[Illustration: _Painted by J. J. Masquerier._
_Engraved by W. T. Fry._
_William Spence, Esq^r., F.L.S._]
_Published by Longman & C^o. London, July 1825._
AN
INTRODUCTION
TO
ENTOMOLOGY:
OR
ELEMENTS
OF THE
_NATURAL HISTORY OF INSECTS_:
WITH PLATES.
BY WILLIAM KIRBY, M.A. F.R. AND L.S.
RECTOR OF BARHAM,
AND
WILLIAM SPENCE, ESQ. F.L.S.
IN FOUR VOLUMES.
VOL. IV.
_FIFTH EDITION._
LONDON:
PRINTED FOR
LONGMAN, REES, ORME, BROWN, AND GREEN,
PATERNOSTER ROW.
1828.
PRINTED BY RICHARD TAYLOR,
RED LION COURT, FLEET STREET.
CONTENTS OF VOL. IV.
Letter. Page.
XXXVII. Internal Anatomy and Physiology of
Insects. _Sensation_ 1-33
XXXVIII. Internal Anatomy and Physiology of
Insects continued. _Respiration_ 34-80
XXXIX. Internal Anatomy and Physiology of
Insects continued. _Circulation_ 81-101
XL. Internal Anatomy and Physiology of
Insects continued. _Digestion_ 102-126
XLI. Internal Anatomy and Physiology of
Insects continued. _Secretion_ 127-151
XLII. Internal Anatomy and Physiology of
Insects continued. _Reproduction_ 152-173
XLIII. Internal Anatomy and Physiology of
Insects concluded. _Motion_ 174-203
XLIV. Diseases of Insects 204-240
XLV. Senses of Insects 241-264
XLVI. Orismology, or Explanation of Terms 265-363
XLVII. System of Insects 364-428
XLVIII. History of Entomology 429-485
XLIX. Geographical Distribution of Insects;
their Stations and Haunts; Seasons;
Times of Action and Repose 486-527
L. On Entomological Instruments; and
the best Methods of collecting,
breeding, and preserving Insects 528-559
LI. Investigation of Insects 560-573
Appendix 575-584
Authors quoted 585-602
Explanation of the Plates 603-614
Indexes 615-683
AN
INTRODUCTION
TO
ENTOMOLOGY.
LETTER XXXVII.
_INTERNAL ANATOMY AND PHYSIOLOGY
OF INSECTS._
SENSATION.
Having given you this full account of the _external_ parts of
insects, and their most remarkable variations; I must next direct
your attention to such discoveries as have been made with regard
to their _Internal Anatomy and Physiology_: a subject still more
fertile, if possible, than the former in wonderful manifestations of
the POWER, WISDOM and GOODNESS of the CREATOR.
The vital system of these little creatures, in all its great features,
is perfectly analogous to that of the vertebrate animals. _Sensation_
and _perception_ are by the means of _nerves_ and a _common sensorium_;
the _respiration_ of air is evident, being received and expelled by
a particular apparatus; _nutrition_ is effected through a _stomach_
and _intestines_; the analogue of the _blood_ prepared by these organs
pervades every part of the body, and from it are secreted various
peculiar substances; _generation_ takes place, and an intercourse
between the sexes, by means of appropriate _organs_; and lastly,
_motion_ is the result of the action of _muscles_. Some of these
functions are, however, exercised in a mode apparently so dissimilar
from what obtains in the higher animals, that upon a first view we are
inclined to pronounce them the effect of processes altogether peculiar.
Thus, though insects respire _air_, they do not receive it by the
_mouth_, but through little orifices in the _sides_ of the body; and
instead of _lungs_, they are furnished with a system of air-vessels,
ramified _ad infinitum_, and penetrating to every part and organ of
their frame; and though they are nourished by a fluid prepared from
the food received into the stomach, this fluid, unlike the blood of
vertebrate animals, is _white_, and the mode in which it is distributed
to the different parts of the system, except in the case of the true
_Arachnida_, in which a circulation in the ordinary way has been
detected, is altogether obscure.
In order that you may more clearly understand the variations
that occur in insects, and in what respects they differ amongst
themselves, and from the higher animals, in the vital functions
and their organs, I shall consider them as to their organs of
_sensation_, _respiration_, _circulation_, _nutrition_, _generation_,
_secretion_, and _muscular motion_.
* * * * *
_Organs of Sensation._--The nervous system of animals is one of
the most wonderful and mysterious works of the CREATOR. Its pulpy
substance is the _visible_ medium by which the governing principle[1]
transmits its commands to the various organs of the body, and they
move instantaneously--yet this appears to be but the conductor of some
higher principle, which can be more immediately acted upon by the mind
and by the will. This principle, however, whatever it be, whether we
call it the nervous _fluid_, or the nervous _power_[2], has not been
detected, and is known only by its effects. The system of which we are
speaking may therefore be deemed the foundation and root of the animal,
the centre from which emanate all its powers and functions.
