the development of the limbs becomes more and more extended, till
we arrive at crocodiles, in which they attain a considerable degree
of perfection. As a consequence of this greater development of the
skeleton, we find the trunk divisible into separate regions. We now,
for the first time, meet with a distinct neck, separating the head
from the thorax, which is itself distinguishable from the abdomen;
and a distinct sacrum is interposed between the lumbar and the caudal
vertebræ.
The number of ribs differs in different species of Sauria: they are
always articulated to the extremities of the transverse processes of
the vertebræ, of which they appear to be continuations. Processes of
this description also occur in the neck, attached to the transverse
processes of the cervical vertebræ; and these have been regarded as
_cervical ribs_. Their presence are impediments to the flexions of
the neck; whence arises the difficulty which the crocodile appears to
have in bending the neck, while turning round upon the animal he is
pursuing. In the thorax, the ribs are connected with a broad sternum;
but there are other ribs, both before and behind, which have no such
termination, and therefore bear the name of _false ribs_.
The toes are usually provided with membranes spread between them,
to assist in swimming. The form of the tail, which is generally
compressed vertically, like that of fishes, though perhaps not to
an equal degree, is another indication of their being formed for an
aquatic life: for where the tail has this shape, we always find that
the chief muscular power is bestowed upon it as an instrument of
aquatic progression, producing, by its lateral flexions, a horizontal
movement of the body. Crocodiles and alligators, for instance, which
have this conformation, are comparatively weak when on land, and as
soon as they have seized their prey their efforts are always directed
to drag it with them into the water; knowing that when in their own
element they can readily master its struggles, and dispose of it as
they please.
In the Gecko tribe we find a particular mechanism provided for
effecting the adhesion of the feet to the objects to which they
are applied. It is somewhat analogous to that employed in the case
of the house-fly, already mentioned. Each foot has five toes; all,
except the thumb, terminated by a sharp curved claw. On the under
surface of each toe there are as many as sixteen transverse slits,
leading to the same number of cavities, or sacs; these open forward,
and their external edge is serrated, appearing like the teeth of a
small-toothed comb. All these parts, together with the cavities, are
covered or lined with cuticle. Below them are large muscles which
draw down the claw; and from the tendons of these muscles arise two
sets of smaller muscles, situated so as to be put upon the stretch,
when the former are in action. By the contractions of these muscles
the orifices of the cavities, or sacs to which they belong, are
opened, and the serrated edges applied accurately to the surfaces
with which the feet are in contact. Sir Everard Home, in his account
of this structure, compares it to the sucking disk of the Remora. By
its means the animal is enabled to walk securely upon the smoothest
surfaces, even in opposition to the tendency of gravity. It can run
very quickly along the walls or ceiling of a building, in situations
where it can not be supported by the feet, but must depend altogether
upon the suspension derived from a succession of rapid and momentary
adhesions.
Although the Sauria are better formed for progressive motion than
any of the other orders of reptiles, yet the greater shortness and
oblique position of their limbs, compared with those of mammiferous
quadrupeds, obliges them in general to rest the weight of the trunk
of the body on the ground, when they are not actually moving. None
of these reptiles has any other kind of pace than that of walking or
jumping; being incapable of performing either a trot or a gallop, in
consequence of the obliquity of the plane in which their limbs move.
The chameleon walks with great slowness and apparent difficulty; and
we have seen that, in consequence of the structure of the bones of
its neck, the crocodile, though capable of swift motion in a straight
line, is unable to turn itself round quickly. The general type of
these reptiles, having reference to an amphibious life, has not
attained that exclusive adaptation to a terrestrial existence which
we find in the higher orders of the Mammalia.
