry on the outside certain appendages or
filaments which have received the name of _palpi_.
When speaking of sucking insects, and in general of the various orders
of insects, we shall speak more in detail of the various parts of the
mouth.
The thorax (Fig. 6), the second primary division of the body of
insects, plays almost as important a part as the head. It consists of
three segments or rings, which are in general joined together--the
prothorax, the mesothorax, and the metathorax, each of which bears a
pair of legs. The wings are attached to the two posterior segments.
All insects have six true legs. There is no exception whatever to this
rule, though some may not be developed.
From the segments to which they are attached, the legs are called
anterior, posterior, and intermediate. The legs are composed of four
parts: the trochanter, a short joint which unites the thigh to the
body; the thigh or _femur_; the _tibia_, answering to the shank in
animals; and the _tarsus_, or foot, composed of a variable number of
pieces placed end to end, and called the _phalanges_.
We take as examples the hind leg of a _Heterocerus_ (Fig. 7), and the
front leg of a _Zophosis_ (Fig. 8) (genera of beetles).
We shall not dwell on the different parts, as they perform functions
which will occupy us later, when speaking of the various species of
the great class of insects.
[Illustration: Fig. 7.--Hind leg of a Heterocerus.]
[Illustration: Fig. 8.--Front leg of a Zophosis.]
The functions which the legs of insects have to perform consist in
walking, swimming, or jumping.
In walking, says M. Lacordaire, insects move their legs in different
ways. Some move their six legs successively, or only two or three at
a time without distinction, but never both legs of the same pair
together, consequently one step is not the same as another. The walk
of insects is sometimes very irregular, especially when the legs are
long; and they often hop rather than walk. Others have one kind of
step, and walk very regularly. They commence by moving the posterior
and anterior legs on the same side and the intermediate ones on the
opposite side. The first step made, these legs are put down, and the
others raised in their turn to make a second.
Running does not change the order of the movements, it only makes them
quicker--very rapid in some species, and surpassing in proportion that
of all other animals; but in others the pace is slow. Some insects
rather crawl than walk.
[Illustration: Fig. 9.--Posterior leg of a jumping insect.]
In swimming, the posterior legs play the principal part. The other
legs striking the water upwards or downwards, produce an upward or
downward motion. The animal changes its course at will by using the
legs on one side only, in the same way as one turns a rowing boat with
one oar without the aid of a rudder. Swimming differs essentially from
walking, for the foot being surrounded by a resisting medium, the
legs on both sides are moved at the same time.
The act of jumping is principally performed by the hind legs. Insects
which jump have these legs very largely developed, as in Fig. 9. When
about to jump they bring the tibia into contact with the thigh, which
is often furnished with a groove to receive it, having on each side a
row of spines. The leg then suddenly straightens like a spring, and
the foot being placed firmly on the ground, sends the insect into the
air, and at the same time propels forward. The jump is greater in
proportion as the leg is longer.
To treat here in a general manner of the wings of insects would be
useless. We shall refer to them at length in their proper place, when
treating of the various types of winged insects.
In the perfect insect the abdomen does not carry either the wings or
the legs. It is formed of nine segments, which are without appendages,
with the exception of the posterior ones, which often carry small
organs differing much in form and function. These are saws, probes,
forceps, stings, augers, &c. We shall consider these different organs
in their proper places.
With vertebrate animals, which have an interior skeleton suited to
furnish points of resistance for their various movements, the skin is
a more or less soft covering, uniformly diffused over the exterior of
the body, and intended only to protect it against external injury. In
insects the points of resistance are changed from the interior to the
exterior. The skin is altered by Nature to fit it to this purpose. It
is hard, and presents between the segments only membranous intervals,
which allow the hard parts to move in all directions.
We are examining a perfect insect; we have glanced at its skeleton,
and the different appendages which spring from it. The principal
organs which are contained in the body remain to be examined.
We will first study the digestive apparatus. This apparatus consists
of a lengthened tubular organ, swollen at certain points, forming
more or less numerous convolutions, and provided with two distinct
orifices. This alimentary canal is always situated in the median line
of the body, traverses its whole length, and is at first surrounded
by, and then passes above, the nervous ganglia.[1]
[1] _Ganglion_--a mass, literally a knot, of nervous matter.
