the bases of the legs in each pair are closely approached to one another.
They are so set into the body that the basal joints, or, as they are
called, the coxal joints, nearly touch. This is the case in almost all
Chilognatha, except in the Polyxenidae, and it is a fact connected with
some important features in the internal anatomy. Then we have the
peculiarity in the Chilognatha which has formed the basis of most
classifications which have placed these animals in a group by themselves.
This is the possession in most segments of two pairs of legs. This
characteristic has caused the group to be called by some naturalists
Diplopoda. As a general rule, the first four segments {54}have only three
pairs of legs between them, one of them being without a pair of legs. This
legless or apodal segment is usually the third. From the fifth segment to
the end of the body all the segments have two pairs of legs each. The legs
are shorter than those of the Chilopods, and are all nearly equal in size.
This is not the case in the other Orders. The legs are commonly wanting in
the seventh segment of the male, and are replaced by a copulatory organ.
This peculiarity is related to the different position of the generative
openings in the Chilognatha. Another anatomical feature peculiar to the
Chilognatha is the possession of the stink glands—the _glandulae
odoriferae_ before mentioned. This, however, is a character which does not
hold for all the Chilognatha, since the Polyxenidae have none of these
glands. All the other families, however, possess them, and they are present
in none of the other Orders.
As regards the internal anatomy of the Chilognatha, the _digestive canal_
differs mainly in the glands which supply it with secretions. It receives
the saliva from two long tubular salivary glands, which open at the base of
the four-lobed plate which has been mentioned as the third of the mouth
appendages. The secretion of these glands is used, as has already been
said, in the process of preparing the nest for the eggs. We cannot fail to
be reminded of a similar function of salivary glands in those swallows,
which prepare the nests of which bird's-nest soup is made with the
secretion of the salivary glands. Another feature in the form of the
digestive tube is that in many cases, if not in all, it is marked with
constrictions which correspond with the segments of the body.
The _heart_ in the Chilognatha is not such a highly developed organ as in
the other Orders. The muscles which have already been mentioned as the
_alary_ muscles (or wing-shaped muscles) are not so highly developed, and
consist for the most part of a few muscular fibres. The muscular walls of
the heart, which consist of three layers, have the muscles less strongly
developed, and are in general adapted for a less energetic circulation.
The _tracheae_, which open into the stigmata, as has already been said,
branch into tufts of fine tubes, but the ramifications of these tufts never
join (or anastomose, as it is called), and consequently we never get, as in
the other Orders, long tracheal trunks running along the body.
{55}The _nervous system_, in addition to the existence of the visceral
nerve system described by Newport, shows a marked peculiarity in the form
of the ventral ganglionic chain. As has already been said, the nerve system
consists of a brain or mass of ganglia fused together and connected with
the ventral nervous cord by a collar of nervous substance surrounding the
oesophagus, and generally known as the _circumoesophageal_ collar. The
ventral nerve cord is a stout cord of nervous substance passing along the
whole length of the animal, and situated below (or ventral to) the
digestive tube and the generative system. This cord is enlarged at certain
points, and these enlargements or swellings are called ganglia, while from
the ganglia pass off nerves which supply the different organs of the body.
In the Chilognatha the cord has a compressed appearance as if the ganglia
were pressed into one another in such a way that it is very hard to
distinguish any ganglia at all. If we use the microscope and examine
sections cut transversely through the cord, we see that it is not a simple
cord. Even if we examine the nerve cord with a simple lens, we see that a
furrow runs longitudinally down it, and the use of the compound microscope
shows us that this furrow represents a division into two cords in such a
way that the single stout cord as it appeared to the naked eye is in
reality two cords running side by side, and so compressed together that the
substance is partly fused together. The ganglia too are double, being
swellings of the two cords and not a single enlargement on a single cord.
As we shall see in the other Orders, this arrangement constitutes a
characteristic distinction.
The _generative organs_ consist of a long tubular ovary or testis lying
along almost the whole length of the body and placed between the digestive
organ and the nervous system. Near its exit from the body the long tube
divides into two short tubes, the oviducts in the female or the vasa
_deferentia_ in the male. These ducts open in the third segment of the
body, unlike those of Myriapods belonging to other Orders. The accessory
glands present in most other Myriapods are not present in the Chilognatha.
The general arrangement of the organs of the Chilognatha may be seen from
Fig. 34, which represents a transverse section through the body of
_Polyxenus_ (Fig. 18). A comparison of {56}these two figures (Figs. 34 and
18) will show the position of the organs mentioned in this account. The
heart is shown with the suspensory and alary muscles attached.
[Illustration: FIG. 34.—Transverse section through _Polyxenus lagurus_:
_g.n.c_, _f.n.c_, ganglionic and fibrous parts of nerve cord; _Rec.sen_,
receptaculum seminis; _ori.dct_, oviduct; _Spmzoa_, spermatoza. (From
Heathcote, Anatomy of _Polyxenus lagurus_.)]
ORDER II. CHILOPODA.
The shape of the body differs from that of the Order which has been just
described (Chilognatha), inasmuch as it is not cylindrical but flattened,
the back, however, being more arched than the ventral surface. In this
respect, however, it cannot be said to differ from the other Orders which
we have yet to describe.
The segments are not formed by a single ring of the exoskeleton, which in
this Order is formed of chitin, and is tough and flexible rather than hard
and strong; but of two or three plates which form a covering to the
segment. The back is covered by a large plate known as the _tergum_, the
sides by two plates known as _pleura_, and the ventral part by a plate
called the _sternum_. The pleura and sternum are, however, in most cases
fused together or indistinguishable. In this, as in most of the anatomical
peculiarities, there is a much greater difference between the two Orders
Chilopoda and Chilognatha than between {57}the Chilopoda and the other
three Orders which have still to be described.
The Chilopoda have only one pair of appendages to each segment of the body
instead of two pairs like the Chilognatha.
The antennae of the Chilopoda are as a rule very long, and are always
longer than in the Chilognatha which we have just described. They differ
from those of the Schizotarsia (the third Order, which will be the next to
be described) in having the basal joints nearer together; in other words,
they are differently placed on the head. They differ from those of the
Pauropoda (the fifth Order) in being straight and not branched. As a rule
the antennae of the Chilopoda taper towards the extremity.
[Illustration: FIG. 35.—Mouth parts of _Lithobius_ (Latzel). A, Head of
_Lithobius_ seen from the under surface after removal of poison claws; _a_,
second maxilla; _b_, _c_, the two shafts of the first maxilla. B, One of
the mandibles. C, The two poison claws.]
The mouth parts are more numerous than in the Order we have just described
(the Chilognatha). They consist of—
1. An upper lip. This is a transverse plate as just described in the
case of the Chilognatha, but it is not always fused with the rest of the
head. It is also usually composed of three pieces, two lateral and a
middle piece.
2. A pair of _jaws_ or _mandibles_, which are not of so simple a form as
those of the Chilognatha, but rather resemble those of some of the
Crustacea.
3 and 4. Two pairs of appendages called maxillae resembling feet, but
used to aid the act of eating instead of locomotion. They are very
different in different Chilopods, but are mostly slender and weak and
usually provided with feelers (or palps) growing out of the main stem.
{58}5. The next pair of appendages are the first pair of the legs of the
body, which are also metamorphosed to serve a function different from the
ambulatory function of the other limbs. These are the _poison claws_, and
the possession of these forms another distinction between the Order we
are now discussing and that of the Chilognatha. At the same time the
third Order, that of the Schizotarsia, has poison claws, so that this
feature does not separate the Chilopoda from all the other Orders. These
poison claws are large curved claws connected with poison glands, the
secretion of which flows through a canal which opens near the point.
The _legs_ are longer than those of the Chilognatha, but not so long as
those in the next Order to be described (the Schizotarsia). Their number is
very various, from 15 pairs in _Lithobius_ to 173 in the Geophilidae.
Latzel notes a curious point in the number of the legs in this Order,
namely, the number of pairs of legs is always an uneven one. There are
always one pair to each segment. The last pair of legs is always longer
than the other pairs, and this is a peculiarity of the Order.
The _digestive tube_ resembles that of the other Orders, but the salivary
glands are not long and tubular but short (Fig. 28, _d_). It is, moreover,
not marked with constrictions corresponding with the segments of the body.
The _tracheal_ system or the system of respiration may be said to be more
highly developed in this Order than in any other. The tracheal branches
anastomose with one another (that is, the branches join), and in some cases
form long tracheal stems running along the body almost for its whole
length. The number of the tracheal openings or stigmata varies and does not
correspond with the number of segments.
The _nervous system_ differs considerably from that in the Order
Chilognatha; it resembles that in the Schizotarsia, and differs again from
that in the other two Orders, Symphyla and Pauropoda. The brain shows some
differences from other Orders chiefly in the development of the different
lobes which are connected with the sense organs, the eyes and antennae, for
instance; but the most marked difference is in the ventral ganglionic cord.
First, the ganglionic swellings are much more clearly marked than in the
Chilognatha. Secondly, the first three ganglia differ {59}from the others
in being nearer to one another and forming a single mass when seen by the
naked eye, though when examined by the aid of a microscope we can see all
the different parts are there. Thirdly, the division into two cords
mentioned in the Chilognatha is carried to a much greater extent. The
ganglia in each segment can be seen plainly to be double, and the cords
connecting the ganglia are two in number. We can plainly see that the
ventral nervous system of the Chilopoda consists of two cords lying
parallel to one another, and each having a ganglionic enlargement in every
segment. Whether a visceral nervous system is present in the group is
doubtful.
The eighth family of the Chilognatha, the Polyxenidae, show an approach to
the Chilopod nervous system.
The generative system differs chiefly in the opening of the genital
apparatus at the end of the body instead of in the third segment; though
this difference only separates the Order from the Chilognatha and not from
the other Orders. They also have two pairs of large accessory glands (as
they are called) connected with the genital openings.
ORDER III. SCHIZOTARSIA.
The third Order of Myriapods, the Schizotarsia, show a much greater
resemblance to the Chilopoda than to the first Order, the Chilognatha.
There are, however, important differences to distinguish them from all the
other Orders.
The shape of the body is short, thick, and very compact. The composition of
the individual segments resembles that found in Chilopoda rather than that
of Chilognatha.
The antennae are very long, longer than in any of the Chilopods, and are
composed of a great number of very small joints. The mouth parts show a
greater length and slenderness than do those of the other Orders mentioned
as yet. They consist of—
1. An _upper lip_ partly free, but fused at the sides with the rest of
the head. The upper lip is in three parts, as in the Chilopoda, but with
the middle part very small and the lateral pieces large.
2. A pair of _jaws_ or _mandibles_. These are provided not only with
teeth, as in the other Myriapods, but also with a sort of comb of stiff
bristles.
{60}3 and 4. Two pairs of _maxillae_ or foot jaws distinguished by their
length and slenderness.
5. The _poison claws_ long, slender, and not sharply curved. The bases of
the poison claws hardly fused together and short.
The _respiratory system_ in the Schizotarsia differs from that in all other
Myriapods in the fact before mentioned, that they breathe by means of lungs
and not by tracheae. There are, as before mentioned, eight dorsal scales in
these animals; each dorsal scale except the last bears one of the peculiar
organs which I have called lungs. At the hinder end of the scale there is a
slit which leads into an air sac, from which a number of short tubes
project into the blood in the space round the heart and serve to aerate it
before it enters the heart. The heart, therefore, sends aerated blood to
the organs, while in the tracheal-breathing Myriapods the blood is aerated
in the organs themselves by means of tracheae.
The poison claws are followed by segments bearing fifteen pairs of true
ambulatory legs. These are covered by eight large dorsal plates, increasing
in size from before to the middle of the body, the middle plate being the
largest, and then diminishing in size.
The _nervous system_ resembles that of the Chilopoda, but there is a
special pair of nerves which supply the sense organ, which has been
mentioned as peculiar to the Order. The ventral nerve cord shows a very
clear division into two, the ganglia of the two cords being almost entirely
separate. The first few ganglia are fused, as has been mentioned in the
Chilopoda.
The _digestive tube_ resembles that of the Chilopoda. The legs are very
long and slender, and the joints are beset with bristles. Both sexes have
small hook-like appendages at the sides of the genital openings.
The eyes have already been mentioned as being more highly developed than in
other classes, in correspondence with the more active habits of the animal.
The generative organs open at the hind end of the body, as in Chilopoda.
The heart is highly developed, quite as much so as the Chilopod heart, the
alary muscles being strong and broad, and the arteries being quite as
perfect as those in any Myriapod. The muscular coats which govern the
pulsations by their contractions are powerful and well developed.