The order of Chelonian Reptiles, which comprises all the tribes of
tortoises and turtles, appears to constitute an exception to the
general laws of conformation which prevail among vertebrated animals:
for instead of presenting a skeleton wholly internal, the trunk of
the body is found to be inclosed on every side in a bony case, which
leaves openings only for the head, the tail, and the fore and hind
extremities. That portion of this osseous expansion which covers
the back is termed the Carapace; and the flat plate which defends
the lower part of the body is termed the Plastron. It is a form of
structure that reminds us of the defence provided for animals very
low in the scale of organization, such as the echinus, the crustacea,
and the bivalve mollusca. Yet the substance which forms these
strong bucklers, both above and below, is a real osseous structure,
developed in the same manner as other bones, subject to all the
changes and having all the properties of these structures. The great
purpose which Nature seems to have had in view in the formation of
the Chelonia is security; and for the attainment of this object she
has constructed a vaulted and impenetrable roof, capable of resisting
enormous pressures from without, and proof against any ordinary
measures of assault. It is to the animal a strong castle, into which
he can retire on the least alarm, and defy the efforts of his enemies
to dislodge or annoy him.
These considerations supply us with a key to many of those apparent
anomalies which can not fail to strike us in viewing the dispositions
of the parts of the skeleton and the remarkable inversion they appear
to have undergone, when compared with the usual arrangement. We
find, however, on a more attentive examination, that all the bones
composing the skeleton in other vertebrated animals exist also in
the tortoise; and that the bony case which envelops all the other
parts is really formed by an extension of the spinous processes of
the vertebræ and ribs on the one side and of the usual pieces which
compose the sternum on the other. The upper and lower plates thus
formed are united at their edges by expansions of the sternocostal
appendices, which become ossified. Thus, no new element has been
created; but advantage has been taken of those already existing in
the general type of the vertebrata, to modify their forms by giving
them different degrees of relative development, and converting them,
by these transformations, into a mechanism of a very different kind,
and subservient to other objects than those to which they are usually
applied. It is scarcely possible to have stronger proofs, if such
were wanting, of the unity of plan which has regulated the formation
of all animal structures than those afforded by the skeleton of the
tortoise.
The first step taken to secure the relative immobility of the trunk
is to unite in one rigid, bony column all its vertebræ, and to allow
of motion only in those of the neck and of the tail. The former,
accordingly, are all anchylosed together, leaving, indeed, traces
of their original forms as separate vertebræ, but exhibiting no
sutures at the place of junction. The canal for the spinal marrow is
preserved, as usual, above the bodies of these coalesced vertebræ,
and is formed by their united leaves; the arches being completed by
the spinous processes. But these processes do not terminate in a
crest as usual; they are further expanded in a lateral direction,
forming flat pieces along the back, which are united to one another
by sutures, and which are also joined to the expanded ribs, so as to
form the continuous plane surface of the carapace. The transverse
processes of the vertebræ are well marked, but, though firmly united
to the ribs, do not give rise to them; for the ribs, which are
flattened and expanded, so as to touch one another along their whole
length, are inserted below, between the bodies of every two adjoining
vertebræ; while above they are united by suture with the plates of
the spinous processes. This change in the situation of the ribs is
the consequence of the change in their office. When designed to be
very movable, we find them attached either to the extremities of
the transverse processes or to the articular surfaces of a single
vertebra; but where solidity and security are aimed at, they are
always inserted between the bodies of two vertebræ. It is remarkable,
indeed, that a great number of the peculiarities which distinguish
the conformation of the chelonia from that of other reptiles indicate
an approach to the structure of birds; as if Nature had intended this
small group of animals to be an intermediate link of gradation to
that new and important type of animals destined for a very different
mode of existence.
It is to be noticed, also, that as the plates, which form this
investing case, are bony structures, they could not with any safety
have been exposed to the action of the atmosphere. Hence we find them
covered throughout with a thin, horny plate, originally a production
of the integument. This substance is commonly known by the name of
tortoise-shell.
The immobility of the trunk is compensated, as far as regards the
safety of the head, by the great flexibility of the neck; which is
composed of seven vertebræ, unencumbered by processes, and capable
of taking a double curvature like the letter S, when the head is to
be retracted within the carapace. These vertebræ are joined by the
ball and socket articulation common to all the _existing_ species
of reptiles.[7] The articulation of the head with the neck is
effected in the same manner; but it is interesting to remark that the
occipital condyle, which is situated at the lower margin of the great
aperture, though presenting a single convex surface, yet has that
surface evidently divided into three parts; the two upper portions
being lateral and the lower portion in the middle.