In its most complicated form the alimentary canal is composed of an
_oesophagus_, or gullet, of a crop, of a gizzard, of a chylific
ventricle or stomach, a small intestine, a large intestine, divers
appendages, salivary, biliary, and urinary glands. The oesophagus
is often not wider than a hair, and part of it in many species is
enlarged into a pouch, which is called the crop, because it occupies
the same position, and performs analogous functions with that organ in
birds. It is enough to say that the food remains there some time
before passing on to the other parts of the intestinal canal, and
undergoes a certain amount of preparation. It is in the gizzard, when
one exists, that the food, separated by the masticatory organs of the
mouth, undergoes another and more complete grinding. Its structure is
suited to its office. It is, in fact, very muscular, often half
cartilaginous, and strongly contractile. Its interior walls are
provided with a grinding apparatus, which varies according to the
species, and consists of teeth, plates, spines, and notches, which
convert the food into pulp. It only exists among insects which live
on solid matters, hard vegetables, small animals, tough skin, &c. This
apparatus is absent in sucking insects and those which live on soft
substances, such as the pollen of flowers, &c.
[Illustration: Fig. 10.--Digestive apparatus of Carabus auratus.]
The chylific ventricle or stomach is never absent; it is the organ
which performs the principal part in the act of digestion.
Two kinds of appendages belong to the chylific ventricle, but only in
certain families. The first are papillæ, in the form of the fingers of
a glove, which bristle over the exterior of this organ, and in which
it is believed that the food begins to be converted into chyle. The
second are cæca, and larger and less numerous.
They have been considered as secretory organs, answering to the
pancreas in vertebrate animals.
Fig. 10, which represents the digestive apparatus of _Carabus
auratus_, a common beetle, presents to the eyes of the reader the
different organs of which we are speaking.
A is the mouth of the insect, B the oesophagus, C the crop, D the
gizzard, E the chylific ventricle, F and G the small and large
intestines, and H the anus.
It is not necessary to consider the other parts of the alimentary
canal in insects, but only to refer to some of the appendages of this
apparatus.
The salivary glands pour into the digestive tube a liquid, generally
colourless, which, from the place where it is secreted, and its
alkaline nature, corresponds to the saliva in vertebrate animals. It
is this liquid which comes from the tongue of sucking insects in the
form of drops.
These glands are always two in number. Their form is as variable
as complicated. The most simple is that of a closed flexible tube,
generally rolled into a ball, and opening on the sides of the
oesophagus.
At the posterior extremity of the chylific ventricle are inserted
a variable number of fine tubes, usually elongated and flexible,
and terminating in _culs-de-sac_ at one end. Their colour, which
depends on the liquid they may contain, is sometimes white, but more
frequently brown, blackish, or green. They appear to be composed of a
very slight and delicate membrane, as they are very easily torn, and
nothing is more difficult than to unroll and to disengage them from
the fatty or other tissues by which they are enveloped.
[Illustration: Fig. 11.--Posterior extremity of the chylific
ventricle, surrounded by the Malpighian vessels.]
The function of these vessels is uncertain. Cuvier and Léon Dufour
supposed them to be analogous to the liver, and on that account they
have been called biliary vessels; and they are often termed the
Malpighian vessels, after the name of their discoverer.
According to M. Lacordaire, their functions vary with their position.
When they enter the chylific ventricle, they furnish only bile; bile
and a urinary liquid when they enter the posterior part of the
ventricle and the intestine; and urine alone when they are placed near
the posterior extremity of the alimentary canal.
Fig. 11 represents part of the preceding figure more highly magnified,
showing the manner in which these tubes enter the chylific ventricle.
In our rapid description of the digestive apparatus of insects, it
only remains for us to mention certain purifying organs which secrete
those fluids, generally blackish, caustic, or of peculiar smell, which
some insects emit when they are irritated, and which cause a smarting
when they get into one's eyes.
Less well developed than the salivary organs, they are often of a very
complicated structure. In Fig. 12 is represented the secretory
apparatus of the _Carabus auratus_, which will serve for an example: A
represents the secretory sacs aggregated together like a bunch of
grapes, B the canal, C the pouch which receives the secretion, D the
excretory duct.
[Illustration: Fig. 12.--Secretory apparatus of Carabus auratus.]