{61}ORDER IV. SYMPHYLA.
We next come to one of the last two Orders which have been recently added
to the Myriapoda. These little animals have a great resemblance to the
Thysanura among the Insects, and especially to _Campodea_ among the
Thysanura. It will be well, therefore, to begin our account with a few of
the reasons which have induced naturalists to include them among the
Myriapods rather than among the Thysanura.
1. _Campodea_ has three pairs of mouth appendages, while _Scolopendrella_
has only two.
2. _Scolopendrella_ has broad plates covering the back, not only on the
anterior (thoracic) segments, but on the whole body.
3. The terminal appendages of _Scolopendrella_ differ from those in
_Campodea_.
4. _Scolopendrella_ has a sense organ which is absent in _Campodea_.
5. _Campodea_ breathes by means of three stigmata in the anterior part of
the body. The stigmata of _Scolopendrella_ are hard to see, and are not
in the same position.
6. _Scolopendrella_ has twelve pairs of legs, and _Campodea_, like all
Insects, has only three.
I will now go on to an account of their anatomy. The body is small and
slender, and is covered with a delicate shell or exoskeleton of chitin,
which is so thin as to be almost transparent.
The _antennae_ are long, and are composed of many joints of equal size.
The _mouth parts_ consist of—
1. An upper lip.
2. A pair of mandibles.
3. A pair of maxillae.
The segments are not all of equal size. Some are larger than others. The
larger and smaller segments are arranged alternately, and the smaller do
not bear legs. As before stated, there are twelve leg-bearing segments.
At the end of the body there are two hook-like appendages which are pierced
by a canal, through which is poured the secretion of a pair of glands. Near
the sides of these appendages are a pair of sense organs, consisting of
long hairs connected with nerves.
{62}The digestive canal is a long straight tube passing through the length
of the body. In the middle it is much enlarged, so as to form a stomach
with a glandular coat. Posterior to the stomach the digestive tube receives
the contents of two Malpighian tubes which act as kidneys.
The tracheal system consists of a single pair of stigmata on the under
surface of the head, and the tracheae connected with them.
ORDER V. PAUROPODA.
The Pauropoda, which form the fifth Order of Myriapods, are as yet very
imperfectly known. _Pauropus_ was discovered by Sir John Lubbock, and its
discovery was announced by him in 1866. He found this little Centipede in
his kitchen garden among some Thysanura, and at first considered it as a
larval form, but continued observation showed that it was a mature
creature. He described it as a small, white, bustling, intelligent little
creature about 1/25 inch in length.
The antennae are very curious and highly characteristic of the Order. They
resemble those of Crustacea rather than those of Myriapoda. Each antenna is
composed in the following manner. First there is a shaft of four joints.
From the fourth joint of this shaft spring two branches; one of these two
branches is narrower than the other, and ends in a long thin bristle
composed of a great number of joints. The other and broader branch bears
two such bristles, and between them a small pear-shaped or globular body,
the function of which is unknown.
The mouth parts consist of two minute pairs of appendages, the anterior
pair toothed and the posterior pointed. The body is rather narrower in
front; the segment behind the head has one pair of legs, the second, third,
fourth, and fifth behind the head two each. The posterior legs are the
longest; the genital organs open at the base of the second pair of legs,
between these and the third pair. The manner of breathing is as yet
unknown, tracheae not having been discovered.
_Pauropus_ at first looks most like a Chilopod, but differs from that
Order—
1. In the form of the antennae.
2. In the absence of poison claws and in the form of the mouth parts.
{63}3. The opening of the generative organs being in the front part of
the body.
It differs from Chilognatha in the following respects:—
1. The legs are not of equal length, the posterior legs being the
longest, as in Chilopods.
2. The mouth parts differ from those of Chilognaths almost as much as
from those of Chilopods.
3. The form of the antennae.
Only a few Pauropoda have been discovered as yet.
EMBRYOLOGY.
The preceding account of the anatomy of Myriapods would be incomplete
without some reference to the wonderful manner in which the different
organs of the body are built up; the whole of the complex organism
proceeding by a gradual and regulated process of development from a simple
cell called the ovum derived from the female body, and united with a cell
from the male body (called the spermatozoon). I hope to be able to give my
readers some idea of the interest which the pursuit of the difficult study
of embryology adds to anatomy, by offering us a key to the interpretation
of the relations between our knowledge of the forms at present living on
the earth and those which, we learn from Palaeontology, have inhabited our
planet in past ages.
[Illustration: FIG. 36.—Young ovum of _Julus terrestris_: _nucl_,
nucleolus; _nu_, nucleus; _R_, first appearance of yolk; _F_, follicle
cells.]
Like all living creatures with which we are acquainted, the starting-point
of Myriapod life is the _ovum_, as it is called. This _ovum_ is a _cell_
resembling the cells of which the body of all living animals are built up,
and which may be compared to the bricks of which a building is composed.
This cell or ovum is a small sphere of living transparent substance called
protoplasm, and it is _nucleated_—that is, it contains a small spot of
denser protoplasm called the nucleus, and within that a still smaller spot
of still more dense protoplasm called the nucleolus. In the process of
impregnation the ovum unites with the male cell, and the cell so formed is
called the impregnated ovum. This ovum has the property of dividing into
two cells, each resembling the {64}parent cell from which it is derived;
each of these cells has, like the parent cell, the same property of
dividing into two more, and so on. Thus from this continual process of
division or reproduction of every living cell, the materials are provided
for the building up of the body.
The regularity of the process of the division of the ovum, or, as it is
called, _segmentation of the ovum_, is interfered with by the presence of
_food yolk_. The cells formed by the process of cell division just
described need nourishment, and this nourishment is supplied to them by the
food yolk formed in the body of the ovum before the process of segmentation
begins. It is easy to understand that this yolk, which is not _alive_ like
the cells, cannot divide like them, and therefore the segmentation of the
ovum in Myriapods is _irregular_, as it is called.
[Illustration: FIG. 37.—Later stage: _nu_, nucleolus; _c.p_, nucleus;
_y.sp_, yolk spherules; _ch_, shell.]
I will now go back a little and describe what happens to the ovum before
the process of segmentation is complete. It increases in size and forms the
supply of food yolk which is to provide the nutriment of the ovum. Then
after impregnation the egg-shell is formed round it, and it becomes what we
know as the egg. This egg is not a perfect sphere, but is oval (in most
Myriapods) in shape. The egg is laid, and the process of segmentation
begins shortly after it is laid, as has already been described.
When it has been laid for about 36 hours, if we take an egg and, after
proper preparation, cut it into thin slices known to {65}microscopists by
the name of sections, and examine it by means of the microscope, we shall
see that segmentation has resulted in this. Just beneath the egg-shell
there is a thin layer of cells, one cell thick, which completely surrounds
the egg. Inside this coat of cells is the food yolk, with a few cells
scattered about in it at rare intervals, something like the raisins in a
plum-pudding.
With the next process the formation of the young Myriapod may be said to
begin. A strip along the length of the oval-shaped egg is thickened, and
this thick mass of cells represents the future ventral surface of the
animal. The rest of the thin layer of cells already mentioned just below
the shell will form the shell or exoskeleton of the future animal. The
thick strip of cells at the ventral surface has by this time split into
layers, so that, resorting to our microscope again, a section through the
short axis of the oval-shaped egg—a transverse section—will show us—
1. The egg-shell.
2. A layer of cells completely surrounding the egg, thin everywhere but
on the ventral surface. This layer is known to embryologists as the
_epiblast_. The thick part of the epiblast on the ventral surface gives
rise to the nervous system.
3 and 4. Two layers of cells connected in the middle, along the line of
the thick strip, but separate elsewhere, and not extending round the
whole of the inside. These layers constitute what is known as the
mesoblast, and give rise to the muscles and most of the internal organs.
5. The scattered cells in the yolk. They are known as the hypoblast and
give rise to the digestive canal.
After this point is reached the formation of the organs begins. The
segments are formed in order from before backwards. First the head, then
the next segment, and so on. When the number of segments with which the
animal will be hatched are formed, another process begins, and the tail end
of the animal, which can already be distinguished, is bent towards the
head. This is a process that takes place in many animals besides Myriapods,
and is called the formation of the ventral flexure. Shortly after this the
animal bursts the shell and comes {66}into the outer world. The various
processes may be understood by reference to the Figs. 36, 37, 38, 39, which
are successive stages in the development of a Chilognath. Figs. 37, 38, are
thin slices through the shorter diameter of the egg, which, as before
mentioned, is an oval in shape. Fig. 39 is a section through the longer
diameter of an egg in a more advanced stage of development, in fact just
about to burst the shell. The body of the future animal is marked by
constrictions, the future segments. Some of the organs are already formed,
as the brain and the digestive tube, the openings of which will form the
mouth (_st_) and the anus (_pr_).
[Illustration: FIG. 38.—Transverse section through next stage: _mk_,
keel-like mass of cells from which the mesoblast is produced; _ec_,
epiblast. (From Heathcote, Post. Emb. Dev. of _Julus terrestris_; _Phil.
Trans._ vol. 179, 1888, B.)]
[Illustration: FIG. 39.—Longitudinal section through later stage: _Segs._
2, 3, etc., segments; _Ceph. Seg_, head; _mes_, mesoblast; _en_, hypoblast;
_st_, future mouth; _pr_, future anus; _mesen_, gut; _mem.ex_, as in Fig.
41. (From Heathcote, Post. Emb. Dev. of _Julus terrestris_.)]
Myriapods are hatched at different stages of development. The Chilognatha
have only three appendages, which are so little developed that they are
only small shapeless stumps, while {67}the Chilopoda have the full number
of legs in some cases; in others only a small number of legs, but yet more
than the three pairs of legs of the Chilognatha, and fully developed
instead of stump-like. The eyes are usually developed late in the life of
the young animal. The bursting of the egg-shell is assisted in some
Myriapods by a special kind of spike on the back part of the head.
The Fig. 40 shows a young Chilognath which has just burst the shell and
come into the outer world. It is still surrounded with a membrane which has
been formed by its skin or epiblast within the egg. One eye-spot has been
formed.
[Illustration: FIG. 40.—Young _Julus terrestris_ just hatched.]
Fig. 41 shows a longitudinal section through the young Chilognath shown in
Fig. 40, and the next (Fig. 42) a transverse section through the same. In
comparing the two Figs. 41 and 42 it must be remembered that they are
_sections_ in different planes through the animal shown in Fig. 40, and
therefore they only show a small portion, a thin slice, of the organs.
[Illustration: FIG. 41.—Longitudinal section through late stage:
_Sup.oe.gl_, First appearance of brain; _st_, mouth; _pr_, anus; _mesen_,
gut; _n_, nerve cord; _n.gang_, nerve ganglion; _mem.ex_, membrane
surrounding the animal; _v.f_, ventral flexure; _mes_, mesoblast cells.
(Heathcote, Post. Emb. Dev. of _Julus terrestris_.)]
{68}The first appearance of the mouth appendages has been already
mentioned, and these are shown in Fig. 43, where the small stumps that
later on change to jaws are shown. The figure shows the head of a young
Chilognath seen from the lower side, and the second pair of stumps fuse
together later on and produce the broad plate already mentioned as the
characteristic mouth appendage of the Order.
[Illustration: FIG. 42.—_G_, gut; _Malp.T_, Malpighian tube; _N.C_, nerve
cord; _Tr.I_, deep invagination by which the tracheae are formed; _y.s_,
yolk spherules still present; _L_, first appearance of legs; _S.S_, part of
mesoblast. (Heathcote, Post. Emb. Dev. of _Julus terrestris_.)]
[Illustration: FIG. 43.—Under surface of the head of a young _Julus
terrestris_: _pro.m_, rudimentary jaws; _Deut.m_, rudimentary mouth plate;
_an_, antennae.]
After the animal is hatched it has still, in the case of most Myriapods
(those which are not hatched with all the segments complete), to undergo a
further development, and in particular the eyes are still unformed. The
process of development of the eye has only been followed out as yet in the
Chilognatha, and in only one form, _Julus_, and is so curious that a short
account may be of interest here. The development of the eye begins (in
_Julus_) on the fourth day after hatching, and continues until the animal
is full grown. A single {69}ocellus or eye-spot appears first, and the rest
are added one by one until the full number are reached.
The first appearances connected with the formation of the eye take place in
the cellular layer just beneath the chitinous exoskeleton. This layer,
called the hypodermis, plays an important part in the organisation of the
animal. It forms the inner layer of what we may call the skin of the
animal, and the cells of which it is composed secrete the chitin of which
the shell or exoskeleton of the animal is composed, and which is moulted
every year.
