Philip Henry Gosse.

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tubes, which are larger and longer than those of the first
pair. Finally, the third pair resemble palpi, for each con-
sists of two lengthened joints and are bluntly pointed.
The spinning tubes in these are limited, as it appears to
me, to one or two at the extreme end of each spinneret,
the whole surface besides being covered with the ordinary
long bristles. Strictly speaking, however, they are three-



SPIDERS AND MITES

jointed, for all the spinnerets spring from wart-like sock-
ets, which may be considered as basal joints; and as the
circlet of bristles in the first pair doubtless indicates a
short joint, sunken as it were within the preceding, this
pair is likewise three-jointed; the middle pair appears to
be but two-jointed.

The minute horny tubes are themselves composed of
two joints, the basal one thick, the terminal one very
slender, and perforated with an orifice of excessive tenu-
ity; through which the gum oozes, at the will of the
animal, as an equally attenuated thread. On our Clubiona^
the number of tubes in all the spinnerets is about three
hundred; but in the Garden Spider (Epeira) they exceed
a thousand.

This remarkable multiplicity of the strands with which
the apparently simple and certainly slender thread of the
Spider is composed, has attracted the attention of those
philosophers who seek to discover the reasons of the phe-
nomena they see in nature. The explanation was first
suggested, I believe, by Mr. Rennie, 1 but it has been
amplified with much force by Professor Jones, in the
following words:

"A very obvious reflection will here naturally suggest
itself, in connection with this beautiful machinery; why,
in the case of the Spider, it has been found necessary to
provide a rope of such complex structure, when in so
many Insects a simple, undivided thread, drawn from the
orifice of a single tube, like the thread of the Silkworm,
for instance, was sufficient for all required purposes.
And here, as in every other case, it will be found, on

1 "Insect Architecture," 337.



246 EVENINGS AT THE MICROSCOPE

consideration, that a complicated apparatus has been sub-
stituted for a simple one only to meet the requirements
of strict necessity. The slow-moving Caterpillar, as it
leisurely produces its silken cord, gives time enough for
the fluid of which it is formed to harden by degrees into
a tenacious filament, as it is allowed to issue by instal-
ments from the end of the labial pipe; but the habits of
the Spider require a different mode of proceeding, as its
line must be instantly converted from a fluid into a strong
rope, or it would be of no use for the purposes it is in-
tended to fulfil. Let a fly, for example, become entangled
in the meshes of a Spider's web; no time is to be lost;
the struggling victim, by every effort to escape, is tearing
the meshes that entangle it, and would soon succeed in
breaking loose did not its lurking destroyer at once rush
out to complete the capture and save its net, spun with
so much labor, from ruin. With the rapidity of thought
it darts upon its prey; and before the eye of the spectator
can comprehend the manoeuvre, the poor fly is swathed
in silken bands, until it is as incapable of moving as an
Egyptian mummy. To allow the Spider to perform such
a feat as this, its thread must evidently be instantaneously
placed at its disposal, which would have been impossible
had it been a single cord, but being subdivided into nu-
merous filaments, so attenuated as we have seen them to
be, there is no time lost in the drying; from being fluid
they are at once converted into a solid rope, ready for im-
mediate service." 1

No doubt you have often admired the exquisite reg-
ularity of those Spiders' webs which are called geometric;

1 "Nat. Hist, of Anim.," ii. 339.



SPIDERS AND MITES 247

that of our abundant Garden Spider, for instance. You
have observed the cables which stretch from wall to wall,
or from bush to bush, in various directions, to form the
scaffolding, on which the net is afterward to be woven;
then you have marked the straight lines, like the spokes
of a wheel, that radiate from the centre to various points
of these outwork cables, and finally the spiral thread that
circles again and again round the radii, till an exquisite
net of many meshes is formed.

But possibly you are not aware that these lines are
formed of two quite distinct sorts of silk. It has been
shown that the cables and radii are perfectly unadhesive,
while the concentric or spiral circles are extremely viscid.
Now the microscope, or a powerful lens, will reveal the
cause of this difference; the threads of the cables and radii
are perfectly simple, while the spiral threads are closely
studded with minute globules of fluid, like drops of dew,
which, from the elasticity of the thread, are easily sepa-
rated from each other. These are globules of viscid gum,
as is easily proved by touching one or two with the finger,
to which they will instantly adhere ; or by throwing a little
fine dust over the nest, when the spirals will be found
clogged with dirt, while the radii and cables remain un-
soiled. It is these viscid threads alone that have the
power of detaining the vagrant flies which accidentally
touch the net.

The diversity of the secreting organs already alluded
to, as well as in the spinnerets, is no doubt connected with
this difference in the character of the silk ; and it is worthy
of remark that this diversity is greatest in such Spiders,
as the Epeirce, which spin geometric nets.