Sometimes the secretion is liquid, and has a foetid or ammoniacal
odour; sometimes, as in the Bombardier beetle (_Brachinus crepitans_),
it is gaseous, and is emitted, with an explosion, in the form of a
whitish vapour, having a strong pungent odour analogous to that of
nitric acid, and the same properties. It reddens litmus paper, and
burns and reddens the skin, which after a time becomes brown, and
continues so for a considerable time.
About the middle of the seventeenth century Malpighi at Bologna, and
Swammerdam at Utrecht, discovered a pulsatory organ occupying a median
line of the back, which appeared to them to be a heart, in different
insects. Nevertheless, Cuvier, having declared some time afterwards
that there was no circulation, properly so called, among insects, his
opinion was universally adopted.
But in 1827 a German naturalist named Carus discovered that there were
real currents of blood circulating throughout the body, and returning
to their point of departure. The observations of Carus were repeated
and confirmed by many other naturalists, and we are thus enabled to
form a sufficiently exact idea of the manner in which the blood
circulates.
The following summary of the phenomena of circulation among insects is
borrowed from "Leçons sur la Physiologie et l'Anatomie comparée," by
M. Milne-Edwards:--
The tube which passes under the skin of the back of the head, and
front part of the body, above the alimentary canal, has been known for
a long time as the dorsal vessel. It is composed of two very distinct
portions: the anterior, which is tubular and not contractile; and the
posterior, which is larger, of more complicated structure, and which
contracts and dilates at regular intervals.
This latter part constitutes, then, more particularly the heart of
the insect. Generally it occupies the whole length of the abdomen,
and is fixed to the vault of the tegumentary skeleton by membranous
expansions, in such a manner as to leave a free space around it, but
shut above and below, so as to form a reservoir into which the blood
pours before penetrating to the heart. This reservoir is often called
the auricle, for it seems to act as an instrument of impulsion, and to
drive the blood into the ventricle or heart, properly so called.
The heart is fusiform, and is divided by numerous constrictions into
chambers. These chambers have exits placed in pairs, and membranous
folds which divide the cavity in the manner of a portcullis. The lips
of the orifices, instead of terminating in a clean edge, penetrate
into the interior of the heart in the form of the mouth-piece of a
flute. The double membranous folds thus formed on each side of the
dorsal vessel are in the shape of a half moon, and separate from each
other when this organ dilates; but the contrary movement taking place,
the passage is closed.
By the aid of this valvular apparatus, the blood can penetrate into
the heart from the pericardic chamber, the empty space surrounding the
heart, but cannot flow back from the heart into that reservoir.
The anterior or aortic portion of the dorsal vessels shows neither
fan-shaped lateral expansions, nor orifices, and consists of a
single membranous tube. The whole of the blood set in motion by the
contractions of the cardial portion of the dorsal vessel runs into the
cavity of the head, and circulates afterwards in irregular channels
formed by the empty spaces left between the different organs. It is
the unoccupied portions of the great visceral cavity which serve as
channels for the blood, and through them run the main currents to the
lateral and lower parts of the body. These currents regain the back
part of the abdomen, and enter the heart after having passed over the
internal organs. These principal channels are in continuity with other
gaps between the muscles, or between the bundles of fibres of which
these muscles are composed.
The principal currents send into the network thus formed, minor
branches, which having ramified in their turn among the principal
parts of the organism, re-enter some main current to regain the dorsal
vessel.
In the transparent parts of the body the blood may be seen circulating
in this way to a number of inter-organic channels, penetrating
the limbs and the wings, when these appendages are not horny, and,
in short, diffusing itself everywhere. "If, by means of coloured
injections," says M. Milne-Edwards, "one studies the connections which
exist between the cavities in which sanguineous currents have been
found to exist and the rest of the economy, it is easy to see that the
irrigatory system thus formed penetrates to the full depth of every
organ, and should cause the rapid renewal of the nourishing fluid in
all the parts where the process of vitality renders the passage of
this fluid necessary."
We shall see presently, in speaking of respiration, that the relations
between the nourishing fluid and the atmospheric air are more direct
and regular than was for a long time supposed.