The first process in the formation of the eye-spot is the thickening of the
hypodermis beneath the chitin, just in the place where the eye will come.
At the same time the cells of this thickened mass of hypodermis secrete a
quantity of pigment of a dark red brown colour. Next the cells of the thick
mass of hypodermis begin to separate from one another in such a way that a
vesicle is formed. This vesicle is hollow inside, and the thick walls are
formed from the cells of the thickened hypodermic mass. This can be seen
from Fig. 44, which represents a section through an ocellus when it is
partly formed. From this vesicle the eye is formed.
The wall of the vesicle nearest the exoskeleton gives rise to the lens of
the eye, while the other walls of the vesicle form the retinal parts of the
eye. The cells from the brain grow out and form the optic nerve connecting
the retina with the brain. The whole eye spot is covered internally by a
thin membrane, formed not from the hypodermis but by cells from the inside
of the body (mesoblast cells).
[Illustration: FIG. 44.—Section through eye when first forming: _Hyp_,
hypodermis; _Ln_, lens; _F.W.V_, front wall of optic vesicle; _b.w.v_, back
wall of vesicle; _cap_, capsule.]
In the Chilognatha, the first Order of Myriapods, the young {70}animal
leaves the egg with three pairs of appendages; the first have already the
form of antennae, the second will form the jaws, but have not yet taken
their proper form, while the third pair will fuse together and alter their
shape so as to form the curious plate that has already been mentioned as
forming the second pair of mouth appendages. Behind the mouth appendages
will come the first three pairs of legs. The whole young animal on leaving
the egg is enveloped in two membranes. These membranes are secreted by the
outside layer of cells in the same way that the shell or exoskeleton of the
animal will be eventually formed, and represent the first two moults of the
animal, which continues to moult its shell every year throughout life.
Of the Chilopoda, the second Order of Myriapods, all the families leave the
egg-shell with the full number of legs, with the exception of the
Lithobiidae, which have seven pairs of legs including the poison-claws. The
Schizotarsia, the third Order, also have seven pairs of legs when hatched.
The legs make their appearance not one by one but in batches (in _Julus
terrestris_ in batches of five). The addition of legs and segments to the
body takes place, not at the end of the body, but between the end segment
and the penultimate.
This is a short sketch of the gradual development of the Myriapoda from the
ovum to the fully-grown animal. It is, I am aware, a short and insufficient
account of all the beautiful processes by which the different organs take
their rise, but space is insufficient here, and too much detail would be
out of place in a work of this nature, which only aims at giving an outline
sketch of the group, which shall be intelligible to the general reader who
has not made a special study of such matters. Before leaving the subject,
however, I must mention a few of the points of interest which are to be
learned from the examination of the course of development which has been
sketched here. One of the greatest puzzles in the natural history of the
Order Chilognatha has always been the double segments, as they are called;
that is, in fact, the possession of two pairs of legs to each segment,
which is, as we have already said, a distinguishing characteristic of the
Order. As we have seen, the Chilognatha at an early stage of existence do
not possess this characteristic, which is only peculiar to the adult
{71}and half-grown forms. Now what does this mean? Does each double segment
in the full-grown Millepede represent two segments which have become fused
together, or is each double segment, so called, a real segment resembling
the segments present in the other Orders (for instance, Chilopoda), which
has grown an extra pair of legs? Both these views have been advocated by
distinguished naturalists. Neither of them is, in my opinion, quite right
when viewed in the light cast on the subject by recent investigations into
the life history of the Chilognatha.
A close examination into the minutiae of the growth of the different organs
has shown us that the double characters of the double segments are more
deeply seated than was imagined. The circulatory system, the nerve cord,
and the first traces of segmentation in the mesoblast all show this double
character, and the only single part about the segment is the broad plate
covering the segment. Now in some of the most ancient of the fossil
Myriapods this broad plate shows traces of a division, as if it were in
reality two plates fused together. We have also to consider that the life
history of the Chilognatha allows us to believe that the peculiar
cylindrical shape of the body shown in the greatest degree in the Julidae
is attained by the unequal development of the dorsal and ventral surfaces
of the body; the ventral surface being compressed together till it is
extremely narrow, and the dorsal surface, as it were, growing round it till
the originally dorsal surface forms almost a complete ring round the body.
Taking all this into consideration, we are justified, in my opinion, in
concluding that each double segment in the Chilognatha is not two segments
fused together, nor a single segment bearing two pairs of legs, but is two
complete segments perfect in all particulars, but united by a large dorsal
plate which was originally two plates which have been fused together, and
which in most Chilognatha surrounds almost the whole of two segments in the
form of a ring.
Again in the Chilopoda we see that a great distinctive feature that
separates them from the Chilognatha is the character of the ventral nerve
cord, the cord being double and not single, a character connected with the
fact that the bases of the legs are widely separated from one another, and
not closely approached to each other, as in the Chilognatha. As we before
said, a more {72}minute anatomical examination showed us that this
difference was not so great as appeared at first sight, the cord showing
traces of a duplication. Well, are these traces superficial, or do they
represent a state of affairs more or less similar to that in the Chilopoda?
Embryology helps us to answer this question also. In the early stages of
the Chilognatha we find that the nerve cord has exactly the form of that in
Chilopoda, showing us that the appearances in the anatomy had led us to a
right conclusion, and giving us a valuable confirmation of our views. These
two examples will serve to show the kind of interest which attaches to
embryology.
PALAEONTOLOGY.
We have seen that embryology enables us to look at the structure of the
Myriapods from a new standpoint, and to correct and supplement the
knowledge gained from an examination of the adult animal. In the same way a
study of the forms of Myriapods which have become extinct on the globe, and
have been preserved to us in a fossil form, gives a further opportunity of
considering the relations of one form to another, and again of the
relations of our group to other groups of animals now existing on the
earth. Myriapod fossils have been found in strata of great antiquity. The
oldest of such fossils must have been among the first land animals. The
figure below shows a fossil Myriapod found in America, belonging to the
Order of the Protosyngnatha which are only found in the Palaeozoic strata.
It is a good example of the manner in which Myriapods were protected by
bundles of bristles in the same way as the _Polyxenus_ of the present time.
The oldest fossil Myriapods which have been discovered at the present time
are two species which have been found in the Old Red Sandstone in Scotland.
To realise the antiquity of these Myriapods, it will be worth while
recalling the typical fossils found in the Old Red Sandstone, so as to see
what the contemporaries of these ancient Myriapods were like. Among the
plants there were Algae, Ferns, and Conifers, belonging to the lower
divisions of the plant tribe. Among the animals there were Sponges, Corals,
Starfish, Worms, Shell-fish, and Fishes, but none of the more highly
organised of the animal or vegetable tribe {73}had appeared on the earth.
The Myriapods of the Old Red Sandstone, as has been before said, differ
considerably from those of the present day, and as we proceed towards the
species found in the more recent strata we find them more and more like the
ones at present living, till we get to the _Polyxenus_ and other species
found in amber, which are hardly to be distinguished from living forms.
The next oldest fossil Myriapods are found in the coal measures, when both
the animal and vegetable kingdoms were represented by more numerous and
more specialised forms. The fossil fauna of this period is characterised by
the number of gigantic Amphibia, many remains of which have been found. The
great forests and the abundant vegetation of this time must have been
favourable to the existence of our class, and accordingly we find no less
than 32 species of fossil Myriapods. Of these most have been found in
America, some in Great Britain, and some in Germany. One well-preserved
fossil of _Xylobius sigillariae_ was found by Dr. Dawson in America in the
stump of a tree in the remains of a fossil forest. The eyes, head, and legs
were plainly seen under the microscope. All these fossils belong to the
earliest or Palaeozoic period.
[Illustration: FIG. 45.—_Palaeocampa anthrax._ (After Meek and Worth.) From
Mazon Creek, Illinois.]
The figure below (Fig. 46) shows a fossil also from the coal formations of
Illinois, America, belonging to the family of the Euphoberiidae mentioned
further on. It shows a nearer approach to the Julidae of the present time.
The limbs, however, were of very curious shape, and may possibly have been
adapted to locomotion in water as well as on land, and the small supposed
branchiae on the ventral surface shown in Fig. 46, B, may possibly have
been an arrangement to render respiration in the water possible.
{74}In the secondary period the Myriapods were scantily represented, or, at
any rate, geologists have failed to find their fossils. The class is
represented by a single specimen found in the chalk in Greenland. This
fossil, which has been included in the Julidae under the name of _Julopsis
cretacea_, may perhaps belong to the Archipolypoda.
Passing on to the Tertiary or Recent period, we find the Myriapods again
numerous, and more nearly resembling those living at the present time. They
belong mostly to the Chilognatha and Chilopoda. They have been found in the
fresh-water gypsum of Provence in France, the brown coal of Germany, and
the green river formations of America. Several have been found in amber.
[Illustration: FIG. 46.—_Acantherpestes major._ (After Meek and Worth.)
Mazon Creek, America. A, The whole animal; B, branchiae on the ventral
surface.]
Fossil Myriapods have been divided into four Orders, two of which coincide
with the Orders of living Myriapods; the differences between the fossils
and the living Myriapods having been held insufficient to warrant the
establishment of a new Order. These two Orders are the Chilopoda and the
Diplopoda or Chilognatha (Diplopoda is another name used by some writers
for the group which we have hitherto called Chilognatha). The other two
Orders have sufficient differences from living forms to render it necessary
to include them in separate Orders.
The fossil Myriapods, then, are arranged as follows:—
Order I. Protosyngnatha.
Order II. Chilopoda.
Order III. Archipolypoda.
Order IV. Chilognatha (or Diplopoda).
The following table will show the species that have been discovered in the
different strata:—
Devonian, or } {75}
Old Red Sandstone } 2 species of _Archipolypoda_
Carboniferous { 1 species _Protosyngnatha_
{ 31 species _Archipolypoda_
Permian (Rothliegendes of Germany), 4 specimens belonging to the
_Julidae_ or _Archipolypoda_.
Cretaceous, 1 species { _Archipolypoda_ or
{ _Chilognatha_
Oligocene { 17 species _Chilopoda_
{
{ 23 species { _Diplopoda_
{ (_Chilognatha_)
Miocene, 1 species { _Diplopoda_
{ (_Chilognatha_)
I will now give a short account of the different Orders, and the fossil
forms which are included in them.
ORDER I. PROTOSYNGNATHA.
This Order is represented by a single fossil (Fig. 45), discovered in the
coal at Mazon Creek, Illinois, America, by Meek and Worth. It differs
greatly from any of those in existence at the present day. The body is
cylindrical, and composed of ten segments. The cephalic appendages (that
is, the antennae and mouth parts) are inserted into a single unsegmented
cephalic mass (the head). Each segment behind the head bears a single
dorsal and ventral plate of equal breadth and length. The limbs are placed
in these plates with a wide space between the base of each leg and that of
the opposite one of the pair. Along the back, bundles of bristles are
arranged in longitudinal rows.
ORDER II. CHILOPODA.
The fossil forms of this Order resemble those of the Chilopoda of the
present day. The oldest of them are found in amber. The following families
have been found:—
_Lithobiidae._ Several species have been found in amber.
_Scolopendridae._ One species in amber, several species in later Tertiary
formations.
_Geophilidae._ Three species in amber.
Two species resembling the Schizotarsia of the present day have been found
in amber.
{76}ORDER III. ARCHIPOLYPODA.
The most numerous of the fossil families. With a few exceptions, all the
Palaeozoic (that is, the oldest) Myriapods belong to this Order. The
Carboniferous Archipolypoda seem to be much more numerous in the coal of
America than in that of England. They resemble for the most part the
Myriapods of the present day, except that all the segments without
exception bear legs.
The families are three in number.
Family 1. _Archidesmidae._
Resemble the _Polydesmidae_ of the present day. Two species have been
found by Page in the Old Red Sandstone of Forfarshire. He named them
_Kampecaris_. One found by Peach in the same formation is called
_Archidesmus_.
Family 2. _Euphoberiidae._
They show some resemblance to the _Julidae_ of the present day, but the
dorsal scutes, or plates of the back, are more or less perfectly divided
into two divisions corresponding with the pairs of legs. The following
are the principal fossils of this family:—
_Acantherpestes._ Found by Meek and Worth in the coal at Mazon Creek in
America (Fig. 46).
_Euphoberia._ About 12 species found at the same place as the last
named.
_Amylispes._ Found by Scudder, Mazon Creek, America.
_Eileticus._ Scudder, Mazon Creek, America.
Family 3. _Archijulidae._
The dorsal plates nearly consolidated, but the division still apparent.
Fossil forms are—
_Trichijulus._ Scudder, Mazon Creek, America.