The singular conformation of the bones of the head, in the turtle,
affords fresh evidence in support of the theory that these bones were
originally vertebræ. The brain of the tortoise is exceedingly small;
and yet the skull, when viewed from above, presents an appearance of
great breadth, as if it inclosed a cavity of large dimensions. This
great breadth of the head in the turtle gives the animal an aspect of
superior intelligence, to which character, from the really diminutive
size of its brain, it is in no respect entitled. As the turtle is
unable to withdraw its head within the carapace, such extraordinary
protection appears to have been necessary: for it is not met with in
the tortoise, which has a carapace sufficiently capacious to give
shelter to the head whenever occasion may require.
All the feet are joined obliquely to the limbs which support them,
giving the animal an apparent awkwardness of gait, as if it were
obliged to walk upon club feet. The impulse which they give being
lateral and oblique, renders them more efficacious for progression in
the water than on land: this circumstance, in conjunction with the
constitutional torpor of the animal, sufficiently accounts for the
excessive and, indeed, proverbial tardiness of its movements.
Security appears still to be the object aimed at in the mechanism of
all other parts of the skeleton. After the head has been drawn in by
the double or serpentine flexion of the neck, the knees are brought
together and the whole limb withdrawn within the shell, the forelegs
folding completely over the head, so as to cover and protect it most
effectually.
Considerable differences may be noticed in the structure of the
several species of Chelonia, according to the diversity of their
habits. Tortoises which live on land require more complete protection
by means of their shell than turtles, or Emydes, which dwell only in
the water: hence the convexity of their carapace, the solidity of its
ossification, its immovable connection with the plastron, and the
complete shelter it affords to the head and limbs. Turtles, on the
other hand, receiving support from the element in which they reside,
require less provision to be made for these objects. Previously to
the retraction of the head and limbs within the shell, the air is
expelled from the large cavities of the lungs by the vigorous actions
of the abdominal muscles, which exist in these animals as well as in
all the vertebrata, although here they are covered by the bones, and
compress the lungs by pushing the abdominal viscera against them.
This sudden expulsion of air is the cause of the long-continued
hissing sound which the tortoise emits while preparing to retreat
into its stronghold.
The ribs, though they first assume the form of broad plates
immovably united to the spine, when they have proceeded a certain
distance separate from each other and resume their usual form; the
intervening spaces between two adjacent ribs being here filled up
by membrane. The plastron is united with the carapace by membrane
likewise; and the sternum, instead of forming one broad plate of
bone, has the intervals between its imperfectly developed elements
also membraneous. All this renders the whole shell less compact,
more flexible, and more feeble: but the movements of the animal are
quicker and more energetic.
These characteristic differences between the aquatic Chelonia and
those that live on land are still more strongly marked in the genus
Trionyx, or soft tortoise, which is destitute of scales, and in which
many of the pieces that are bony in the tortoise are replaced by
simple cartilage or membrane.
The enormous weight of the shell of the turtle would be a serious
impediment to the motion of this animal in the water, were there
not some provision made for diminishing the specific gravity in the
body. This purpose is answered by the great capacity of the lungs,
which, when inflated with air, nearly fill the thorax, and give great
buoyancy to the whole mass. Thus, wherever there exists a supposed
inconvenience, dependent on the fulfilment of one condition, we are
certain to meet with a compensation in the structure of some other
part and in the mode of executing some other function. An express
provision for giving buoyancy has been made in the construction of
the shell of a species of tortoise inhabiting the coasts of the
Seychelle Islands. The under surface of the shell, instead of being
gently concave, as in land tortoises, has a deep circular concavity
in the centre, above four inches in depth, which, when the animal
goes into the water, retains a large volume of air, buoying up the
whole mass while it remains in that element. The greater size of
turtles, when compared with tortoises, is a further instance of the
superior facility with which organic growth proceeds in aquatic than
in land animals formed on the same model of construction.