In short, insects possess an active circulation, although we find
neither arteries nor veins, and although the blood put in motion by
the contractions of the heart, and carried to the head by the aortic
portion of the dorsal vessel, can only distribute itself in the
different parts of the system to return to the heart, by the gaps left
between the different organs, or between the membranes and fibres of
which these organs are composed.
Fig. 13 (page 14), which shows both the circulating and breathing
systems of an insect, enables us to recognise the different organs
which we have described, as helping to keep up both respiration and
circulation.
The knowledge of the respiration of the insect is comparatively a
modern scientific acquisition. Malpighi was the first to prove, in
1669, that insects are provided with organs of respiration, and that
air is as indispensable to them as it is to other living beings. But
the opinion of this celebrated naturalist has been contradicted,
and his views were long contested. Now, however, one can easily
recognise the apparatus by the aid of which the respiration of the
insect is effected.
[Illustration: Fig. 13.--Organs of circulation and breathing in an
insect. A, abdominal portion of the dorsal vessel. B, aortic or
thoracic portion. C, air-vessels of the head; D, of the abdomen.]
The respiratory apparatus is essentially composed of membranous
ducts of great tenuity, their ramifications spread everywhere in
incalculable numbers, and bury themselves in the different organs,
much in the same way as the fibrous roots of plants bury themselves in
the soil. These vessels are called tracheæ. Their communications with
the air are established externally in different ways, according to the
character of the medium in which the insect lives.
It is well known that a vast number of insects live in the air. The
air penetrates into the tracheæ by a number of orifices placed at the
sides of the body, which are termed spiracles. On close examination
these may be seen in the shape of button-holes in a number of
different species. Let us dwell for a moment on the breathing
apparatus of the insect, that is to say, on the tracheæ.
This apparatus is sometimes composed of elastic tubes only, sometimes
of a collection of tubes and membranous pouches. We will first treat
of the former.
The coats of these breathing tubes are very elastic, and always
preserve a cylindrical form, even when not distended. This state of
things is maintained by the existence, throughout the whole length of
the tracheæ, of a thread of half horny consistency, rolled up in a
spiral, and covered externally by a very delicate membranous sheath.
The external membrane is thin, smooth, and generally colourless, or of
a pearly white. The cartilaginous spiral is sometimes cylindrical and
sometimes flat. It only adheres slightly to the external membrane,
but is, on the other hand, closely united to the internal one. This
spiral thread is only continuous in the same trunk; it breaks off when
it branches, and each branch then possesses its own thread, in such a
way that it is not joined to the thread of the trunk from which it
issued, except by continuity, just as the branch of a tree is attached
to the stem which supports it. This thread is prolonged, without
interruption, to the extreme points of the finest ramifications.
The number of tracheæ in the body of an insect is very great. That
patient anatomist, Lyonet, has proved this in his great work on the
Goat-moth Caterpillar, _Cossus ligniperda_. Lyonet, who congratulated
himself with having finished his long labours without having had to
destroy more than eight or nine of the species he wished to describe,
had the patience to count the different air-tubes in that caterpillar.
He found that there were 256 longitudinal and 1,336 transverse
branches; in short, that the body of this creature is traversed in all
directions by 1,572 aeriferous tubes which are visible to the eye by
the aid of a magnifying glass, without taking into account those which
may be imperceptible.
The complicated system of the breathing apparatus which we are
describing is sometimes composed of an assemblage of tubes and
membranous pouches, besides the elastic tubes which we have already
mentioned. These pouches vary in size, and are very elastic, expanding
when the air enters, and contracting when it leaves them, as they are
altogether without the species of framework formed by the spiral
thread of the tubular tracheæ, of which they are only enlargements.
Fig. 13 is explanatory of these organs of respiration.
The respiratory mechanism of an insect is easily understood. "The
abdominal cavity," says M. Milne-Edwards, "in which is placed the
greater part of the respiratory apparatus, is susceptible of being
contracted and dilated alternately by the play of the different
segments of which the skeleton is composed, and which are placed in
such a manner that they can be drawn into each other to a greater or
less extent. When the insect contracts its body, the tracheæ are
compressed and the air driven out. But when, on the other hand, the
visceral cavity assumes its normal size, or dilates, these channels
become larger, and the air with which they are filled being rarefied
by this expansion, is no longer in equilibrium with the outer air with
which it is in communication through the medium of the spiracles. The
exterior air is then impelled into the interior of the respiratory
tubes, and the inspiration is effected."