_Xylobius._ Dawson. Found in the coal in Nova Scotia. Two species found
at Mazon Creek, America.
ORDER IV. CHILOGNATHA.
Families corresponding to those of the present day. The oldest specimens
come from the chalk in Greenland; most of the others from amber.
Family 1. _Glomeridae._ One form, _G. denticulata_, has been found in
amber.
Family 2. _Polydesmidae._ Two species in amber.
Family 3. _Lysiopetalidae._ A number of species, amongst which are 6
_Craspedosoma_, mostly from amber.
{77}Family 4. _Julidae._ A number of species of this family have been
found, some in amber, some in other Tertiary strata. Amongst the latter a
probable example of _Julus terrestris_, living at the present time.
Family 5. _Polyxenidae._ Five species have been found in amber.
Now that we have considered the structure of the Myriapods and the groups
into which they are subdivided or classified, we may proceed to consider
what position they hold in the household of nature. That they present
certain features of similarity to other classes has been already mentioned,
and that this is the fact cannot be doubted when we look back at the way in
which they have been classified in the works of early writers. For example,
Lamarck, the great French naturalist, classifies them with spiders in his
well-known work, _La Philosophie Zoologique_, under the name of _Arachnides
antennistes_. Cuvier, the comparative anatomist, unites them with the
Insects, making them the first Order, while the Thysanura is the second. We
have already seen that one Order of Myriapods, the Symphyla, bears a great
resemblance to the Thysanura. The English naturalist Leach was the first to
establish Myriapods as a class, and his arrangement has been followed by
all naturalists after his time. But while their peculiarities of structure
and form are sufficiently marked to separate them as a class, it cannot be
denied that the older naturalists were right to recognise that they have
many essential characteristics in common with other classes of animals. And
recent investigations have emphasised this fact. For instance, let us
consider the recent discoveries of the Orders of Symphyla and Pauropoda,
Orders which, while bearing so many of the characters of Myriapods that
naturalists have agreed to place them in that class, yet resemble in many
important points the Insect Order of Thysanura. This seems to justify
Cuvier in claiming the close relationship for them that he did.
Recent investigations have also brought out more prominently the
resemblances to the Worms. Of late, considerable attention has been
directed to _Peripatus_ (see pp. 1-26), and the resemblances to the
Myriapods in its anatomy and development are such that Latzel has actually
included it in the Myriapods as an Order, Malacopoda. Now _Peripatus_ also
shows resemblances to the annelid Worms, and thus affords us a connexion to
the Worm type hardly less striking than that to the Insect. This
{78}resemblance to the Worms, which Myriapods certainly bear, was noticed
by the ancient writers, and as they had for the most part only external
appearances to consider, they pushed this idea to extremes in actually
including some of the marine Worms (Annelida) among the Centipedes. Pliny
talks of a marine _Scolopendra_ as a very poisonous animal, and there is
little doubt that he meant one of the marine worms. An old German
naturalist, Gesner, in a very curious book published in 1669 gives an
account of an annelid sea-worm which he calls _Scolopendra marina_, and
which is in all probability the sea Scolopendra which Pliny mentions. From
Gesner's account it seems to have been used as a medicine (externally
only). "The use of this animal in medicine. The animal soaked in oil makes
the hair fall off. So do its ashes mixed in oil." It was also pounded up
with honey.
This idea of Centipedes living in water survived among later naturalists.
Charles Owen, the author before quoted, mentions them as amphibious in
1742. "The Scolopendra is a little venomous worm and amphibious. When it
wounds any, there follows a blueness about the affected part and an itch
all over the body like that caused by nettles. Its weapons of mischief are
much the same with those of the spider, only larger; its bite is very
tormenting, and produces not only pruriginous pain in the flesh, but very
often distraction of mind. These little creatures make but a mean figure in
the ranks of animals, yet have been terrible in their exploits,
particularly in driving people out of their country. Thus the people of
Rhytium, a city of Crete, were constrained to leave their quarters for them
(Aelian, lib. xv. cap. 26)."
Myriapods have been considered to bear resemblances to the Crustacea, and
this to a certain extent is true, though only to a certain extent, the
resemblances being confined to the more general characteristics that they
share with other groups of animals.
Of late years attempts have been made to speculate about the origin of the
Myriapods—that is, to endeavour to obtain by means of investigation of
their anatomy, embryology, and palaeontological history, some idea of the
history of the group. Such attempts at research into the _phylogeny_, as it
is called, of a group must be more or less speculative until our knowledge
is much greater than {79}it is at present. But such inquiries have their
value, and the schemes of descent and phylogenetic trees, at any rate,
indicate a real relation to different groups, even if they do not provide
us with a real and actual history of the animals.
There have been two main theories about the descent of the Myriapoda. One
of these derives them directly from the Insecta through the forms known as
the Thysanura, which resemble in such a degree the Myriapod Orders of
Symphyla and Pauropoda. The other theory holds that the Myriapods, as well
as the Insecta, have been derived from some ancestor bearing a resemblance
to _Peripatus_. In other words, one theory claims that the relationship of
Myriapoda to Insecta is that of father and son; the other that the
relationship between the two is that of brother to brother. The arguments
by which these theories are respectively supported consist for the most
part of an analysis of the different characters of the anatomy and
embryology and the determination of the most primitive among them. For
example, the supporters of the theory that the Thysanura are the most
nearly allied to the Myriapod ancestor lay great weight on the fact that
some Myriapods are born with three pairs of legs only, and they compare
this stage in the life history of the Myriapoda to the metamorphosis and
larval stage of Insects. For the supporters of this view the Orders of
Symphyla and Pauropoda are the most primitive of the Myriapods. On the
other hand, the followers of the other theory do not allow that the
characters in which the Myriapods are like Insects are primitive ones, but
they lay more stress on the characters found in the early development, such
as the character of the process of the formation of the body segments, the
mesoblastic segmentation, and the origin of the various organs of the body.
It may be easily understood that such differences in the estimation of the
primitive characters of the embryology of a group may arise. Embryology has
been compared by one of the greatest of modern embryologists to "an ancient
manuscript with many of the sheets lost, others displaced, and with
spurious passages interpolated by a later hand." What wonder is it that
different people examining such a record should come to different
conclusions as to the more doubtful and difficult portions of it. It is
this very difficulty which makes the principal interest in the study, and
although our knowledge of the language in {80}which this manuscript is
written is as yet imperfect, still we hope that constant study may teach us
more and more, and enable us to read the great book of nature with more and
more ease and certainty.
If any of my readers should wish for a more full account of the natural
history of this group I must refer them to the following works, which I
have used in compiling the above account. In the first of these there is an
excellent bibliography of the subject:—
LATZEL, Die Myriapoden der Oesterreichisch-Ungarischen Monarchie, Wien,
1880.
ZITTEL, Handbuch der Palaeontologie, 1 Abth, II. Bd., Leipzig, 1881-1885.
KORSCHELT AND HEIDER, Lehrbuch der vergleichenden Entwicklungsgeschichte
der wirbellosen Thiere, Jena 1891.
INSECTA
BY
DAVID SHARP, M.A., M.B., F.R.S.
{83}CHAPTER III
CHARACTERISTIC FEATURES OF INSECT LIFE–SOCIAL INSECTS–DEFINITION OF THE
CLASS _INSECTA_–COMPOSITION OF INSECT SKELETON–NUMBER OF SEGMENTS–NATURE OF
SCLERITES–HEAD–APPENDAGES OF THE MOUTH–EYES–THORAX–ENTOTHORAX–LEGS–WINGS–
ABDOMEN OR HIND BODY–SPIRACLES–SYSTEMATIC ORIENTATION.
Insects form by far the larger part of the land animals of the world; they
outnumber in species all the other terrestrial animals together, while
compared with the Vertebrates their numbers are simply enormous. Yet they
attract but little attention from the ordinary observer, this being
probably primarily due to the small size of the individual Insect, which
leads the unreflecting to treat the creature as of little importance. "It
can be crushed in a moment" is perhaps the unformulated idea that underlies
the almost complete neglect of knowledge concerning Insects that prevails
even in the educated classes of society. The largest Insects scarcely
exceed in bulk a mouse or a wren, while the smallest are almost or quite
imperceptible to the naked eye, and yet the larger part of the animal
matter existing on the lands of the globe is in all probability locked up
in the forms of Insects. Taken as a whole they are the most successful of
all the forms of terrestrial animals.
In the waters of the globe the predominance of Insect life disappears. In
the smaller collections of fresh water many Insects find a home during a
portion of their lives, and some few contrive to pass their whole existence
in such places; but of the larger bodies of fresh water they invade merely
the fringes, and they make only the feeblest attempt at existence in the
ocean; the genus _Halobates_ containing, so far as we know, the sole
Insects {84}that are capable of using the ocean as a medium of existence at
a distance from the shore.
It will probably be asked, how has it come about that creatures so
insignificant in size and strength have nevertheless been so successful in
what we call the struggle for existence? And it is possible that the answer
will be found in the peculiar relations that exist in Insects between the
great functions of circulation and respiration; these being of such a
nature that the nutrition of the organs of the body can be carried on very
rapidly and very efficiently so long as a certain bulk is not exceeded.
Rapidity of growth is carried to an almost incredible extent in some
Insects, and the powers of multiplication—which may be considered as
equivalent to the growth of the species—even surpass the rapidity of the
increase of the individual; while, as if to augment the favourable results
attainable by the more usual routine of the physiological processes,
"metamorphosis" has been adopted, as a consequence of which growth and
development can be isolated from one another, thus allowing the former to
go on unchecked or uncomplicated by the latter. A very simple calculation
will show how favourable some of the chief features of Insect life are. Let
it be supposed that growth of the individual takes time in proportion to
the bulk attained, and let A be an animal that weighs one ounce, B a
creature that weighs ten ounces, each having the power of producing 100
young when full grown; a simple calculation shows that after the lapse of a
time necessary for the production of one generation of the larger creature
the produce of the smaller animal will enormously outweigh that of its
bulkier rival. Probably it was some consideration of this sort that led
Linnaeus to make his somewhat paradoxical statement to the effect that
three flies consume the carcase of a horse as quickly as a lion.[16]
Astonishing as may be the rapidity of the physiological processes of
Insects, the results attained by them are, it must be admitted, scarcely
less admirable: the structures of the Insect's body exhibit a perfection
that, from a mechanical point of view, is unsurpassed, while the external
beauty of some of the creatures makes them fit associates of the most
delicate flowers or no mean rivals of the most gorgeous of the feathered
world. The words {85}of Linnaeus, "Natura in minimis maxime miranda," are
not a mere rhetorical effort, but the expression of a simple truth. Saint
Augustine, too, though speaking from a point of view somewhat remote from
that of the great Swedish naturalist, expressed an idea that leads to a
similar conclusion when he said, "Creavit in coelum angelos, in terram
vermiculos; nec major in illis nec minor in istis."
The formation of organised societies by some kinds of Insects is a
phenomenon of great interest, for there are very few animals except man and
Insects that display this method of existence. Particulars as to some of
these societies will be given when we treat of the Termitidae, and of the
Hymenoptera Aculeata; but we will take this opportunity of directing
attention to some points of general interest in connexion with this
subject. In Insect societies we find that not only do great numbers of
separate individuals live together and adopt different modes of industrial
action in accordance with the position they occupy in the association, but
also that such individuals are profoundly modified in the structures of
their body and in their physiological processes in such ways as to
specially fit them for the parts they have to play. We may also see these
societies in what may be considered different stages of evolution; the
phenomena we are alluding to being in some species much less marked than
they are in others, and these more primitive kinds of societies being
composed of a smaller number of individuals, which are also much less
different from one another. We, moreover, meet with complex societies
exhibiting some remarkably similar features among Insects that are very
different systematically. The true ants and the white ants belong to groups
that are in structure and in the mode of growth of the individual
essentially dissimilar, though their social lives are in several important
respects analogous.
It should be remarked that the phenomena connected with the social life of
Insects are still only very imperfectly known; many highly important points
being quite obscure, and our ideas being too much based on fragments
gathered from the lives of different species. The honey bee is the only
social Insect of whose economy we have anything approaching to a wide
knowledge, and even in the case of this Insect our information is neither
so complete nor so precise as is desirable.
The various branches of knowledge connected with Insects {86}are called
collectively Entomology. Although entomology is only a department of the
great science of zoology, yet it is in practice a very distinct one; owing
to its vast extent few of those who work at other branches of zoology also
occupy themselves with entomology, while entomologists usually confine
themselves to work in the vast field thus abandoned to them.