THE CLASSIFICATION AND ORIGIN OF INSECTS
--LORD AVEBURY
About sixty years ago the civil and ecclesiastical authorities of
St. Fernando in Chili arrested a certain M. Renous on a charge of
witchcraft because he kept some caterpillars which turned into
butterflies. This was no doubt an extreme case of ignorance; it is
now almost universally known that the great majority of insects quit
the egg in a state very different from that which they ultimately
assume; and the general statement in works on entomology has been
that the life of an insect may be divided into four periods.
Thus, according to Kirby and Spence, “the states through which the
insects pass are four: the _egg_, the _larva_, the _pupa_, and the
_imago_.” Burmeister, also, says that, excluding certain very rare
anomalies, “we may observe four distinct periods of existence in
every insect--namely, those of the egg, the larva, the pupa, and
the imago, or perfect insect.” In fact, however, the various groups
of insects differ widely from one another in the metamorphoses they
pass through: in some, as in the grasshoppers and crickets, the
changes consist principally in a gradual increase of size, and in the
acquisition of wings; while others, as, for instance, the common fly,
acquire their full bulk in a form very different from that which they
ultimately assume, and pass through a period of inaction in which not
only is the whole form of the body altered, not only are legs and
wings acquired, but even the internal organs themselves are almost
entirely disintegrated and re-formed.
The following list gives the orders or principal groups into which
the Class Insecta may be divided. I will not, indeed, here enter upon
my own views, but will adopt the system given by Mr. Westwood in his
excellent _Introduction to the Modern Classification of Insects_. He
divides insects into thirteen groups, and with reference to eight of
them it may be said that there is little difference of opinion among
entomologists. These orders are by far the most numerous, and I have
placed them in capital letters. As regards the other five there is
still much difference of opinion. It must also be observed that Prof.
Westwood omits the parasitic Anoplura, as well as the Thysanura and
Collembola.
ORDERS OF INSECTS ACCORDING TO WESTWOOD
1. HYMENOPTERA Bees, Wasps, Ants, etc.
2. Strepsiptera Stylops, Zenos, etc.
3. COLEOPTERA Beetles.
4. Euplexoptera Earwigs.
5. ORTHOPTERA Grasshoppers, Crickets, Cockroaches, etc.
6. Thysanoptera Thrips.
7. NEUROPTERA Ephemeras, etc.
8. Trichoptera Phryganea.
9. DIPTERA Flies and Gnats.
10. Aphaniptera Fleas.
11. HETEROPTERA Bugs.
12. HOMOPTERA Aphis, Coccus, etc.
13. LEPIDOPTERA Butterflies and Moths.
Of these thirteen orders, the eight which I have placed in capital
letters--namely, the first, third, fifth, seventh, ninth, eleventh,
twelfth, and thirteenth--are much the most important in the number
and variety of their species; the other five form comparatively
small groups. The Strepsiptera are minute insects, parasitic on
Hymenoptera: Rossi, by whom they were discovered, regarded them as
Hymenopterous; Lamarck placed them among the Diptera; by others they
have been considered to be most closely allied to the Coleoptera, but
they are now generally treated as an independent order.
The Euplexoptera or Earwigs are only too familiar to most of us.
Linnæus classed them among the Coleoptera, from which, however, they
differ in their transformations. Fabricius, Olivier, and Latreille
regarded them as Orthoptera; but Dr. Leach, on account of the
structure of their wings, considered them as forming the type of
a distinct order, in which view he has been followed by Westwood,
Kirby, and many other entomologists.
The Thysanoptera, consisting of the Linnæan genus Thrips, are minute
insects well known to gardeners, differing from the Coleoptera in the
nature of their metamorphoses, in which they resemble the Orthoptera
and Hemiptera.
The Trichoptera, or Caddis worms, offer many points of resemblance
to the Neuroptera, while in others they approach more nearly to the
Lepidoptera. According to Westwood, the genus Phryganea “forms the
connecting link between the Neuroptera and Lepidoptera.”