The respiratory movements can be accelerated or diminished, according
to the wants of the animal; in general, there are from thirty to fifty
to the minute. In a state of repose the spiracles are open, and all
the tracheæ are free to receive air whenever the visceral cavity is
dilated, but those orifices may be closed, and the insect thus
possesses the faculty of stopping all communication between the
respiratory apparatus and the surrounding atmosphere.
Some insects live in the water; they are therefore obliged to come to
the surface to take the air they are in need of, or else to possess
themselves of the small amount contained in the water. Both these
methods of respiration exist under different forms in aquatic insects.
To inhale atmospheric air, which is necessary for respiration, above
the water, certain insects employ their elytra[2] as a sort of
reservoir; others make use of their antennæ, the hairs of which retain
the globules of air. In this case it is brought under the thorax,
whence a groove carries it to the spiracles. Sometimes the same result
is obtained by a more complicated arrangement, consisting of
respiratory tubes which can be thrust into the air, which it is their
function to introduce into the organisation.
[2] The horny upper wings with which some insects are provided
are called elytra.--ED.
Insects which breathe in the water without rising to the surface are
provided with gills--organs which, though variable in form, generally
consist of foliaceous or fringed expansions, in the midst of which the
tracheæ ramify in considerable numbers. These vessels are filled with
air, but it does not disseminate itself in them directly, and it is
only through the walls of these tubes that the contained gas is
exchanged for the air held in suspension by the surrounding water. The
oxygen contained in the water passes through certain very permeable
membranes of the gill, and penetrates the tracheæ, which discharge, in
exchange, carbonic acid, which is the gaseous product of respiration.
[Illustration: Fig. 14.--Branchiæ, or gills, of an aquatic larva
(_Ephemera_). A, foliaceous laminæ, or gills.]
Fig. 14 represents the gills or breathing apparatus in an aquatic
insect. We take as an example _Ephemera_.[3] It may be observed that
the gills or foliaceous laminæ are placed at the circumference of the
body, and at its smallest parts.
[3] May-fly family.--ED.
We have now seen that the respiratory apparatus is considerably
developed in insects; it is, therefore, easy to foresee that those
functions are most actively employed by them. In fact, if one
compares the oxygen they imbibe with the heavy organic matter of which
their body is composed, the amount is enormous.
Before finishing this rapid examination of the body of an insect, we
shall have to say a few words on the nervous system.
This system is chiefly composed of a double series of ganglions, or
collections of nerves, which are united together by longitudinal
cords. The number of these ganglions corresponds with that of the
segments. Sometimes they are at equal distances, and extend in a chain
from one end of the body to the other; at others they are many of them
close together, so as to form a single mass.
The cephalic ganglions are two in number; they have been described by
anatomists under the name of brain. "This expression," says M.
Lacordaire, "would be apt to mislead the reader, as it would induce
him to suppose the existence of a concentration of faculties to
control the feelings and excite the movements, which is not the
case."[4] The same naturalist observes, "All the ganglions of the
ventral chain are endowed with nearly the same properties, and
represent each other uniformly."
[4] "Introduction à l'Entomologie," tome ii. p. 192. 8vo.
Paris. 1838.
The ganglion situated above the oesophagus gives rise to the optic
nerves, which are the most considerable of all those of the body, and
to the nerves of the antennæ. The ganglion beneath the oesophagus
provides the nerves of the mandibles, of the jaws, and of the lower
lip. The three pairs of ganglions which follow those placed
immediately below the oesophagus, belong to the three segments of
the thorax, and give rise to the nerves of the feet and wings. They
are in general more voluminous than the following pairs, which occupy
the abdomen.
Fig. 15 represents the nervous system of the _Carabus auratus_: A is
the cephalic ganglion; B, the sub-oesophagian ganglion; C, the
prothoracic ganglion; D and E are the ganglions of the mesothorax and
metathorax. The remainder, F F, are the abdominal ganglions.
Before finishing these preliminary observations, it is necessary to
say that the preceding remarks only apply absolutely to insects
arrived at the perfect state. It is important to make this remark, as
insects, before arriving at that state, pass through various other
stages. These stages are often so different from each other, that it
would be difficult to imagine that they are only modifications of the
same animal; one would suppose that they were as many different kinds
of animals, if there was not abundant proof of the contrary.