Immense is the number of globules of viscidity that



248 EVENINGS AT THE MICROSCOPE

stud the spiral circles of one of these nets. Mr. Black-
wall, the able and learned historian of the tribe, has esti-
mated that as many as 87,360 such pearly drops occurred
in a net of average dimensions, and 120,000 in a large net
of fourteen or sixteen inches diameter; and yet a Spider
will construct such a net, if uninterruped, in less than
three-quarters of an hour.

Scarcely less admirable is the ease and precision with
which the little architect traverses her perpendicular or
diagonal web of rope; a skill which leaves that of the
mariner who leaps from shroud to backstay in a ship's
rigging immeasurably behind. To understand it, however,
in some measure, look at this last joint of one of the feet
of our well- used Clubiona. It is a cylindrical rod, ending
in a rounded point; every part of its surface is studded
with stiff, rather long, horny bristles, which, springing
from the side, arch inward toward the point. Now this
array of spines effectually prevents a false step, for if any
part of . the leg, which is sufficiently long, only strikes the
thread, the latter is certain to slip in between the bristles,
and thus to catch the leg. But more precision than this
is requisite; especially when we observe with what deli-
cacy of touch the hinder feet are often used to guide the
thread as it issues from the spinnerets, and particularly
with what lightning-like rapidity the larger net- weavers
will, with the assistance of these feet, roll a dense web of
silk around the body of a helpless fly, swathing it up, like
an Egyptian mummy, in many folds of cloth, in an instant.

Look, then, at the extreme tip of the ultimate joint.
Two stout hooked claws of dark horny texture are seen
proceeding from it side by side, and a third of smaller size,
and more delicate in appearance, is placed between them,



SPIDERS AND MITES 249

and on a lower level. The former have their under or
concave surface set with teeth (eighteen on each in this
example) very regularly cut, like those of a comb, which
are minute at the commencement of the series near the
base of the claw, and gradually increase in length to the
tip. These are doubtless sensible organs of touch, feeling
and catching the thread; and they, moreover, act as combs,
cleansing their limbs, and probably their webs, from the
particles of dust and other extraneous matter which are
continually cleaving to them.

There are Spiders in the sea also. I can show you one
which is sufficiently common on the southern shores,




CLAWS OP SPIDEU.



sprawling and crawling sluggishly among the filamentous
seaweeds and branching flexible zoophytes. Here it is,
Nymphon by name.

Its most prominent characteristic is the excessive slen-
derness of all its parts, but especially its eight legs, which
are exceedingly lengthened, comprising each eight joints,
and no thicker than the finest thread. On the other hand,
the body is reduced to a minimum; the abdomen, which
in the Spiders and Harvestmen of the land is so bulky as
to constitute the chief volume of the animal, is here so
minute that you will have some difficulty in finding it at
all; it is, in fact, that tiny atom of a point that projects
between the hindmost pair of limbs. The thorax, indeed,
is a little more developed; but even this has scarcely any



250 EVENINGS AT THE MICROSCOPE

appreciable breadth or thickness, being scarcely more than
the extended line formed by the successive points of
origin of the limbs.

The head, however, is distinct and well furnished. It
is crowned with a short column, much as in the Harvest-
man, on the summit of which are placed four black eyes,
set in square; these, under the magnifying power which
we are applying to them, gleam like diamonds, the light
being highly refracted through them. It is the high re-
fractive power of these eyes, as of those which we have
lately been examining, which makes them appear black;
for, as I have explained, they are really transparent lenses,
covered with polished corneas, and furnished with the
other essentials requisite for the transmission of the rays
of light to the optic nerve, or, as in this case, direct to
the brain.

In front, you see the head projects into a stout oval
or cylindrical proboscis, terminating in a small mouth and
stout jaws, and furnished at the sides with a pair of spine-
like palpi, and a pair of pincer-claws (modified antennae)
somewhat resembling the nippers of a Crab or Lobster.

Such is the outward form of this tiny speck, the whole
body of which scarcely equals in dimensions a quarter of
an inch of sewing cotton. And now I will beg your atten-
tion to the singular manner in which digestion is carried
on in this atom, which you will discern plainly enough
through its brown but translucent skin. If you look care-
fully at either of the long, many- jointed legs, you will see
that it is permeated by a central vessel, the walls of which
contract periodically with a pulsation closely resembling
that of a heart, by which granules or pellucid corpuscles,
floating in a clear fluid, are forced forward. There is no



SPIDERS AND MITES 251 -

uniformity in the direction of the pulsatory waves; some-
times, as in the limb we are watching, they proceed from
the body toward the extremity; but, in some of the others,
we shall probably find, even at the same moment, that the
waves have a retrograde course; and this contrariety may
occur in two contiguous limbs on the same side of the ani-
mal. By continuing our observation for some minutes, we
shall find also that its force is varying and uncertain;
strong and regular at one time, weak and vacillating at
another, and sometimes even quite intermitted, or, at least,
quite imperceptible.