Comparative anatomists have considered the nervous system of
animals as formed upon _four_ primary types, which may be
called the _molecular_, the _filamentous_, the _ganglionic_,
and _cerebro-spinal_[3]. The _first_ is where invisible nervous
molecules are dispersed in a gelatinous body, the existence of
which has only been ascertained by the nervous irritability of such
bodies, their fine sense of touch, their perceiving the movements
of the waters in which they reside, and from their perfect sense
of the degrees of light and heat[4]. Of this description are the
infusory animals, and the _Polypi_. The nervous molecules in these
are conjectured to constitute so many ganglions, or centres of
sensation and vitality[5]. The _second_, the filamentous, is where
the nervous system consists of nervous threads radiating from the
mouth, as in the _Radiata_, or star-fish and sea-urchins[6]. The
_third_, the ganglionic, is where the nervous system consists of
a series of ganglions connected by nervous threads or a medullary
chord, placed, except the first ganglion, below the intestines, from
which proceed nerves to the various parts of the body. This system
may be considered as divisible into two--the _proper ganglionic_,
in which it is ganglionic with the ganglions arranged in a series
with a double spinal chord. This prevails in the classes _Insecta_,
_Crustacea_, _Arachnida_, &c., and the _improper ganglionic_, in
which it is ganglionic with the ganglions dispersed irregularly,
but connected by nervous threads, as in the _Mollusca_[7]. In the
_fourth_, the cerebro-spinal, the nervous tree may be said to be
double, or to consist of _two_ systems--the first taking its origin
in a brain formed of two hemispheres contained in the cavity of the
head, from which posteriorly proceeds a spinal marrow, included
in a dorsal vertebral column. These send forth numerous nerves to
the organs of the senses and the muscles of the limbs. The second
consists of two principal ventral chords, which by their ganglions,
but without any direct communication, anastomose with the spinal
nerves and some of those of the brain, and run one on each side from
the base of the skull to the extremity of the _sacrum_. This system
consists of an assemblage of nervous filaments bearing numerous
ganglions, from which nervous threads are distributed to the organs
of nutrition and reproduction[8]. Its chords are called the _great
sympathetic_, the _intercostal_, or _trisplanchnic_ nerves[9].
While the first of these two systems is the messenger of the will,
by means of the organs of the senses connects us with the external
world, and is subject to have its agency interrupted by sleep or
disease[10]; the latter is altogether independent of the will and of
the intellect, is confined to the internal organic life, its agency
continues uninterrupted during sleep, and is subject to no paralysis.
While the former is the seat of the intellectual powers, the latter
has no relation to them, but is the focus from whence _instincts_
exclusively emanate: from it proceed spontaneous impulses and
sympathies, and those passions and affections that excite the agent
to acts in which the will and the judgement have no concern[11].
It is probable, though the above appear to exhibit the _primary_ types
of nervous systems, that others exist of an _intermediate_ nature, with
which future investigators may render us better acquainted[12]: but as
our business is solely with that upon which _insects_ in this respect
have been modelled, without expatiating further in this interesting
field, I shall therefore now confine myself to them.
We have before seen[13] that the nervous system of insects belongs to
the _ganglionic_ type: but it requires a more full description, and
this is the place for it. It originates in a small brain placed in
the head, and consisting almost universally of two lobes, sometimes
extremely distinct. It is placed over or upon the _œsophagus_ or
gullet, and from its posterior part proceeds a double nervous chord,
which embracing that organ as a collar dips below the intestines, and
proceeds towards the anus, forming knots or ganglions at intervals,
in many cases corresponding in number with the segments of the body,
and sending forth nerves in pairs, the ramifications of which are
distributed to every part of the frame. In the perfect insect the
bilobed ganglion of the head or the brain is usually of greater volume
than in the larva, and the ganglions of the spinal chord are fewer,
which gives a more decided character of _centricity_ to the whole
nervous system[14]. This may be considered more particularly with
respect to its _substance_ and _colour_; its _tunics_, and _parts_.
I. _Substance and Colour._--The nervous apparatus of insects is
stated by those who have examined it most narrowly, though consisting
of a cortical and medullary part, the latter more delicate and
transparent than the former, to be less tender and less easy to
separate than the human brain[15]. It has a degree of tenacity,
and does not break without considerable tension; in general, it
is clammy and flabby, and under a microscope a number of minute
grains are discoverable in it, and when left to dry upon glass, it
appears to contain a good deal of oil, which does not dry with the
rest[16]. That of the ganglions differs from the substance of the
rest of the spinal chord, in being filled with very fine aërial
vessels, which are not discoverable in the latter[17]. With regard
to _colour_, Lyonet states that the chords of the spinal marrow in
the larva of the great goat-moth are of a blueish gray, and have some
transparence[18]; Malpighi and Swammerdam observed that the cortical
part of the ganglions of that of the silk-worm and the hive-bee had a
reddish hue, while the medullary part was white[19]; Cuvier relates
that the brain and the third ganglion in _Hypogymna dispar_, with
us a scarce moth, differed in colour from all the rest, being quite
white, while the others were more or less tinted, and examined under
a lens appeared variegated by reddish sinuous markings, resembling
blood vessels as they are seen in injected glands[20].