Before passing to the consideration of the natural history and structure of
the members of the various Orders of Insects we will give a verbal
diagrammatic sketch, if we may use such an expression, with a view to
explaining the various terms that are ordinarily used. We shall make it as
brief as possible, taking in succession (1) the external structure, (2)
internal structure, (3) development of the individual, (4) classification.
In the course of this introductory sketch we shall find it necessary to
mention the names of some of the Orders of Insects that will only be
explained or defined in subsequent pages. We may therefore here state that
the term "Orthoptera" includes grasshoppers, locusts, earwigs, cockroaches;
"Neuroptera" comprises dragon-flies, May-flies, lacewings, stone-flies and
caddis-flies; to the "Hymenoptera" belong bees, wasps, ants, sawflies, and
a host of little creatures scarcely noticed by the ordinary observer:
"Coleoptera" are beetles; "Lepidoptera," butterflies and moths; "Diptera,"
house-flies, blue-bottles, daddy-longlegs, and such; "Hemiptera" or
"Rhynchota" are bugs, greenfly, etc.
CLASS INSECTA: OR INSECTA HEXAPODA.
_Definition_.—Insects are small animals, having the body divided into three
regions placed in longitudinal succession—head, thorax, and abdomen: they
take in air by means of tracheae, a system of tubes distributed throughout
the body, and opening externally by means of orifices placed at the sides
of the body. They have six legs, and a pair of antennae; these latter are
placed on the head, while the legs are attached to the thorax, or second of
the three great body divisions; the abdomen has no true legs, but not
infrequently has terminal appendages and, on the under surface,
protuberances which serve as feet. Very frequently there are two pairs of
wings, sometimes only one pair, in other cases none: the wings are always
placed on the thorax. Insects are transversely segmented—that is to say,
the body has the form of a succession of {87}rings; but this condition is
in many cases obscure; the number of these rings rarely, if ever, exceeds
thirteen in addition to the head and to a terminal piece that sometimes
exists. Insects usually change much in appearance in the course of their
growth, the annulose or ringed condition being most evident in the early
part of the individual's life. The legs are usually elongate and apparently
jointed, but in the immature condition may be altogether absent, or very
short; in the latter case the jointing is obscure. The number of jointed
legs is always six.
EXTERNAL STRUCTURE.
The series of rings of which the external crust or skeleton of Insects is
composed exhibits great modifications, not only in the various kinds of
Insects but even in the different parts of the same individual, and at
successive periods of its development; so that in the majority of mature
Insects the separate rings are readily distinguished only in the hind body
or abdomen. The total number of the visible rings, segments, somites, or
arthromeres, as they are variously called by different writers, is
frequently thirteen in addition to the head. This latter part is considered
to be itself composed of the elements of several rings, but morphologists
are not yet agreed as to their number, some thinking this is three while
others place it as high as seven; three or four being, perhaps, the figures
at present most in favour, though Viallanes, who has recently discussed[17]
the subject, considers six, the number suggested by Huxley, as the most
probable. Cholodkovsky is of a similar opinion. However this may be, the
three rings behind the head constitute the thorax, which is always largely
developed, though, like the head, its segmentation is usually very much
obscured by unequal development of different parts, or by consolidation of
some of them, or by both of these conditions. The third great division of
the body, the abdomen, is also usually much modified by one or more of the
terminal segments being changed in form, or even entirely withdrawn into
the interior of the body. The existence of ten segments in the hind body
can, however, be very frequently actually demonstrated, so that it is
correct to speak of ten as the normal number.
{88}[Illustration: FIG. 47—Diagram of exterior of insect: the two vertical
dotted lines indicate the divisions between H, head; T, thorax; and A,
abdomen: _a_, antenna; _b_, labrum; _c_, mandible; _d_, maxillary palpus;
_e_, labial palpus; _f_, facetted eye; _g_, pronotum; _h_, mesonotum; _i_,
metanotum; _k_, wings; _l_{1}_ to _l_{10}_, abdominal segments; _m_, the
internal membranous portions uniting the apparently separated segments;
_n_, cerci; _o_, stigma; _p_, abdominal pleuron bearing small stigmata;
_q_{1}_, _q_{2}_, _q_{3}_, pro-, meso-, meta-sterna; _r_{1}_, mesothoracic
episternum; _s_{1}_, epimeron, these two forming the mesopleuron; _r_{2}_,
_s_{2}_, metathoracic episternum and epimeron; _t_, coxa; _v_, trochanter;
_w_, femur; _x_, tibia; _y_, tarsus; _z_, gula.]
It is no reproach to morphologists that they have not yet agreed as to the
number of segments that may be taken as typical for an Insect, for all the
branches of evidence bearing on the point are still imperfect. It may be
well, therefore, to state the most extreme views that appear to be at all
admissible. Hagen[18] has recently stated the opinion that each thoracic
segment consists really of three segments—an anterior or wing-bearer, a
middle or leg-bearer, and a posterior or stigma-bearer. There seems to be
no reason for treating the stigma as being at all of the nature of an
appendage, and the theory of a triple origin for these segments may be
dismissed. There are, however, several facts that indicate a duplicity in
these somites, among which we may specially mention the remarkable
constancy of two pleural pieces on each side of each thoracic segment. The
hypothesis of these rings being each the representative of two segments
cannot therefore be at present considered entirely untenable, and in that
case the maximum and minimum numbers that can be suggested appear to be
twenty-four and eleven, distributed as follows:—
Maximum. Minimum. {89}
Head 7 3
Thorax 6 3
Abdomen 11 5
-- --
Total 24 11
Although it is not probable that ultimately so great a difference as these
figures indicate will be found to prevail, it is certainly at present
premature to say that all Insects are made up of the same number of primary
segments.
A brief account of the structure of the integument will be found in the
chapter dealing with the post-embryonic development.
The three great regions of the Insect body are functionally as well as
anatomically distinct. The head bears the most important of the sense
organs, viz. the antennae and ocular organs; it includes the greater of the
nerve-centres, and carries the mouth as well as the appendages, the trophi,
connected therewith. The thorax is chiefly devoted to the organs of
locomotion, bearing externally the wings and legs, and including
considerable masses of muscles, as well as the nerve centres by which they
are innervated; through the thorax there pass, however, in the longitudinal
direction, those structures by which the unity of the organisation is
completed, viz. the alimentary canal, the dorsal vessel or "heart" for
distributing the nutritive fluid, and also the nerve cords. The abdomen
includes the greater part of the organs for carrying on the life of the
individual and of the species; it also frequently bears externally, at or
near its termination, appendages that are doubtless usually organs of sense
of a tactile nature.
In the lower forms of Insect life there is little or no actual internal
triple division of the body; but in the higher forms such separation
becomes wonderfully complete, so that the head may communicate with the
thorax only by a narrow isthmus, and the thorax with the abdomen only by a
very slender link. This arrangement is carried to its greatest extreme in
the Hymenoptera Aculeata. It may be looked on as possibly a means for
separating the nutrition of the parts included in the three great body
divisions.
Along each side of the body extends a series of orifices for the admission
of air, the stigmata or spiracles; there are none of these on the head, but
on each side of most of the other segments {90}there is one of these
spiracles. This, however, is a rule subject to many exceptions, and it is
doubtful whether there is ever a spiracle on the last abdominal segment.
Even in the young stage of the Insect the number of these stigmata is
variable; while in the perfect Insect the positions of some of the stigmata
may be much modified correlatively with the unequal development or
consolidation of parts, especially of the thorax when it is highly modified
for bearing the wings.
The segments of the Insect are not separate parts connected with one
another by joints and ligaments; the condition of the Insect crust is in
fact that of a continuous long sac, in which there are slight constrictions
giving rise to the segments, the interior of the sac being always traversed
from end to end by a tube, or rather by the invaginated ends of the sac
itself which connect with an included second sac, the stomach. The more
prominent or exposed parts of the external sac are more or less hard, while
the constricted parts remain delicate, and thus the continuous bag comes to
consist of a series of more or less hard rings connected by more delicate
membranes. This condition is readily seen in distended larvae, and is shown
by our figure 48 which is taken from the same specimen, whose portrait,
drawn during life, will be given when we come to the Coleoptera, family
Cleridae. The nature of the concealed connexions between the apparently
separate segments of Insects is shown at _m_, Fig. 47, p. 88.
[Illustration: FIG. 48—_Tillus elongatus_, fully distended larva.]
As the number of segments in the adult Insect corresponds—except in the
head—with the number of divisions that appear very early in the embryo, we
conclude that the segmentation of the adult is, even in Insects which
change their form very greatly during growth, due to the condition that
existed in the embryo; but it must not be forgotten that important
secondary changes occur in the somites during the growth and development of
the individual. Hence in some cases there appear to be more than the usual
number of segments, e.g. _Cardiophorus_ larva, and in others the number of
somites is diminished by {91}amalgamation, or by the extreme reduction in
size of some of the parts.
Besides the division of the body into consecutive segments, another feature
is usually conspicuous; the upper part, in many segments, being
differentiated from the lower and the two being connected together by
intervening parts in somewhat the same sort of way as the segments
themselves are connected. Such a differentiation is never visible on the
head, but may frequently be seen in the thorax, and almost always in the
abdomen. A dorsal and a ventral aspect are thus separated, while the
connecting bond on either side forms a pleuron. By this differentiation a
second form of symmetry is introduced, for whereas there is but one upper
and one lower aspect, and the two do not correspond, there are two lateral
and similar areas. This bilateral symmetry is conspicuous in nearly all the
external parts of the body, and extends to most of the internal organs. The
pleura, or lateral regions of the sac, frequently remain membranous when
the dorsal and ventral aspects are hard. The dorsal parts of the Insect's
rings are also called by writers terga, or nota, and the ventral parts
sterna.
The appendages of the body are:—(1) a pair of antennae; (2) the trophi,
constituted by three pairs of mouth-parts; (3) three pairs of legs; (4) the
wings[19]; (5) abdominal appendages of various kinds, but usually jointed.
Before considering these in detail we shall do well to make ourselves more
fully acquainted with the elementary details of the structure of the trunk.
In the adult Insect the integument or crust of the body is more or less
hard or shell-like, sometimes, indeed, very hard, and on examination it
will be seen that besides the divisions into segments and into dorsal,
ventral, and pleural regions, there are lines indicating the existence of
other divisions, and it will be found that by dissection along these lines
distinct pieces can be readily separated. Each hard piece that can be so
separated is called a sclerite, and the individual sclerites of a segment
have received names from entomotomists. The sclerites are not really
{92}quite separate pieces, though we are in the habit of speaking of them
as if such were the case. If an Insect be distended by pressure from the
interior, many of the sclerites can be forced apart, and it is then seen
that they are connected by delicate membrane. The structure is thus made up
of hard parts meeting one another along certain lines of union—sutures—so
that the original membranous continuity may be quite concealed. In many
Insects, or in parts of them, the sclerites do not come into apposition by
sutures, and are thus, as it were, islands of hard matter surrounded by
membrane. A brief consideration of some of the more important sclerites is
all that is necessary for our present purpose: we will begin with the head.
[Illustration: FIG. 49.—Capsule of head of beetle, _Harpalus caliginosus_:
A, upper; B, under surface: _a_, clypeus; _b_, epicranium; _c_,
protocranium; _d_, gula; _e_, facetted eye; _f_, occipital foramen; _g_,
submentum; _h_, cavity for insertion of antenna.]
The head is most variable in size and form; as a part of its surface is
occupied by the eyes and as these organs differ in shape, extent, and
position to a surprising degree, it is not a matter for astonishment that
it is almost impossible to agree as to terms for the areas of the head. Of
the sclerites of the head itself there are only three that are sufficiently
constant and definite to be worthy of description here. These are the
clypeus, the epicranium, and the gula. The clypeus is situate on the upper
surface of the head-capsule, in front; it bears the labrum which may be
briefly described as a sort of flap forming an upper lip. The labrum is
usually possessed of some amount of mobility. The clypeus itself is
excessively variable in size and form, and sometimes cannot be delimited
owing to the obliteration of the suture of connexion with the more
posterior part of the head; it is rarely or never a paired piece.
Occasionally there is a more or less distinct piece interposed between the
clypeus and the labrum, and which is the source of considerable difficulty,
as it may be taken for the clypeus. Some authors call the clypeus the
epistome, but it is better to use this latter term for the purpose of
indicating the part that is immediately behind the labrum, whether that
part be the clypeus, or some other sclerite; the {93}term is very
convenient in those cases where the structure cannot be, or has not been,
satisfactorily determined morphologically.