[Illustration: Fig. 15.--Nervous system of Carabus auratus.]
The successive stages through which an insect passes are four in
number:--the egg; the larva; the pupa, nymph, or chrysalis; and the
perfect insect, or imago.
The egg state, which is common to them, as to all other articulate
animals, it is unnecessary to explain. Nearly all insects lay eggs,
though some few are viviparous. There often exists in the extremity of
the abdomen of the female a peculiar organ, called the ovipositor,
which is destined to make holes for the reception of the eggs. By a
wonderful instinct the mother always lays her eggs in a place where
her young, on being hatched, can find an abundance of nutritious
substances. It will not be needless to observe that in most cases,
these aliments are quite different to those which the mother seeks for
herself.
In the second stage, that is to say, on leaving the egg--the larva
period--the insect presents itself in a soft state, without wings, and
resembles a worm. In ordinary language, it is nearly always called a
worm, or grub, and in certain cases, a caterpillar.
Linnæus was the first to use the term "larva"--taken from the Latin
word _larva_, "a mask"--as he considered that, in this form, the
insect was as it were masked. During this period of its life the
insect eats voraciously, and often changes its skin. At a certain
period it ceases to eat, retires to some hidden spot, and, after
changing its skin for the last time, enters the third stage of its
existence, and becomes a chrysalis. In this state it resembles a mummy
enveloped in bandages, or a child in its swaddling clothes. It is
generally incapable of either moving or nourishing itself. It
continues so for days, weeks, months, and sometimes even for years.
While the insect is thus apparently dead, a slow but certain change is
going on in the interior of its body. A marvellous work, though not
visible outside, is being effected, for the different organs of the
insect are developing by degrees under the covering which surrounds
them. When their formation is complete, the insect disengages itself
from the narrow prison in which it was enclosed, and makes its
appearance, provided with wings, and capable of propagating its kind;
in short, of enjoying all the faculties which Nature has accorded to
its species. It has thrown off the mask; the larva and pupa has
disappeared, and given place to the perfect insect.
To show the reader the four states through which the insect passes in
succession, in Fig. 16 is represented the insect known as the
_Hydrophilus_,[5] firstly, in the egg state; secondly, as the larva,
or caterpillar; thirdly in the pupa; and fourthly as the perfect
insect or imago. The different degrees of transformation and evolution
which we have just described, are those which take place either
completely or incompletely in all insects. Their metamorphoses are
then at an end. There are certain insects, however, that show no
difference in their various stages, except by absence of wings in
the larva; and in these the chrysalis is only characterised by the
growth of the wings, which, at first folded back and hidden under
the skin, afterwards become free, but are not wholly developed till
the last skin is cast. These insects are said to undergo incomplete
metamorphoses, the former complete metamorphoses. Some never possess
wings; indeed, there are others which undergo no metamorphosis, and
are born possessed of all the organs with which it is necessary they
should be provided.
[5] A kind of water-beetle.--ED.
[Illustration: Fig. 16.--Hydrophilus in its four states. A, eggs; B,
larva; C, pupa; D, imago, or perfect insect.]
Some curious researches have been lately made on the strength
of insects. M. Felix Plateau, of Brussels, has published some
observations on this point, which we think of sufficient interest to
reproduce here.
In order to measure the muscular strength of man, or of animals--as
the horse, for instance--many different dynamometric apparatuses have
been invented, composed of springs, or systems of unequal levers. The
Turks' heads which are seen at fairs, or in the Champs Élysées, at
Paris, and on which the person who wishes to try his strength gives a
strong blow with his fist, represent a dynamometer of this kind. The
one which Buffon had constructed by Régnier the mechanician, and which
is known by the name of Régnier's Dynamometer, is much more precise.
It consists of an oval spring, of which the two ends approach each
other; when they are pulled in opposite directions, a needle, which
works on a dial marked with figures, indicates the force exercised on
the spring. It has been proved, with this instrument, that the
muscular effort of a man pulling with both hands is about 124 lbs.,
and that of a woman only 74 lbs. The ordinary effort of strength of a
man in lifting a weight is 292 lbs.; and a horse, in pulling, shows a
strength of 675 lbs.; a man, under the same circumstances, exhibiting
a strength of 90 lbs.