II. _Tunics._--The coats that inclose the various branches of the
nervous system in insects seem analogous to those of vertebrate
animals. The first thing that strikes the eye, when these parts in
a recent subject are submitted to a microscope, is a tissue of very
delicate vessels, which ramify beyond the reach of the assisted
sight; these are merely air-vessels or _bronchiæ_ derived originally
from the _tracheæ_ of the animal: but besides these is an exterior
and an interior tunic; the first corresponding with the _dura mater_
of anatomists; and the other, which is the most delicate and
incloses the cortical and medullary parts, with the _pia mater_[21].
III. _Parts._--The nervous system of insects consists of the _brain_;
the _spinal marrow_ and its _ganglions_; and the _nerves_.
i. _Brain_.[22] Linné denied the existence of a _brain_ in insects,
and most modern physiologists seem to be of the same opinion. A
part however, analogous to this important organ--at least in its
situation, and in its emission of nerves to the principal organs of
the _senses_, in which respect it certainly differs very materially
from the upper cervical ganglion, which Dr. Virey regards as its
analogue[23]--is certainly to be found in them; and as Messrs. Cuvier
and Lamarck distinguish this part by the name of _brain_, we may
continue to call it by that name without impropriety. The _brain_
of insects, then, is distinguished from the succeeding ganglions of
the spinal chord by its _situation_ in the head, the middle of the
internal cavity of which it occupies, and by being the only ganglion
_above_ the œsophagus. It is usually small, though in some cases
larger than they are[24]. It consists of two lobes, more or less
distinct and generally of a spherical form. In _Oryctes nasicornis_
and _Pontia Brassicæ_ the lobes are separated both before and
behind[25]; while in the larva of _Dytiscus marginalis_, but not in
the imago, in which there are two large hemispheres separated by a
furrow, the brain is undivided[26]. Cuvier mentions the larva of a
saw-fly in which this part is formed of _four_ nearly equal spherical
bulbs[27]: in the Scorpion (to judge by the figure of Treviranus[28])
the two lobes represent an equilateral triangle, the exterior angle
of which terminates in several lesser spherical bulbs; in _Acrida
viridissima_, _Nepa cinerea_, _Clubiona atrox_, and the common Louse,
the lobes are pear-shaped[29].
ii. _The spinal marrow and its ganglions_[30]. From the _posterior_
part of the brain of insects, but in the ground and water beetles
(_Eutrech_in_a_ and _Eunech_in_a_) from its _sides_ below[31], issue
two chords which diverging embrace the _œsophagus_, and dipping
below it and the intestines,--a situation they maintain to the end
of their course,--and in their further progress uniting at intervals
and dilating into several knots or ganglions, compose their spinal
marrow. This part is so named, from a supposed analogy to the spinal
marrow of vertebrate animals, which however admits of some degree of
doubt; yet, since it mixes the functions of that organ with those
of the great sympathetic nerves, the denomination is not wholly
improper, and may be retained. Though this chord is usually _double_
when it first proceeds from the brain, and surrounds the _œsophagus_
like a collar, yet in some insects it may be called a _single_ chord.
This is the case with that of the common louse, in which Swammerdam
could perceive no opening for the transmission of the part just
named[32]; if he was not mistaken in this, the brain, as well as
the rest of the spinal marrow in that animal, would be below the
intestines; from the figures of Treviranus it should seem that the
spiders, at least _Clubiona atrox_, are similarly circumstanced[33];
in the cheese-maggot, which turns to a two-winged fly (_Tyrophaga
Casei_), the chord is also single, but it has a small orifice
through which the gullet passes[34]. At the union of the chords in
other cases below that organ, a knot or ganglion is usually formed,
and an alternate succession of internodes and ganglions commonly
follows to the end. The internodes also may generally be stated to
consist of a _double_ chord, though in many cases the two chords
unite and become one, or are distinguished only by a longitudinal
furrow, and even where they are really distinct and separable, in the
body of the insect they lie close together[35]. In the rhinoceros
beetle (_Oryctes nasicornis_) and _Acrida viridissima_ &c. _all_
the internodes consist of a double chord[36]; but in many other
insects numerous variations in this respect occur.--Thus in the
stag-beetle the _last_ internode is single[37]; in the caterpillar
of the cabbage butterfly (_Pontia Brassicæ_) the _five first_ are
double, and the _six last_ single[38]; in that of the great goat-moth
(_Cossus ligniperda_) the _three first_ only are double, but the
others terminate in a fork[39]; in the cockroaches (_Blatta_) the
_four first_, in _Hydrophilus piceus_ the _three first_, and in
_Eristalis tenax_ the _two first_ only are double, the rest being all
single[40]. A singular variation takes place in _Hypogymna dispar_;
_all_ the internodes are single, except the _second_, the chords of
which at first are separate, and afterwards united[41]; and, to name
no more, in _Clubiona atrox_ there is only _one_ internode, which is
single, with a longitudinal furrow[42]. In some, as in the louse, the
grub of _Oryctes nasicornis_, and the cheese-maggot, there are no
internodes, the spinal marrow being formed of knots separated only by
slight or deep constrictions[43].