In Figure 50 the parts usually visible on the anterior aspect of the head
and its appendages are shown so far as these latter can be seen when the
mouth is closed; in the case of the Insect here represented the bases of
the mandibles are clearly seen (_g_), while their apical portions are
entirely covered by the labrum, just below the lower margin of which the
tips of the maxillae are seen, looking as if they were the continuations of
the mandibles.
The labrum is a somewhat perplexing piece, morphologists being not yet
agreed as to its nature; it is usually placed quite on the front of the
head, and varies extremely in form; it is nearly always a single or
unpaired piece; the French morphologist Chatin considers that it is really
a paired structure.
[Illustration: FIG. 50.—Front view of head of field-cricket (_Gryllus_):
_a_, epicranium; _b_, compound eye; _c_, antenna; _d_, post-: _e_,
ante-clypeus; _f_, labrum; _g_, base of mandible; _h_, maxillary palpus;
_i_, labial palpus; _k_, apex of maxilla.]
The gula (Fig. 49, B _d_, and Fig. 47, _z_) is a piece existing in the
middle longitudinally of the under-surface of the head; in front it bears
the mentum or the submentum, and extends backwards to the great occipital
foramen, but in some Insects the gula is in front very distant from the
edge of the buccal cavity. The epicranium forms the larger part of the
head, and is consequently most inconstant in size and shape; it usually
occupies the larger part of the upper-surface, and is reflected to the
under-surface to meet the gula. Sometimes a transverse line exists (Fig.
49, A) dividing the epicranium into two parts, the posterior of which has
been called the protocranium; which, however, is not a good term. The
epicranium bears the antennae; these organs do not come out between the
epicranium and the clypeus, the foramen for their insertion being seated
entirely in the epicranium (see Fig. 50). In some Insects there are traces
of the epicranium being divided longitudinally along the middle line. When
this part is much modified the antennae may appear to be inserted on the
lateral portions of the head, or even {94}on its under-side; this arises
from extension of some part of the epicranium, as shown in Fig. 49, B,
where _h_, the cavity of insertion of the antenna, appears to be situate on
the under-surface of the epicranium, the appearance being due to an
infolding of an angle of the part.
There is always a gap in the back of the head for the passage of the
alimentary canal and other organs into the thorax; this opening is called
the occipital foramen. Various terms, such as frons, vertex, occiput,
temples, and cheeks, have been used for designating areas of the head. The
only one of these which is of importance is the gena, and even this can
only be defined as the anterior part of the lateral portion of the
head-capsule. An extended study of the comparative anatomy of the
head-capsule is still a desideratum in entomology. The appendages of the
head that are engaged in the operations of feeding are frequently spoken of
collectively as the trophi, a term which includes the labrum as well as the
true buccal appendages.
The appendages forming the parts of the mouth are paired, and consist of
the mandibles, the maxillae, and the labium, the pair in this latter part
being combined to form a single body. The buccal appendages are frequently
spoken of as gnathites. The gnathites are some, if not all, of them
composed of apparently numerous parts, some of these being distinct
sclerites, others membranous structures which may be either bare or
pubescent—that is, covered with delicate short hair. In Insects the mouth
functions in two quite different ways, by biting or by sucking. The Insects
that bite are called Mandibulata, and those that suck Haustellata. In the
mandibulate Insects the composition of the gnathites is readily
comprehensible, so that in nearly the whole of the vast number of species
of that type the corresponding parts can be recognised with something like
certainty. This, however, is not the case with the sucking Insects; in them
the parts of the mouth are very different indeed, so that in some cases
morphologists are not agreed as to what parts really correspond with some
of the structures of the Mandibulata. At present it will be sufficient for
us to consider only the mandibulate mouth, leaving the various forms of
sucking mouth to be discussed when we treat of the Orders of Haustellata in
detail.
The upper or anterior pair of gnathites is the mandibles, (Fig. 50, _g_).
There is no part of the body that varies more than {95}does the mandible,
even in the mandibulate Insects. It can scarcely be detected in some, while
in others, as in the male stag-beetle, it may attain the length of the
whole of the rest of the body; its form, too, varies as much as its size;
most usually, however, the pair of mandibles are somewhat of the form of
callipers, and are used for biting, cutting, holding, or crushing purposes.
The mandibles are frequently armed with processes spoken of as teeth, but
which must not be in any way confounded with the teeth of Vertebrates. The
only Insects that possess an articulated tooth are the Passalidae, beetles
armed with a rather large mandible bearing a single mobile tooth among
others that are not so. Wood Mason and Chatin consider the mandibles to be,
morphologically, jointed appendages, and the latter authority states that
in the mandible of _Embia_ he has been able to distinguish the same
elements as exist in the maxillae. In aculeate Hymenoptera the mandibles
are used to a considerable extent for industrial purposes.
[Illustration: FIG. 51.—Mandibles, maxillae, and labium of _Locusta
viridissima_: A, mandibles; B, maxillae (lateral parts) and labium (middle
parts) united: _a_, cardo; _b_, stipes; _c_, palpiger; _d_, max. palp.;
_e_, lacinia; _f_, galea; _g_, submentum; _h_, mentum; _i_, palpiger; _k_,
labial palpus; _l_, ligula; _m_, paraglossa (galea); _n_, lacinia; _o_,
lingua.]
The maxilla is a complex organ consisting of numerous pieces, viz. cardo,
stipes, palpiger, galea, lacinia, palpus. The galea and lacinia are
frequently called the lobes of the maxilla. The maxilla no doubt acts as a
sense organ as well as a mechanical apparatus for holding; this latter
function being subordinate to the other. In Fig. 68, p. 122, we have
represented a complex maxillary sense-organ.
The labium or lower lip has as its basal portion the {96}undivided mentum,
and closes the mouth beneath or behind, according as the position of the
head varies. In most Insects the labium appears very different from the
maxilla, but in many cases several of the parts corresponding to those of
the maxilla can be clearly traced in the labium.
[Illustration: FIG. 52.—Maxilla and lower lip of Coleoptera. A, Maxilla of
_Passalus_: _a_, cardo; _b_, stipes; _c_, palpiger; _d_, palpus; _e_, inner
or inferior lobe or lacinia; _f_, outer or superior lobe or galea: B,
Labium of _Harpalus caliginosus_: _a_, mentum; _b_, hypoglottis; _c_,
palpiger (support of the labial palp); _d_, palp; _e_, ligula; _f_,
paraglossa.]
The mentum is an undivided, frequently very hard, piece, continuous with
either the submentum or the gula, and anterior to this are placed the other
parts, viz. the labial palpi and their supports, the palpigers; beyond and
between these exists a central piece (Fig. 52, B, _e_), about whose name
some difference of opinion prevails, but which may be called the ligula
(languette of French authors), and on each side of this is a paraglossa. In
the Orthoptera the single median piece—the ligula of Coleopterists—is
represented by two divided parts. In some Insects (many Coleoptera) there
is interposed between the mentum and the palpigers a piece called the
hypoglottis (Fig. 52, B, _b_). It is not so well ascertained as it should
be, that the pieces of the lower lip bearing the same names in different
Orders are in all cases really homologous, and comparison suggests that the
hypoglottis of Coleoptera may possibly represent the piece corresponding to
the mentum of Orthopterists, the so-called mentum of beetles being in that
case the submentum of Orthopterists.
There is another part of the mouth to which we may call special attention,
as it has recently attracted more attention than it formerly did; it is a
membranous lobe in the interior of the mouth, very conspicuous in
Orthoptera, and called the tongue, lingua, or hypopharynx; it reposes, in
the interior of the mouth (Fig. 51, _o_), on the middle parts of the front
of the labium; it is probably not entirely lost in Coleoptera, but enters
into the composition of the {97}complex middle part of the lip by
amalgamation with the paraglossae. It has recently been proposed to treat
this lingua as the morphological equivalent of the labium or of the
maxillae, giving it the name of the endolabium, but the propriety of this
course remains to be proved;[20] the view is apparently suggested chiefly
by the structure of the mouth of _Hemimerus_, a very rare and most peculiar
Insect that has not as yet been sufficiently studied.
As the maxillae and labium are largely used by taxonomists in the
systematic arrangement of the mandibulate Insects, we give a figure of them
as seen in Coleoptera, where the parts, though closely amalgamated, can
nevertheless be distinguished. This Fig. 52 should be compared with Fig.
51.
In speaking of the segments of the body we pointed out that they were not
separate parts but constituted an uninterrupted whole, and it is well to
remark here that this is also true of the gnathites. Although the mouth
parts are spoken of as separate pieces, they really form only projections
from the great body wall. Fig. 51, B, shows the intimate connexion that
exists between the maxillae and labium; the continuity of the mandibles
with the membrane of the buccal cavity is capable of very easy
demonstration.
The head bears, besides the pieces we have considered, a pair of antennae.
These organs, though varying excessively in form, are always present in the
adult Insect, and exist even in the majority of young Insects. They are
very mobile, highly sensitive organs, situate on or near the front part of
the head. The antennae arise in the embryo from the procephalic lobes, the
morphological import of which parts is one of the most difficult points
connected with Insect embryology.
The eyes of Insects are of two sorts, simple and compound. The simple eyes,
or ocelli, vary in number from one to as many as eighteen or twenty; when
thus numerous they are situated in groups on each side of the head. In
their most perfect form, as found in adult aculeate Hymenoptera, in
Orthoptera and Diptera, ocelli are usually two or three in number, and
present the appearance of small, perfectly transparent lenses inserted in
the integument. In their simplest form they are said to consist of some
masses of pigment in connexion with a nerve.
{98}[Illustration: FIG. 53.—Two ommatidia from the eye of _Colymbetes
fuscus_, × 160. (After Exner.) _a_, Cornea; _b_, crystalline cone; _c_,
rhabdom; _d_, fenestrate membrane with nerve structures below it; _e_,
iris-pigment; _f_, retina-pigment.]
The compound, or facetted, eyes are the most remarkable of all the
structures of the Insect, and in the higher and more active forms, such as
the Dragon-flies and hovering Diptera, attain a complexity and delicacy of
organisation that elicit the highest admiration from every one who studies
them. They are totally different in structure and very distinct in function
from the eyes of Vertebrata, and are seated on very large special lobes of
the brain (see Fig. 65), which indeed are so large and so complex in
structure that Insects may be described as possessing special ocular brains
brought into relation with the lights, shades, and movements of the
external world by a remarkably complex optical apparatus. This instrumental
part of the eye is called the dioptric part in contradistinction from the
percipient portion, and consists of an outer corneal lens (_a_, Fig. 53),
whose exposed surface forms one of the facets of the eye; under the lens is
placed the crystalline cone (_b_), this latter being borne on a rod-like
object (_c_), called the rhabdom. There are two layers of pigment, the
outer (_e_), called the iris-pigment, the inner (_f_), the retinal-pigment;
underneath, or rather we should say more central than, the rhabdoms is the
fenestrate membrane (_d_), beyond which there is an extremely complex mass
of nerve-fibres; nerves also penetrate the fenestrate membrane, and their
distal extremities are connected with the delicate sheaths by one of which
each rhabdom is surrounded, the combination of sheath and nerves forming a
retinula. Each set of the parts above the fenestrate membrane constitutes
an ommatidium, and there may be many of these ommatidia in an eye; indeed,
it is said that the eye of a small beetle, _Mordella_, contains as many as
25,000 ommatidia. As a rule the larvae of Insects with a complete
metamorphosis bear only simple eyes. In the young of Dragon-flies, as well
as of some other Insects having a less perfect metamorphosis, the compound
eyes exist in the early stages, but they {99}have then an obscure
appearance, and are probably functionally imperfect.
In the interior of the head there exists a horny framework called the
tentorium, whose chief office apparently is to protect the brain. It is
different in kind according to the species. The head shows a remarkable and
unique relation to the following segments. It is the rule in Insect
structure that the back of a segment overlaps the front part of the one
following it; in other words, each segment receives within it the front of
the one behind it. Though this is one of the most constant features of
Insect anatomy, it is departed from in the case of the head, which may be
either received into, or overlapped by, the segment following it, but never
itself overlaps the latter. There is perhaps but a single Insect
(_Hypocephalus_, an anomalous beetle) in which the relation between the
head and thorax can be considered to be at all similar to that which exists
between each of the other segments of the body and that following it; and
even in _Hypocephalus_ it is only the posterior angles of the head that
overlap the thorax. Although the head usually appears to be very closely
connected with the thorax, and is very frequently in repose received to a
considerable extent within the latter, it nevertheless enjoys great freedom
of motion; this is obtained by means of a large membrane, capable of much
corrugation, and in which there are seated some sclerites, so arranged as
to fold together and occupy little space when the head is retracted, but
which help to prop and support it when extended for feeding or other
purposes. These pieces are called the cervical sclerites or plates. They
are very largely developed in Hymenoptera, in many Coleoptera, and in
Blattidæ, and have not yet received from anatomists a sufficient amount of
attention. Huxley suggested that they may be portions of head segments.