Physiologists have not as yet given their attention to the strength of
invertebrate animals. It is, relatively speaking, immense. Many people
have observed how out of proportion a jump of a flea is to its size. A
flea is not more than an eighth of an inch in length, and it jumps a
yard; in proportion, a lion ought to jump two-thirds of a mile. Pliny
shows, in his "Natural History," that the weights carried by ants
appear exceedingly great when they are compared with the size of these
indefatigable labourers. The strength of these insects is still more
striking, when one considers the edifices they are able to construct,
and the devastations they occasion. The _Termes_, or White Ant,[6]
constructs habitations many yards in height, which are so firmly and
solidly built, that the buffaloes are able to mount them, and use them
as observatories; they are made of particles of wood joined together
by a gummy substance, and are able to resist even the force of a
hurricane.
[6] A neuropterous insect, not a true ant.--ED.
There is another circumstance which is worth being noted. Man is proud
of his works; but what are they, after all, in comparison with those
of the ant, taking the relative heights into consideration? The
largest pyramid in Egypt is only 146 yards high, that is, about ninety
times the average height of man; whereas, the nests of the Termites
are a thousand times the height of the insects which construct them.
Their habitations are thus twelve times higher than the largest
specimen of architecture raised by human hands. We are, therefore,
far beneath these little insects, as far as strength and the spirit
of working go.
The destructive power of these creatures, so insignificant in
appearance, are still more surprising. During the spring of a single
year they can effect the ruin of a house by destroying the beams and
planks. The town of La Rochelle, to which the Termites were imported
by an American ship, is menaced with being eventually suspended on
catacombs, like the town of Valencia in New Grenada. It is well known
what destruction is caused when a swarm of locusts alight in a
cultivated field; and it is certain that even their larvæ do as severe
injury as the perfect insect. All this sufficiently proves the
destructive capabilities of these little animals, which we are
accustomed to despise.
M. Plateau has studied the power of traction in some insects, the
power of pushing in the digging insects, and the lifting power of
others during flight. He has thus been able to make some most
interesting comparisons, of some of which we will relate the results.
The average weight of man being 142 lbs., and his power of traction,
according to Régnier, being 124 lbs., the proportion of the weight he
can draw to the weight of his body is only as 87 to 100. With the
horse the proportion is not more than 67 to 100, a horse 1,350 lbs. in
weight only drawing about 900 lbs. The horse, therefore, can draw
little more than half his own weight, and a man cannot draw the weight
of his own body.
This is a very poor result, if compared with the strength of the
cockchafer. This insect, in fact, possesses a power of traction equal
to more than fourteen times its own weight. If you amuse yourself with
the children's game of making a cockchafer draw small cargoes of
stones, you will be surprised at the great weight which this
insignificant looking animal is able to manage.
To test the power of traction in insects, M. Plateau attached them to
a weight by means of a thread fastened to one of their feet. The
_Coleoptera_ (Beetles) are the best adapted for these experiments.
The following are some of the results obtained by the Belgian
physician:--_Carabus auratus_ can draw seven times the weight of its
body; _Nebria brevicollis_, twenty-five times; _Necrophorus vespillo_,
fifteen times; _Trichius fasciatus_, forty-one times; and _Oryctes
nasicornis_, four times only. The bee can draw twenty times the weight
of its body; _Donacia nymphæ_[7] forty-two times its own weight.
[7] A beetle.--ED.
From this it follows that if the horse possessed the same strength as
this last insect, or if the insect were the size of a horse, they
would either of them be able to draw 155,250 lbs. M. Plateau has
ascertained the pushing power in insects, by introducing them into a
pasteboard tube, the interior of which was made rough, and in which
was fixed a glass plate, which allowed the light to penetrate into the
prison. The animal, if excited, struggled with all its strength
against the transparent plate, which, on being pushed forward, turned
a lever adapted to a miniature dynamometer, which indicated the amount
of effort exercised.
The results thus obtained prove that the pushing power, like the power
of traction, is greater in inverse proportion to the size and weight
of the animal. A few figures will better explain this curious law. In
_Oryctes nasicornis_ the proportion of the pushing power to the weight
of the insect is only three to two; in _Geotrupes stercorarius_ it is
sixteen to two; and in _Onthophagus nuchicornis_ seventy-nine to six.