The last of these small aberrant orders is that of the Aphaniptera,
constituted for the family Pulicidæ. In their transformations, as
in many other respects, they closely resemble the Diptera. Strauss
Durckheim indeed said that “_la puce est un diptère sans ailes_.”
Westwood, however, regards it as constituting a separate order.
As indicated by the names of these orders, the structure of the wings
affords extremely natural and convenient characters by which the
various groups may be distinguished from one another. The mouth-parts
also are very important; and, regarded from this point of view,
the Insecta have been divided into two series--the Mandibulata and
Haustellata, or mandibulate and suctorial groups, between which the
Collembola occupy an intermediate position. These two series are:
MANDIBULATA. HAUSTELLATA.
Hymenoptera. Lepidoptera.
Strepsiptera. Diptera.
Coleoptera. Aphaniptera.
Euplexoptera. Hemiptera.
Orthoptera. Homoptera.
Trichoptera?
Thysanoptera?
Again--and this is the most important from my present point of
view--insects have sometimes been divided into two other series,
according to the nature of their metamorphoses: “Heteromorpha,” to
use the terminology of Prof. Westwood, “or those in which there is no
resemblance between the parent and the offspring; and Homomorpha, or
those in which the larva resembles the imago, except in the absence
of wings. In the former the larva is generally worm-like, of a soft
and fleshy consistence, and furnished with a mouth, and often with
six short legs attached in pairs to the three segments succeeding
the head. In the Homomorpha, including the Orthoptera, Hemiptera,
Homoptera, and certain Neuroptera, the body, legs, and antennæ are
nearly similar in their form to those of the perfect insect, but the
wings are wanting.”
HETEROMORPHA. HOMOMORPHA.
Hymenoptera. Euplexoptera.
Strepsiptera. Orthoptera.
Coleoptera. Hemiptera.
Trichoptera. Homoptera.
Diptera. Thysanoptera.
Aphaniptera.
Lepidoptera.
Neuroptera.
But though the Homomorphic insects do not pass through such striking
changes of form as the Heteromorphic, and are active throughout
life, still it was until within the last few years generally (though
erroneously) considered, that in them, as in the Heteromorpha, the
life fell into four distinct periods; those of (1) the egg, (2)
the larva, characterized by the absence of wings, (3) the pupa with
imperfect wings, and (4) the imago, or perfect insect.
The species belonging to the order Hymenoptera are among the most
interesting of insects. To this order belong the gallflies, the
sawflies, the ichneumons, and, above all, the ants and bees. We are
accustomed to class the Anthropoid apes next to man in the scale
of creation, but if we were to judge animals by their works, the
chimpanzee and the gorilla must certainly give place to the bee and
the ant. The larvæ of the sawflies, which live on leaves, and of the
Siricidæ or long-tailed wasps, which feed on wood, are very much like
caterpillars, having three pairs of legs, and in the former case
abdominal prolegs as well: but in the great majority of Hymenoptera
the larvæ are legless, fleshy grubs; and the various modes by which
the females provide for, or secure to, them a sufficient supply of
appropriate nourishment constitutes one of the most interesting pages
of Natural History.
The species of Hymenoptera are very numerous; in England alone
there are about 3,000 kinds, most of which are very small. In the
pupa state they are inactive, and show distinctly all the limbs of
the perfect insect, incased in distinct sheaths, and folded on the
breast. In the perfect state they are highly organized and very
active. The working ants and some few species are wingless, but
the great majority have four strong membraneous wings, a character
distinguishing them at once from the true flies, which have only one
pair of wings.
The sawflies are so called because they possess at the end of the
body a curious organ, corresponding to the sting of a wasp, but
which is in the form of a fine-toothed saw. With this instrument
the female sawfly cuts a slit in the stem or leaf of a plant, into
which she introduces her egg. The larva much resembles a caterpillar,
both in form and habits. To this group belongs the nigger, or black
caterpillar of the turnip, which is often in sufficient numbers to
do much mischief. Some species make galls, but the greater number of
galls are formed by insects of another family, the Cynipidæ.