I must next say something of the _ganglions_[44]. Lyonet has observed
that, in the caterpillar of the great goat-moth, these in one respect
differ remarkably from the chords that connect them; in the latter
the air-vessels or bronchiæ only cover the _outside_ of the tunic,
while in the former they enter the _substance_ of the ganglion, which
is quite filled with their delicate and numberless branches[45].
Every ganglion may be regarded in some degree as a centre of vitality
or little brain[46], and in many cases, as well as the brain,
they are formed of two lobes[47]. I shall now consider them more
particularly as to their _station_, _number_, and _shape_.
1. With regard to the first head, their _station_, they are most
commonly divided between the trunk and abdomen; but in some cases,
as in _Hydrophilus piceus_ and _Acrida viridissima_, the _first_
ganglion is in the _head_[48]; in others, as in the louse, the
water-scorpion, and the grub of the rhinoceros-beetle, they are
confined to the _trunk_, their functions in the abdomen being
supplied by numerous radiating nerves[49]; in others again, as in
the scorpion, they are all _abdominal_. The ganglions vary also in
their situation with respect to each other. Thus in some, as in the
larva of the Chamæleon-fly (_Stratyomis Chamæleon_), they are so near
as to appear like a string of beads[50]; in that of the ant-lion
(_Myrmeleon_) the two ganglions of the trunk are separated by an
interval from those of the abdomen, which are so contiguous as to
resemble the rattle of the rattle-snake[51]. In others the internodes
are longer, and the ganglions occur at nearly equal intervals, as in
the larva of the _Ephemeræ_[52]; but in the majority they are unequal
in length: thus in the scorpion the three first ganglions are the
most distant[53]; in the hive-bee the third and fourth[54]; and in
the spider the last[55].
2. The ganglions also in different species, and often in the same
insect in its different states, vary in their _number_. Thus in
the grub of the rhinoceros-beetle the whole spinal marrow appears
like a _single_ ganglion divided only by transverse furrows[56];
in the water-scorpion there are _two_[57]; in the louse there
are _three_[58]; in the rhinoceros-beetle there are _four_[59];
_five_ in the stag-beetle[60]; _seven_ in the hive-bee and some
_Lepidoptera_[61]; _eight_ in the grub of the stag-beetle[62]; _nine_
in the great _Hydrophilus_[63]; _ten_ in _Dytiscus_[64]; _eleven_
in the grub of the great _Hydrophilus_[65]; _twelve_ in the grub of
_Dytiscus_ and the caterpillars of _Lepidoptera_[66]; _thirteen_
in the larva of _Æshna_[67]; and _twenty-four_ in _Scolopendra
morsitans_[68]. You must observe that, generally speaking, the number
of ganglions is less in the _imago_ than in the _larva_. With regard
to the distribution of these knots to the different primary parts
of the body, the following table will exhibit it, as far as I am
acquainted with it, at one view. I omit those in which the ganglions
are only in _one_ of these parts.
Head. Trunk. Abdomen.
_Acrida viridissima_ 1 3 6[69]
_Hydrophilus piceus_ 1 6 2
_Clubiona atrox_ 0 2 1
_Gryllotalpa vulgaris_ 0 2 7[70]
_Myrmeleon, Larva_ 0 2 8[71]
_Eristalis tenax_ 0 3 2[72]
_Apis mellifica_ 0 3 4
_Ephemera, Larva_ 0 3 7
_Æshna, Larva_ 0 6 7
3. I am next to say a few words upon the _shape_ of the ganglions.
Most commonly it approaches to a _spherical_ figure, but in many
instances, as I said before, they, as well as the brain, consist
of _two_ lobes: they are, however, seldom all precisely of the same
shape. In the _Dytisci_, and _Carabi_, the last is marked with a
transverse furrow, which seems to indicate the reunion of two[73];
in the stag-beetle, the first ganglion is oval or elliptical, the
second hexagonal; the third and fourth shaped like a crescent, and
the last like an olive[74]; in the caterpillar of the great goat-moth
the first is oblong and constricted in the middle, and the seven last
are rhomboidal[75]; in the great _Hydrophilus_ the _second_, and in
the silk-worm _all_ the ganglions are quadrangular[76]; in _Hypogymna
dispar_ the _third_ is heart-shaped[77]; the great ganglion which
forms the spinal marrow of the cheese-maggot is pear-shaped[78];
that of the grub of the rhinoceros-beetle is fusiform[79]; and in
the scorpion all the ganglions are lenticular[80]. But the most
remarkable in this respect are those of a spider (_Clubiona atrox_):
in this insect the brain sits upon a bilobed ganglion of the ordinary
form, which is immediately followed without any internode by another
bilobed one, terminating on each side in four pear-shaped processes
or fingers, which give it a very singular appearance[81].
iii. The _nerves_[82] of insects, as of other animals, are white
filaments running from the brain and spinal marrow to every part
of the body which they are destined to animate; and their numerous
ramifications, when delineated, form no unpleasing picture[83].