[Illustration: FIG. 54.—Extended head and front of thorax of a beetle,
_Euchroma_: _a_, back of head; _b_, front of pronotum; _c_, chitinous
retractile band; _d_, cervical sclerites.]
THORAX.
The thorax, being composed of the three consecutive rings behind the head,
falls naturally into three divisions—pro-, meso-, {100}and metathorax.
These three segments differ greatly in their relative proportions in
different Insects, and in different stages of the same Insect's life. In
their more highly developed conditions each of the three divisions is of
complex structure, and the sclerites of which it is externally made up are
sufficiently constant in their numbers and relative positions to permit of
their identification in a vast number of cases; hence the sclerites have
received names, and their nomenclature is of practical importance, because
some, if not all, of these parts are made use of in the classification of
Insects. Each division of the thorax has an upper region, called
synonymically dorsum, notum, or tergum; an inferior or ventral region,
called sternum; and on each side a lateral region, the pleuron. These
regions of each of the three thoracic divisions are further distinguished
by joining to their name an indication of the segment spoken of, in the
form of the prefixes pro-, meso-, and meta-; thus the pronotum, prosternum,
and propleura make up the prothorax. The thoracic regions are each made up
of sclerites whose nomenclature is due to Audouin.[21] He considered that
every thoracic ring is composed of the pieces shown in Fig. 55, viz. (1)
the sternum (B', _a_), an unpaired ventral piece; (2) the notum (A),
composed of four pieces placed in consecutive longitudinal order (A'), and
named praescutum (_a_), scutum (_b_), scutellum (_c_), and post-scutellum
(_d_); (3) lateral pieces, of which he distinguished on each side an
episternum (B', _c_), epimeron (_e_), and parapteron (_d_), these together
forming the pleuron. We give Audouin's Figure, but we cannot enter on a
full discussion of his views as to the thorax; they have become widely
known, though the constancy of the parts is not so great as he supposed it
would prove to be. Sometimes it is impossible to find all the elements he
thought should be present in a thoracic ring, while in other cases too many
sclerites exist. As a rule the notum of the meso- and metathoraces is in
greater part composed of two pieces, the scutum and the scutellum; while in
the pronotum only one dorsal piece can be satisfactorily distinguished,
though a study of the development may show that really two are frequently,
if not usually, present. On the other hand, one, or more, of the notal
sclerites in some cases shows evidence of longitudinal division along the
middle. The sternum or ventral piece, though varying greatly in form, is
{101}the most constant element of a thoracic segment, but it has sometimes
the appearance of consisting of two parts, an anterior and a posterior. The
pleuron nearly always consists quite evidently of two parts, the
episternum, the more anterior and inferior, and the epimeron.[22] The
relations between these two parts vary much; in some cases the episternum
is conspicuously the more anterior, while in others the epimeron is placed
much above it, and may extend nearly as far forwards as it. It may be said,
as a rule, that when the sternum extends farther backwards than the notum,
the epimeron is above the episternum, as in many Coleoptera; but if the
sternum be anterior to the notum, then the episternum is superior to the
epimeron, as in dragon-flies. We would here again reiterate the fact that
these "pieces" are really not separate parts, but are more or less
indurated portions of a continuous integument, which is frequently entirely
occupied by them; hence a portion of a sclerite that in one species is
hard, may in an allied form be wholly or partly membranous, and in such
case its delimitation may be very evident on some of its sides, and quite
obscure on another.
[Illustration: FIG. 55.—Mesothorax of _Dytiscus_, after Audouin. A, notum;
A', pieces of the notum separated: _a_, praescutum; _b_, scutum; _c_,
scutellum; _d_, post-scutellum: B, the sternum and pleura united; B', their
parts separated: _a_, sternum; _c_, episternum; _d_, parapteron; _e_,
epimeron.]
{102}The parapteron of Audouin does not appear to be really a distinct
portion of the pleuron; in the case of _Dytiscus_ it is apparently merely a
thickening of an edge. Audouin supposed this part to be specially connected
with the wing-articulation, and the term has been subsequently used by
other writers in connexion with several little pieces that exist in the
pleural region of winged Insects.
The prothorax is even more subject to variation in its development than the
other divisions of the thorax are. In the Hymenoptera the prosternum is
disconnected from the pronotum and is capable, together with the first pair
of legs, of movement independent of its corresponding dorsal part, the
pronotum, which in this Order is always more or less completely united with
the meso-thorax; in the Diptera the rule is that the three thoracic
segments are closely consolidated into one mass. In the majority of Insects
the prothorax is comparatively free, that is to say, it is not so closely
united with the other two thoracic segments as they are with one another.
The three thoracic rings are seen in a comparatively uniform state of
development in a great number of larvae; also in the adult stages of some
Aptera, and among winged insects in some Neuroptera such as the Embiidae,
Termitidae, and Perlidae. In Lepidoptera the pronotum bears a pair of
erectile processes called patagia; though frequently of moderately large
size, they escape observation, being covered with scales and usually
closely adpressed to the sides of the pronotum.
The two great divisions of the body—the mesothorax and the metathorax—are
usually very intimately combined in winged Insects, and even when the
prothorax is free, as in Coleoptera, these posterior two thoracic rings are
very greatly amalgamated. In the higher forms of the Order just mentioned
the mesosternum and mesopleuron become changed in direction, and form as it
were a diaphragm closing the front of the metasternum. The meso- and
meta-thorax frequently each bear a pair of wings.
We have described briefly and figured (Fig. 55) the sclerites of the
mesothorax, and those of the metathorax correspond fairly well with them.
In addition to the sclerites usually described as constituting these two
thoracic divisions, there are some small pieces at the bases of the wings.
Jurine discriminated and named no less than seven of these at the base of
the anterior {103}wing of a Hymenopteron. One of them becomes of
considerable size and importance in the Order just mentioned, and seems to
be articulated so as to exert pressure on the base of the costa of the
wing. This structure attains its maximum of development in a genus (?
nondescript) of Scoliidae, as shown in Fig. 56. The best name for this
sclerite seems to be that proposed by Kirby and Spence, tegula. Some
writers call it paraptère, hypoptère, or squamule, and others have termed
it patagium; this latter name is, however, inadmissible, as it is applied
to a process of the prothorax we have already alluded to.
[Illustration: FIG. 56.—Head and thorax of wasp from Bogota: _t_, tegula;
_b_, base of wing.]
To complete our account of the structure of the thorax it is necessary to
mention certain hard parts projecting into its interior, but of which there
is usually little or no trace externally. A large process in many Insects
projects upwards from the sternum in a forked manner. It was called by
Audouin the entothorax; some modern authors prefer the term apophysis.
Longitudinal partitions of very large size, descending from the dorsum into
the interior, also exist; these are called phragmas, and are of great
importance in some Insects with perfect flight, such as Hymenoptera,
Lepidoptera, and Diptera. There is no phragma in connection with the
pronotum, but behind this part there may be three. A phragma has the
appearance of being a fold of the dorsum; it serves as an attachment for
muscles, and may probably be of service in other ways. More insignificant
projections into the interior are the little pieces called apodemes (Fig.
57, _e_); these are placed at the sides of the thorax near the wings. The
apophyses are no doubt useful in preserving the delicate vital organs from
shocks, or from derangement by the muscular movements and the changes of
position of the body.
[Illustration: FIG. 57.—Transverse section of skeleton of metathorax of
_Goliathus druryi_, seen from behind: _a_, metanotum; _b_, metasternum;
_c_, phragma; _d_, entothorax (apophysis or furca); _e_, apodeme; _f_,
tendon of articulation. (After Kolbe.)]
The appendages of the thorax are (_a_) inferior, the legs; (_b_)
{104}superior, the wings. The legs are always six in number, and are
usually present even in larvae, though there exist many apodal larvae,
especially in Diptera. The three pairs of legs form one of the most
constant of the characters of Insects. They are jointed appendages and
consist of foot, otherwise tarsus; tibia, femur, trochanter, and coxa;
another piece, called trochantin more or less distinctly separated from the
coxa, exists in many Insects. The legs are prolongations of the body sac,
and are in closer relation with the epimera and with the episterna than
with other parts of the crust, though they have a close relation with the
sternum. If we look at the body and leg of a neuropterous Insect (Fig. 58)
we see that the basal part of the leg—the coxa—is apparently a continuation
of one of the two pleural pieces or of both; in the latter case one of the
prolonged pieces forms the coxa proper, and the tip of the other forms a
supporting piece, which may possibly be the homologue of the trochantin of
some Insects. In some Orthoptera, especially in Blattidae, and in
Termitidae, there is a transverse chitinised fold interposed between the
sternum and the coxa, and this has the appearance of being the same piece
as the trochantin of the anterior legs of Coleoptera.
[Illustration: FIG. 58.—Hind leg of _Panorpa_: _a_, episternum; _a′_,
epimeron; _b_, coxa; _b′_, coxal fold of epimeron; _c_, trochanter; _d_,
femur; _e_, tibia; _f_, tarsus.]
Beyond the coxa comes the trochanter; this in many Hymenoptera is a double
piece, though in other Insects it is single; usually it is the most
insignificant part of the leg. The femur is, on the whole, the least
variable part of the leg; the tibia, which follows it, being frequently
highly modified for industrial or other purposes. The joint between the
femur and the tibia is usually bent, and is therefore the most conspicuous
one in the leg; it is called the knee. The other joints have not
corresponding names, though that between the tibia and the tarsus is of
great importance. The spines at the tip of the tibia, projecting beyond it,
are called spurs, or calcares. The tarsus or {105}foot is extremely
variable; it is very rarely absent, but may consist of only one
piece—joint, as it is frequently called[23]—or of any larger number up to
five, which may be considered the characteristic number in the higher
Insect forms. The terminal joint of the tarsus bears normally a pair of
claws; between the claws there is frequently a lobe or process, according
to circumstances very varied in different Insects, called empodium,
arolium, palmula, plantula, pseudonychium, or pulvillus. This latter name
should only be used in those cases in which the sole of the foot is covered
with a dense pubescence. The form of the individual tarsal joints and the
armature or vestiture of the lower surface are highly variable. The most
remarkable tarsus is that found on the front foot of the male _Dytiscus_.
It has been suggested that the claws and the terminal appendage of the
tarsus ought to be counted as forming a distinct joint; hence some authors
state that the higher Insects have six joints to the feet. These parts,
however, are never counted as separate joints by systematic entomologists,
and it has recently been stated that they are not such originally.
The parts of the foot at the extremity of the last tarsal joint proper are
of great importance to the creature, and vary greatly in different Insects.
The most constant part of this apparatus is a pair of claws, or a single
claw. Between the two claws there may exist the additional apparatus
referred to above. This in some Insects—notably in the Diptera—reaches a
very complex development. We figure these structures in _Pelopaeus
spinolae_, a fossorial Hymenopteron, remarking that our figures exhibit the
apparatus in a state of retraction (Fig. 59). According to the nomenclature
of Dahl and Ockler[24] the plate (_b_) on the dorsal aspect is the pressure
plate (_Druck-Platte_), and acts as an agent of pressure on the sole of the
pad (C, _e_); _c_ and _d_ on the underside are considered to be
extension-agents; _c_, extension-plate; _d_, extension-sole
(_Streck-Platte_, _Streck-Sohle_). These agents are assisted in acting on
the pad by means of an elastic bow placed in the interior of the latter.
The pad (_e_) is a very remarkable structure, capable of much extension and
retraction; {106}when extended it is seen that the pressure plate is bent
twice at a right angle so as to form a step, the distal part of which runs
along the upper face of the basal part of the pad; the apical portion of
this latter consists of two large lobes, which in repose, as shown in our
Figure (_f_), fall back on the pad, something in the fashion of the
retracted claws of the cat, and conceal the pressure-plate.
The mode in which Insects are able to walk on smooth perpendicular surfaces
has been much discussed, and it appears highly probable that the method by
which this is accomplished is the exudation of moisture from the foot;
there is still, however, much to be ascertained before the process can be
satisfactorily comprehended. The theory to the effect that the method is
the pressure of the atmosphere acting on the foot when the sole is in
perfect apposition with the object walked on, or when a slight vacuum is
created between the two, has apparently less to support it.
[Illustration: FIG. 59.—Foot of _Pelopaeus_, a fossorial wasp: A, tarsus
entire; B, terminal joint, upper side; C, under side. _a_, claw; _b_, base
of pressure-plate; _c_, extension-plate; _d_, extension-sole; _e_, pad;
_f_, lobe of pad retracted.]
