Chapter VIII: Introduction: Habits–classification–structure–chilognatha–chilopoda (2)

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.

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.

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_).

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.

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.

{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.

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).

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.

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.

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.