In the caterpillar of the goat-moth the accurate Lyonet counted
_forty-five_ pairs of them, and _two_ single ones, making in all
_ninety-two_ nerves; whereas in the _human_ body anatomists count only
_seventy-eight_[84]. From the brain issue several pairs, which go to
the _eyes_, _antennæ_, _palpi_, and other parts of the mouth: sometimes
those that render to the mandibles issue from the first ganglion, as
in the larva of _Dytiscus marginalis_, the stag-beetle, &c.[85]; those
both of _mandibles_ and _palpi_ in the great _Hydrophilus_[86]; and in
_Blatta_ some which act also upon the _antennæ_[87].
The _optic_ are usually the most conspicuous and remarkable of the
nerves. In some insects with large eyes, as many _Neuroptera_,
_Hymenoptera_, and _Diptera_, their size is considerable; in the
hive-bee they present the appearance of a pair of kidney-shaped
lobes, larger than the brain[88]; in the dragon-flies, whose brain
consists of two very minute lobes, these nerves dilate into two large
plates of a similar shape, which line all the inner surface of the
eyes[89]; in the stag-beetle they are pear-shaped, and terminate in
a bulb, from which issue an infinity of minute nerves[90]; it is
probable that this takes place in all cases, and that a separate
nerve renders to every separate lens in a compound eye[91]; the optic
nerve in _Dytiscus_ and _Carabus_ is pyramidal, with the base of the
pyramid at the eye and the summit at the brain[92]; in _Eristalis
tenax_ it is very large, cylindrical, and of a diameter equal to
the length of the last-mentioned part, upon the side of which it is
supported; it terminates in a very large bulb corresponding to the
eye[93]: in _Scolopendra morsitans_ the optic nerves divide into four
branches long before they arrive at the eyes, and in this insect the
nerves which render to the antennæ are so thick as to appear portions
of the brain, which they equal in diameter[94]. Swammerdam discovered
in the grub of the rhinoceros-beetle and in the caterpillar of
the silk-worm, a pair of nerves which he regarded as analogous
to the _recurrent nerves_ in the human subject, and therefore he
distinguishes them by the same name[95]: they issue from the lower
surface of the brain, or that which rests on the _œsophagus_, and at
first go towards the mouth, but afterwards turn back, and uniting
form a small ganglion; this produces a single nerve, which passing
below the brain follows the œsophagus to the stomach, where it swells
into another ganglion, from which issue some small nerves that
render to the stomach, and one more considerable which accompanies
the intestinal canal, producing at intervals lateral filaments which
lose themselves in the tunics of that tube[96]. Lyonet afterwards
discovered these nerves in the caterpillar of the goat-moth[97], and
Cuvier in other insects[98].
The other nerves which issue from the brain exhibit no remarkable
features. Those which originate in the spinal marrow are mostly derived
from the ganglions, and are sometimes interwoven with the muscles, as
the woof with the warp in a piece of cloth[99]; those from the three
or four first commonly rendering to the muscles of the legs, wings,
and other parts of the _trunk_, and those from the remainder to the
_abdomen_. After their origin they often divide and subdivide, and
terminate in numerous ramifications that connect every part of the body
with the _sensorium commune_. A _pair_ of nerves is the most usual
number that proceeds from each side of a ganglion[100]; but this is
by no means constant, since in the louse, the hive-bee, and several
other insects, only a _single_ nerve thus proceeds[101]; and in the
larva of _Ephemeræ_, while _two_ pairs issue from the _six first_
ganglions, only a _single_ one is emitted by the _five last_[102].
In the spinal marrow of the rhinoceros-beetle, both larva and imago,
the nerves consist of simple filaments which diverge like rays in all
directions[103]: the same circumstance distinguishes the cheese-maggot,
only some of the nerves appear to branch at the end[104]: in the
louse, the last ganglion sends forth posteriorly three pairs of nerves
which render to the abdomen[105]. Sometimes, though rarely, nerves
originate in the _internodes_ of the spinal marrow. Cuvier indeed
has asserted that in invertebrate animals _all_ the nerves spring
from the ganglions, and never immediately from the spinal marrow; but
Swammerdam, in describing those of the silk-worm, mentions and figures
four pairs as proceeding from the four anterior internodes, excluding
the first[106]; and at the same time he gives it as his opinion, that
all the nerves in insects really originate from the marrow itself, and
not from the ganglions, which he asserts are of a different substance,
and are inclosed in the marrow for the sake of giving it greater
firmness[107]. In this opinion, however, he seems singular[108]. Those
remarkable nerves described by Lyonet under the name of _spinal bridle_
(_bride épinière_) also take their origin, not from the ganglions, but
from a bifurcation of the spinal marrow. Of these, in the caterpillar
of the goat-moth there are _ten_, the first issuing from the
bifurcation of the internode between the fourth and fifth ganglions,
and the remainder from the succeeding ones. After approaching the
succeeding ganglion, these nerves form a pair of branches that diverge
nearly at right angles from the bridle, and producing several lesser
branches, lose themselves in the sides of the animal[109]. Besides the
nerves above mentioned, two generally issue from the posterior part
of the last ganglion, diverging in opposite and oblique directions:
some of these render to the parts of generation; and in the silk-worm,
and probably other species, the innermost pair is perforated for the
passage of the _vasa deferentia_[110].