The legs of the young Insect are usually more simple than those of the
adult, and in caterpillars they are short appendages, and only imperfectly
jointed. If a young larva, with feet, of a beetle, such as _Crioceris
asparagi_ be examined, it may be seen that the leg is formed by
protuberance of the integument, which becomes divided into parts by simple
creases; an observation suggesting that the more highly developed jointed
leg is formed in a similar manner. This appears to be really the case,
{107}for the actual continuity of the limb at the chief joint—the knee—can
be demonstrated in many Insects by splitting the outer integument
longitudinally and then pulling the pieces a little apart; while in other
cases even this is not necessary, the knee along its inner face being
membranous to a considerable extent, and the membrane continuous from femur
to tibia.
Turning to the wings, we remark that there may be one or two pairs of these
appendages. When there is but one pair it is nearly always mesothoracic,
when there are two pairs one is invariably mesothoracic, the other
metathoracic. The situation of the wing is always at the edge of the notum,
but the attachment varies in other respects. It may be limited to a small
spot, and this is usually the case with the anterior wing; or the
attachment may extend for a considerable distance along the edge of the
notum, a condition which frequently occurs, especially in the case of the
posterior wings. The actual connexion of the wings with the thorax takes
place by means of strong horny lines in them which come into very close
relation with the little pieces in the thorax which we have already
described, and which were styled by Audouin articulatory epidemes. There is
extreme variety in the size, form, texture, and clothing of the wings, but
there is so much resemblance in general characters amongst the members of
each one of the Orders, that it is usually possible for an expert, seeing
only a wing, to say with certainty what Order of Insects its possessor
belonged to. We shall allude to these characters in treating of the Orders
of Insects.
Each wing consists of two layers, an upper and a lower, and between them
there may be tracheae and other structures, especially obvious when the
wings are newly developed. It has been shown by Hagen that the two layers
can be separated when the wings are recently formed, and it is then seen
that each layer is traversed by lines of harder matter, the nervures. These
ribs are frequently called wing-veins, or nerves, but as they have no
relation to the anatomical structures bearing those names, it is better to
make use of the term nervures. The strength, number, form and
inter-relations of these nervures vary exceedingly; they are thus most
important aids in the classification of Insects. Hence various efforts have
been made to establish a system of nomenclature that shall be uniform
throughout the different Orders, but at present success has not
{108}attended these efforts, and it is probable that no real homology
exists between the nervures of the different Orders of Insects. We shall
not therefore discuss the question here. We may, however, mention that
German savants have recently distinguished two forms of nervures which they
consider essentially distinct, viz. convex and concave. These, to some
extent, alternate with one another, but a fork given off by a convex one is
not considered to be a concave one. The terms convex and concave are not
happily chosen; they do not refer to the shape of the nervures, but appear
to have been suggested by the fact that the surface of the wing being
somewhat undulating the convex veins more usually run along the ridges, the
concave veins along the depressions. The convex are the more important of
the two, being the stronger, and more closely connected with the
articulation of the wing.
The wings, broadly speaking, may be said to be three-margined: the margin
that is anterior when the wings are extended is called the costa, and the
edge that is then most distant from the body is the outer margin, while the
limit that lies along the body when the wings are closed is the inner
margin.
The only great Order of Insects provided with a single pair of wings is the
Diptera, and in these the metathorax possesses, instead of wings, a pair of
little capitate bodies called halteres or poisers. In the abnormal
Strepsiptera, where a large pair of wings is placed on the metathorax,
there are on the mesothorax some small appendages that are considered to
represent the anterior wings. In the great Order Coleoptera, or beetles,
the anterior wings are replaced by a pair of horny sheaths that close
together over the back of the Insect, concealing the hind-wings, so that
the beetle looks like a wingless Insect: in other four-winged Insects it is
usually the front wings that are most useful in flight, but the elytra, as
these parts are called in Coleoptera, take no active part in flight, and it
has been recently suggested by Hoffbauer[25] that they are not the
homologues of the front wings, but of the tegulae (see Fig. 56), of other
Insects. In the Orthoptera the front wings also differ in consistence from
the other pair over which they lie in repose, and are called tegmina. There
are many Insects in which the wings {109}exist in a more or less
rudimentary or vestigial condition, though they are never used for purposes
of flight.
The abdomen, or hind body, is the least modified part of the body, though
some of the numerous rings of which it is composed may be extremely altered
from the usual simple form. Such change takes place at its two extremities,
but usually to a much greater extent at the distal extremity than at the
base. This latter part is attached to the thorax, and it is a curious fact
that in many Insects the base of the abdomen is so closely connected with
the thorax that it has all the appearance of being a portion of this latter
division of the body; indeed it is sometimes difficult to trace the real
division between the two parts. In such cases a further differentiation may
occur, and the part of the abdomen that on its anterior aspect is
intimately attached to the thorax may on its posterior aspect be very
slightly connected with the rest of the abdomen. Under such circumstances
it is difficult at first sight to recognise the real state of the case.
When a segment is thus transferred from the abdomen to the metathorax, the
part is called a median segment. The most remarkable median segment exists
in those Hymenoptera which have a stalked abdomen, but a similar though
less perfect condition exists in many Insects. When such a union occurs, it
is usually most complete on the dorsal surface, and the first ventral plate
may almost totally disappear: such an alteration may involve a certain
amount of change in the sclerites of the next segment, so that the
morphological determination of the parts at the back of the thorax and
front of the abdomen is by no means a simple matter. A highly modified
hind-body exists in the higher ants, Myrmicidae. In Fig. 60 we contrast the
simple abdomen of _Japyx_ with the highly modified state of the same part
in an ant.
[Illustration: FIG. 60.—Simple abdomen of _Japyx_ (A) contrasted with the
highly modified one of an ant, _Cryptocerus_ (B). The segments are numbered
from before backwards.]
Unlike the head and thorax, the abdomen is so loosely knitted together that
it can undergo much expansion and contraction. {110}This is facilitated by
an imbricated arrangement of the plates, and by their being connected by
means of membranes admitting of much movement (Fig. 47, _m_, p. 88). In
order to understand the structure of the abdomen it should be studied in
its most distended state; it is then seen that there is a dorsal and a
ventral hard plate to each ring, and there is also usually a stigma; there
may be foldings or plications near the line of junction of the dorsal and
ventral plates, but these margins are not really distinct pieces. The
pleura, in fact, remain membranous in the abdominal region, contrasting
strongly with the condition of these parts in the thorax. The proportions
of the plates vary greatly; sometimes the ventral are very large in
proportion to the dorsal, as is usually the case in Coleoptera, while in
the Orthoptera the reverse condition prevails.
Cerci or other appendages frequently exist at the extremity of the abdomen
(Fig. 47, _n_, p. 88); the former are sometimes like antennae, while in
other cases they may be short compressed processes consisting of very few
joints. The females of many Insects possess saws or piercing instruments
concealed within the apical part of the abdomen; in other cases an elongate
exserted organ, called ovipositor, used for placing the eggs in suitable
positions, is present. Such organs consist, it is thought, either of
modified appendages, called gonapophyses, or of dorsal, ventral, or pleural
plates. The males frequently bear within the extremity of the body a more
or less complicated apparatus called the genital armour. The term
gonapophysis is at present a vague one, including stings, some ovipositors,
portions of male copulatory apparatus, or other structures, of which the
origin is more or less obscure.
The caterpillar, or larva, of the Lepidoptera and some other Insects, bears
a greater number of legs than the three pairs we have mentioned as being
the normal number in Insects, but the posterior feet are in this case very
different from the anterior, and are called false legs or prolegs. These
prolegs, which are placed on the hind body, bear a series of hooks in
Lepidopterous larvae, but the analogous structures of Sawfly larvae are
destitute of such hooks.
Placed along the sides of the body, usually quite visible in the larva, but
more or less concealed in the perfect Insect, are little apertures for the
admittance of air to the respiratory {111}system. They are called spiracles
or stigmata. There is extreme variety in their structure and size; the
largest and most remarkable are found on the prothorax of Coleoptera,
especially in the groups Copridae and Cerambycidae.
The exact position of the stigmata varies greatly, as does also their
number. In the Order Aptera there may be none, while the maximum number of
eleven pairs is said by Grassi[26] to be attained in _Japyx solifugus_: in
no other Insect have more than ten pairs been recorded, and this number is
comparatively rare. Both position and number frequently differ in the early
and later stages of the same Insect. The structure of the stigmata is quite
as inconstant as the other points we have mentioned are.
[Illustration: FIG. 61.—Membranous space between pro- and meso-thoraces of
a beetle _Euchroma_, showing stigma (_st_); _a_, hind margin of pronotum;
_b_, front leg; _c_, front margin of mesonotum; _d_, base of elytra; _e_,
mesosternum.]
The admission of air to the tracheal system and its confinement there, as
well as the exclusion of foreign bodies, have to be provided for. The
control of the air within the system is, according to Landois[27] and
Krancher,[28] usually accomplished by means of an occluding apparatus
placed on the tracheal trunk a little inside of the stigma, and in such
case this latter orifice serves chiefly as a means for preventing the
intrusion of foreign bodies. The occluding apparatus consists of muscular
and mechanical parts, which differ much in their details in different
Insects. Lowne supposes that the air is maintained in the tracheal system
in a compressed condition, and if this be so, this apparatus must be of
great importance in the Insect economy. Miall and Denny[29] state that in
the anterior stigmata of the cockroach the valves act as the occluding
agents, muscles being attached directly to the inner face of the valves,
and in some other Insects the spiracular valves appear to act partially by
muscular agency, but there are many stigmata having valves destitute of
muscles. According to Lowne[30] there exist valves in the blowfly at the
entrance to the trachea proper, and he gives the following as the
arrangement of parts for the admission of air:—there is a spiracle
{112}leading into a chamber, the atrium, which is limited inwardly by the
occluding apparatus; and beyond this there is a second chamber, the
vestibule, separated from the tracheae proper by a valvular arrangement. He
considers that the vestibule acts as a pump to force the air into the
tracheae.
[Illustration: FIG. 62.—Diagrammatic Insect to explain terms of position.
A, apex; B, base: 1, tibia; 2, last abdominal segment; 3, ideal centre.]
SYSTEMATIC ORIENTATION.
Terms relating to position are unfortunately used by writers on entomology
in various, even in opposite senses. Great confusion exists as to the
application of such words as base, apex, transverse, longitudinal. We can
best explain the way in which the relative positions and directions of
parts should be described by reference to Figure 62. The spot 3 represents
an imaginary centre, situated between the thorax and abdomen, to which all
the parts of the body are supposed to be related. The Insect should always
be described as if it were in the position shown in the Figure, and the
terms used should not vary as the position is changed. The creature is
placed with ventral surface beneath, and with the appendages extended, like
the Insect itself, in a horizontal plane. In the Figure the legs are, for
clearness, made to radiate, but in the proper position the anterior pair
should be approximate in front, and the middle and hind pairs directed
backwards under the body. The legs are not to be treated as if they were
hanging from the body, though that is the position they frequently actually
assume. The right and left sides, and the upper and lower faces (these
latter are frequently also spoken of as sides), are still to retain the
same nomenclature even when the position of the specimen is reversed. The
base of an organ is that margin that is nearest to the ideal centre, the
apex that which is most distant. {113}Thus in Fig. 62, where 1 indicates
the front tibia, the apex (A) is broader than the base (B); in the antennae
the apex is the front part, while in the cerci the apex is the posterior
part; in the last abdominal segment (2) the base (B) is in front of the
apex (A). The terms longitudinal and transverse should always be used with
reference to the two chief axes of the body-surface; longitudinal referring
to the axis extending from before backwards, and transverse to that going
across, _i.e._ from side to side.
{114}CHAPTER IV
ARRANGEMENT OF INTERNAL ORGANS–MUSCLES–NERVOUS SYSTEM–GANGLIONIC CHAIN–
BRAIN–SENSE-ORGANS–ALIMENTARY CANAL–MALPIGHIAN TUBES–RESPIRATION–TRACHEAL
SYSTEM–FUNCTION OF RESPIRATION–BLOOD OR BLOOD-CHYLE–DORSAL VESSEL OR
HEART–FAT-BODY–OVARIES–TESTES–PARTHENOGENESIS–GLANDS.
The internal anatomy of Insects may be conveniently dealt with under the
following heads:—(1) Muscular system; (2) nervous system; (3) alimentary
system (under which may be included secretion and excretion, about which in
Insects very little is known); (4) respiratory organs; (5) circulatory
system; (6) fat-body; (7) reproductive system.