Experiments have been made on the lifting power of insects by
fastening a ball of soft wax to a thread attached to the hind legs.
The proportion of the weight lifted has been found equal to that of
the body. That is to say, that the insect, when flying, can lift its
own weight. This is proved by the following calculations:--In the
Neuroptera the proportion is 1 in the Dragon-fly (_Libellula
vulgata_), ·7 in _Lestes sponsa_. In the order Hymenoptera it is ·78
in the bee, and ·63 in _Bombus terrestris_, the humble-bee. In the
Diptera it is ·9 in _Calliphora vomitoria_,[8] 1·84 in the _Syrphus
corollæ_, and 1·77 in the house-fly.
[8] The meat-fly.--ED.
These results show that insects have only sufficient power to sustain
their own weight when flying, as the above calculations exhibit the
maximum of which they are capable, and at the utmost this strength
would only compensate for the fatigue occasioned by the action of
flight.
At the same time it is to be observed that the Diptera, and among
others the house-fly, can sustain their flight longer than the
Hymenoptera and Neuroptera, although one would not think so from their
appearance. In conclusion, if an insect's power of flying is not
considerable, its power of traction and propulsion are immense,
compared with the vertebrate animals; and, in the same group of
insects, those that are the smallest and lightest are the strongest.
The proportion between the muscular strength of insects and the
dimensions of their bodies, would not appear to be on account of
their muscles being more numerous than those of vertebrate animals,
but on account of greater intrinsic energy and muscular activity. The
articulations of insects may be considered as solid cases which
envelop the muscles, and the thickness of these cases appears to
decrease in a singular manner according to the size of the creature.
The relative bulk of the muscles being less in the smaller species
than in the larger, it is necessary to explain the superior relative
strength of the former by supposing them to possess a greater amount
of vital energy.
These astonishing phenomena will perhaps be better understood if we
consider the obstacles which insects have to overcome to satisfy their
wants, to seek their food, to defend themselves against their enemies,
&c.
To meet these requirements they are marvellously constructed for both
labour and warfare, and their strength is superior to that displayed
by all other animals. It is also much greater than that of the
machines we construct to replace manual labour. They represent
strength itself. God's workmen are infinitely more powerful than those
invented by the genius of man, which we call machines.
We think it necessary, in closing this chapter, to give a sort of
general outline of the great class of animals which we are about to
study. If we wished to characterise insects by their exterior aspect,
we might consider them as articulate animals, whose bodies, covered
with tough and membranous integuments, are divided into three distinct
parts: the head, provided with two antennæ, and eyes and mouth of very
variable form; a trunk or thorax, composed of three segments, which
has underneath it always six articulated limbs, and often above it two
or four wings; and an abdomen, composed of nine segments, although
some may not appear to exist at first sight.
If, in addition to these characteristics, one considers that these
animals are not provided with interior skeletons--that their nervous
system is formed of a double cord, swelling at intervals, and placed
along the under-side of the body, with the exception of the first
swellings or ganglions which are under the head--that they are not
provided with a complete circulating system--that they breathe by
particular organs, termed tracheæ, extending parallel to each other
along each side of the body, and communicating with the exterior
air by lateral openings termed spiracles--that their sexes are
distinct--that they are reproduced from eggs--and, in conclusion,
that the different parts we have mentioned are not complete until
the creature has passed through several successive changes, called
metamorphoses, a general idea may be formed of what is meant in
zoology by the word "insect."
Insects, whose general organisation we have briefly traced, have been
classed by naturalists as follows:--
1. APTERA (Fleas and Lice).
2. DIPTERA (Gnats, Flies, &c.)
3. HEMIPTERA (Bugs, &c.)
4. LEPIDOPTERA (Butterflies and Moths).
5. ORTHOPTERA (Grasshoppers, Crickets, Cockroaches, &c.)
6. HYMENOPTERA (Bees, Wasps, &c.)
7. THYSANOPTERA (_Thrips cerealium_).
8. NEUROPTERA (_Libellula_, or Dragon-fly; _Ephemera_,
or May-fly; _Phryganea_, or Alder-fly).
9. COLEOPTERA (Beetles).
We shall commence the history of the various orders by examining the
Aptera.