After duly considering this general outline of the nervous system
of insects, the question will continually occur to you,--is then
what you have called the _brain_ the _sensorium commune_ of these
animals, in the same manner as it is in those with warm blood? To
this query a negative must be returned. In the latter, the brain is
the common centre to which, by means of the nerves and spinal marrow,
all the sensations of the animal are conveyed, and in which all its
perceptions terminate. The nerves and spinal marrow are merely the
_roads_ by which the sensations travel; and if their communication
with the brain, by any means be cut off at the neck, the whole
trunk of the animal becomes paralytic, evidently proving that the
organ by which it feels is the brain. This, however, is so far from
being the case in insects, that in them, if the head be cut off,
the remainder of the body will continue to give proofs of life and
sensation longer than the head: both portions will live after the
separation, sometimes for a considerable period; but the largest
will survive the longest, and will _move_, _walk_, and occasionally
even _fly_, at first almost as actively without the head, as when
united to it. Lyonet informs us, that he has seen motion in the body
of a wasp _three_ days after it had been separated from the head;
and that a caterpillar even _walked_ some days after that operation;
and when touched, the headless animal made the same movements as
when intire[111]. Dr. Shaw has observed--an observation confirmed in
Unzer's _Kleine Schriften_,--that if _Geophilus electricus_ be cut in
two, the halves will live and appear vigorous even for a _fortnight_
afterwards; and what is more remarkable, that the _tail_ part always
survives the _head_ two or three days[112]. The _sensorium commune_
of insects, therefore, does not, as in the warm-blooded animals,
reside in the brain alone, but in the spinal marrow also. It was on
this account probably that Linné denied the existence of a _brain_ in
insects, regarding it merely as the first ganglion of the spine.
Cuvier and other modern physiologists, from the ganglionic structure
of this organ, are of opinion that it is not the analogue of the
_cerebro-spinal_ system of vertebrate animals, but rather of
their _great sympathetic_ nerves. Indeed, considering solely the
_external_ structure of the nervous system of insects, a great
resemblance strikes us between it and these nerves; for besides
its general ganglionic structure, there is also in them an _upper_
ganglion in the neck, seemingly corresponding with what we have
named the brain of insects, from which the nervous chord dips to
the lower part of the neck, where it forms a _second_ ganglion,
which appears to correspond with what we have considered as their
second ganglion[113]. We may observe, however, that at least in
one respect there is even an _external_ resemblance between the
brain of insects and that of vertebrate animals:--it most commonly
consists, as has been stated, like them, of two lobes, often very
distinct; a circumstance which not unfrequently distinguishes the
other ganglions[114], and is not borrowed from the ganglions of
the great sympathetics. With respect to the internal structure
of the ganglions and spinal marrow of insects, we know little to
build any theory upon, except that the internal substance of the
former is filled with air-vessels; at least so Lyonet, as has been
already observed, found in the goat-moth, while only the tunics of
the latter are covered by them. Taking the above resemblance to the
brain of vertebrates into consideration, there appears ground for
thinking that the nervous system of insects, like some of their
articulations[115], is of a _mixed_ kind, combining in it both the
cerebro-spinal and the ganglionic systems; and this will appear
further if we consider its _functions_.
That learned and acute physiologist Dr. Virey, assuming as an
hypothesis, that the structure of the system in question is simply
ganglionic, and merely analogous to the sympathetic system of
vertebrate animals, has built a theory upon the assumption, which
appears evidently contradicted by facts. Because, as he conceives
after Cuvier, insects are not gifted with a real brain and spinal
marrow, he would make it a necessary consequence that they have no
degree of _intellect_, no memory, judgement or free will; but are
guided in every respect by instinct and spontaneous impulses,--that
they are incapable of instruction, and can superadd no acquired
habits to those which are instinctive and inbred[116]. This
consequence would certainly necessarily follow, was their nervous
system perfectly analogous to the sympathetic of warm-blooded
animals. But when we come to take into consideration the _functions_
that in insects this system confessedly discharges, we are led to
doubt very strongly the correctness of the assumption. Now in these
animals the system in question not only renders to the nutritive and
reproductive organs, which is the principal function of the great
sympathetic nerves in the vertebrates; but by the common organs
maintains a connexion with the external world, and acquires ideas
of things without, which in them is a function of the cerebral
system: from the same centre also issue those powers which at the
bidding of the will put the limbs in action, which also belongs to
the cerebral system. That insects have memory, and consequently a
real brain, has been before largely proved, as also that they have
that degree of intellect and judgement which enables them to profit
by the notices furnished by their senses[117]. What can be the use
of eyes,--of the senses of hearing, smelling, feeling, &c. if they
are not instructed by them what to choose and what to avoid? And
if they _are_ thus instructed--they must have sufficient intellect
to apprehend it, and a portion of free will to enable them to act
according to it. With regard to the assertion that they are incapable
of instruction, or of acquiring new habits; few or no experiments
have been tried with the express purpose of ascertaining this point:
but some well-authenticated facts are related, from which it seems to
result that insects may be taught some things, and acquire habits not
instinctive. They could scarcely be brought from their wild state,
and domesticated, as bees have been so universally, and both ants
and wasps occasionally[118], without some departure from the habits
of their wild state; and the fact of the corsair-bees, that acquire
predatory habits before described[119], shows this more evidently:
but one of the most remarkable stories to our purpose upon record,
is that of M. Pelisson, who, when he was confined in the Bastile,
tamed a spider, and taught it to come for food at the sound of an
instrument. A manufacturer also in Paris, fed 800 spiders in an
apartment, which became so tame that whenever he entered it, which he
usually did bringing a dish filled with flies but not always, they
immediately came down to him to receive their food[120].
All these circumstances having their due consideration and weight,
it seems, I think, most probable, that as insects have their
communication with the external world by means of certain organs in
connexion with their nervous system, and appear to have some degree
of intellect, memory, and free will, all of which in the higher
animals are functions of a cerebral system, and at the same time in
other respects manifest those which are peculiar to the sympathetic
system,--it is most probable, I say, as was above hinted, that in
their system _both_ are _united_.
I must bespeak your attention to a circumstance connected with the
subject of this letter, which merits particular consideration: I
mean the gradual change that takes place in the nervous system
when insects undergo their metamorphoses; so that, except in the
_Orthoptera_, _Hemiptera_, and _Neuroptera_ Orders, in which no
change is undergone, the number of ganglions of the spinal chord is
less in the imago than in the larva. There seems an exception indeed
to this rule in the case of the rhinoceros-beetle, in the larva of
which there is only _one_ ganglion, while in the imago there are
_four_[121]. But as this one ganglion occupies the whole spinal
marrow, it is really of greater extent than the four of the imago;
so that even in this case there is a concentration of the cerebral
pulp. In some cases, as in _Dytiscus marginalis_, and _Hydrophilus
piceus_[122], the imago has only _one_ ganglion less than the larva,
but more generally it loses _four_ or _five_. Dr. Herold has traced
the gradual changes that take place in the spinal marrow of the
common cabbage-butterfly (_Pontia Brassicæ_), from the time that it
has attained its full size to its assumption of the imago. Of these I
shall now give you some account.
In the full-grown caterpillar, besides the brain there are _eleven_
ganglions, the chords of the four first internodes being double, and
the rest single: from each ganglion proceed two pairs of nerves, one
from each side. In this the lobes of the brain form an angle with
each other[123]. In two days the double chords mutually recede, so
as to diminish the interval between the ganglions, and the single
ones have become curved: thus the length of the spinal marrow is
shortened about a _fourth_, and the fourth and fifth ganglions
have made an approach to each other[124]. On the eighth day, when
the insect has assumed the pupa but remains still in the skin of
the caterpillar, the flexure of the internodes is much increased;
the first ganglion is now united to the brain, and the fourth and
fifth have joined each other, though they are still distinct; the
spinal marrow has now lost considerably more than a _third_ of
its length[125]. On the fourteenth day, the internodes, except the
double ones, have become nearly straight again; the fourth and
fifth ganglions have coalesced so as to form one, and the sixth and
seventh have each lost their pairs of nerves[126]. Shortly after
this, these last ganglions have nearly disappeared, and the chords
of the three first internodes have again approached each other[127].
The next change exhibited is the absorption of the first ganglion by
the brain, the union of the chords of the first internode, which is
now straight, the approximation of the second and third ganglions,
and the enlargement of the one formed by the union of the fourth
and fifth, at the expense perhaps of the sixth and seventh, which
have now intirely disappeared, and in their place is a very long
internode. These united ganglions retain the pairs of nerves they
had when separate[128]. Just before the assumption of the _imago_,
the direction of the lobes of the brain becomes horizontal, the
second and third ganglions unite, and the internode between the third
and fourth is shortened[129]. Lastly, when the animal is become a
butterfly, the second and third ganglions have coalesced, and are
joined to that formed by the union of the fourth and fifth; a short
isthmus or rather constriction, with an orifice, being their only
separation: each of these united ganglions send forth laterally four
pairs of nerves[130]. In his figure, Dr. Herold has not represented
the orifice for the passage of the gullet, but doubtless one exists,
which for an animal that imbibes only fluid food is probably very
minute. In _Hypogymna dispar_, we learn from Cuvier, this orifice is
of that description, and of a triangular shape[131].
It can admit of no reasonable doubt that one of the principal
intentions of these changes is to accommodate the nervous system to
the altered functions of the animal in its new stage of existence,
in which the antennæ, eyes, and other organs of the senses, as well
as the limbs and muscles moving them, and the sexual organs, being
very different from those of the larva, and if not wholly new, yet
expanded from minute germs to their full size, may well demand
corresponding changes in the structure of the nervous system by which
they are acted upon.
But are these changes also concerned, as Dr. Virey conjectures,
in producing that remarkable alteration which usually takes place
between the _instincts_ of the larva and imago? In order to answer
this question, it will be requisite first to quote the ingenious
illustration with which this able physiologist elucidates his ideas
on this point. "The more readily," he observes, "to comprehend
the action of instinct, let us compare the insect to one of those
hand-organs in which a revolving cylinder presents different tunes
noted at its surface, and pressing the keys of the pipes of the
organ, gives birth to all the tones of a song: if the tune is to
be changed, the cylinder must be pulled out or pushed in one or
more notches, to present other notes to the keys. In the same
manner let us suppose that nature has impressed or engraved certain
determinations or notes of action, fixed in a determinate series
in the nervous system and the ganglions of the caterpillar, by
which alone she lives, she will act according to a certain sequence
of operations; and, so to speak, she will sing the air engraven
within her. When she undergoes her metamorphosis into a butterfly,
her nervous system being, if I may so express myself, pulled out a
notch, like the cylinder, will present the notes of another tune,
another series of instinctive operations; and the animal will even
find itself as perfectly instructed and as capable of employing its
new organs, as it was to use the old ones. The relations will be the
same; it will always be the play of the instrument[132]."
This illustration is doubtless at the first glance very striking and
plausible: but a closer examination will, I think, show, that, as
in so many other instances in metaphysical reasoning, when fanciful
analogies are substituted for a rigid adherence to stubborn facts, it
is satisfactory only on a superficial view, and will not stand the test
of investigation; and as this is a question intimately connected with
what I have advanced on the subject of instinct in a former letter, I
must be permitted to go somewhat into detail in considering it.
To prove his position, Dr. Virey ought at least to be able to show
that, whenever a change takes place in the instincts of insects in
their different states of larva and imago, a corresponding change
takes place in the external structure of the nervous chord. But what
are the facts? In three whole orders, viz. _Orthoptera Hemiptera_,
and _Neuroptera_, as mentioned above[133], the structure of the
nervous chord is _not_ changed; and yet we know that many tribes
of these orders acquire instincts in their imago state altogether
different from those which directed them in their state of larvæ. A
perfect _Locust_, for instance, acquires the new instincts of using
its wings; of undertaking those distant migrations of which so many
remarkable instances were laid before you in a former letter[134];
and, if a female, of depositing its eggs in an appropriate situation.
But if such striking changes in the instinct of these tribes can be
effected without any perceptible alteration in the structure of the
nervous chord, it is contrary to the received rules of philosophical
induction to refer to this alteration the changes in the instincts of
other tribes where it is found. Is it not far more probable that this
alteration has in fact no connexion with the changes of instinct,
but is solely concerned with those remarkable changes in the organs
of sense and motion, which occur in the larva and imago states
of the orders in which it is observed? In a common caterpillar,
the form of the body, the legs, the eyes, and other organs of the
senses, all strikingly differ from those of the imago; whereas, with
the exception of the acquisition of new wings, a perfect locust
differs little from its larva: so that we may reasonably expect a
corresponding change, such as we find it, in the structure of the
nervous chord of the lepidopterous insect, not called for in that of
the neuropterous species, in which accordingly it does not take place.
This reasoning, in opposition to Dr. Virey's theory, that the changes
of instinct depend on the altered structure of the nervous system,
becomes greatly strengthened when we advert to the higher classes of
animals, which surely in any investigation of the nature of instinct
ought to be closely kept in view; for the faculty, though often less
perfect in them than in insects, is still of the _same kind_, and may
consequently be expected to follow the same general laws. In a young
swallow, for example, all its instincts are not developed at once any
more than in an insect. The instinct which leads it to migrate does
not appear for some months after its birth, and that of building a
nest still later. But we have not the slightest ground for believing
that these new instincts are preceded by any change in the structure
of the great sympathetic nerve, or of any other portion of the
nervous system: and the same may be said as to the sexual instincts
developed in quadrupeds some years subsequent to their birth. If,
then, these remarkable changes in the instinct of the higher classes
of animals can take place independently of any visible change in the
nerves, what substantial reason can be assigned why they may not also
in the class of insects?
On the whole, I think you will agree with me, that there is nothing
in Dr. Virey's hypothesis which should lead me to alter the opinion
I have already so strongly expressed in a former letter[135],
as to the insufficiency of the mechanical theories of instinct
hitherto promulgated, adequately to explain _all_ the phenomena;
and unless they do this they are evidently of small value. Such
theories as I have there adverted to may often seem to be supported
by a few insulated facts, but with others, far more numerous, they
are utterly at variance; and, to omit many other instances, I am
strongly inclined to doubt the possibility of satisfactorily
explaining the _variety_ of instincts exercised by a bee[136], or the
_extraordinary_ development of new ones in particular circumstances
only[137], on any merely mechanical grounds.
And after all, even suppose it could be demonstratively shown that
_every_ instinct is as clearly dependent on secondary causes, as I have
formerly admitted that _some_ doubtless seem to be, yet what would
this teach us as to the essential nature of instinct? We have advanced
indeed a step; but still, as I have before observed in referring to the
theories of Brown and Tucker, we have only placed the world upon the
tortoise, and instinct, as to its _essence_, which is what we want to
detect, is as mysterious as ever: just as, though we can clearly prove
that the mind is acted upon by the senses, yet this throws no light
upon the essential nature of the mind, which we are forced to admit
is inscrutable, as if to teach us humility, and prevent our vainly
fancying, that though allowed to discover some of the arcana of nature,
we shall ever be able to penetrate into her inmost sanctuaries.