By selecting a limb in which the movements are strong,
you may trace the vessel to its termination in a blind sac
in the last joint but one of the limb; and then follow it
up to its junction with a great vessel which runs longitudi-
nally through the trunk, of which all the vessels that per-
meate the limbs are branches, and whence the circulating
globules all proceed. This great vessel is the stomach and
this circulation is the provision for dispersing the nutritive
properties of the food to all parts of the system. There is
in these humble and simply organized animals no proper
blood, or, at least, none included in a system of arteries
and veins ; but the products of digestion are carried to the
most distant parts of the body, through this extraordinary
development of the stomach or intestine (both in one), and
through this series of blind canals, by means of their own
irregular contractions, aided by the muscular movements
of the body and limbs.

You would scarcely forgive me if I took no opportu-
nity of showing you the Cheese-mite, that first object of
wonder to every child that looks through a magnifying
glass. And no place so suitable for its introduction, as in



252 EVENINGS AT THE MICROSCOPE

connection with its cousins, the Spiders and Harvestmen.
Well, fortunately, we need not search far for specimens;
for here, in the cavity of this almost defunct skeleton of
a cheese, we can find as many millions as you can reason-
ably desire to select from. Here is a fat one; we'll take
him.

You see with a pocket lens that it has a plump, pol-
ished oval body, of a pellucid white hue, and eight short
red legs; but for more than this we must go to the tube.
Look at him now, as he lies on his back, helplessly sprawl-
ing and throwing his feeble legs about, in the live -box.

His oval body is divided by a transverse furrow into
thorax and abdomen, like a Beetle's; and there is another
division between the head and thorax, wherein it differs
from the Spiders. The first two pairs of legs are separated
by an interval from the last two pairs; they are all of a
translucent pale red hue, as is also the head : each consists
of seven short joints, the last of which has a sort of heart-
shaped pad, something like a horse's hoof, and a single
hooked claw, which works against its sole.

The structure of the head cannot be seen satisfactorily
otherwise than by crushing the Mite in the compressorium;
a process which, when we remember how many thousands
we crush down in our oral compressorium every time we
eat ripe cheese, need not excite much compunction. We
must put a drop of water between the plates, in order to
wash away the opaque granules which will escape from
the bodies of the animals, when the skin, and all the solid
parts, will be left beautifully clear and distinct. More-
over, by putting half a dozen specimens in at once, we
shall secure them pressed in various aspects, and be pretty
sure of some perfectly flat and symmetrical



SPIDERS AND MITES



253



I have one under such conditions; the parts of the
mouth nicely expanded, and the whole well displayed.
iNow for a higher power; for, to discern this properly, we
cannot do with less than 600 diameters.

Viewed from beneath, we see a broad labium, nearly
square, divided at the tip into two blunt points, with a
sharp notch between them. The two lateral edges are, as
it were, buttressed by a pair of palpi, which are thick, and
consist of four joints each; these are distinguished by the
bristles at each joint, though the whole are united; sol-
dered, as it were, to the sides of the lip.

The upper portion of the mouth is formed by two stout




HEAD OF CHEESE-MITE



mandibles, which are jointed to the front of the head, and
can be either widely expanded or brought together so as
to form a covering to the labium. They are pincer-form,
like the claws of a crab, the two fingers being strongly
toothed on their opposing surfaces. They thus form effec-
tive prehensile instruments. These mandibles can be ad-
vanced separately or together, and the whole head can be
elevated or depressed.

In the water of ponds we may frequently see, playing



254 EVENINGS AT THE MICROSCOPE

among the sub-aquatic vegetation, bright-colored Mites;
sometimes rich velvety green, sometimes purple, but more
commonly brilliant scarlet; often curiously marked with
sinuous patterns or spots of black. They swim rapidly
and evenly by means of rapid rowings with their legs,
which are thickly fringed with long hairs. I have one
here, which seems to be the Hydrachna histrionica. It is
a little, flat, circular, cushion- or cake-like creature, scar-
let, with four clouds of black on its back, about one-sixth
of an inch in diameter. You may notice the effective oars
which the legs form by means of their thick fringe of hair,
and in particular the power which the hind pair possess
by reason of the enormously dilated hip -joint, affording
space for broad and powerful muscles.

The structure of the mouth differs greatly from the
same parts in the Cheese-mite. The palpi here are long
and perfectly free throughout; the fourth joint is long
and slender, and is curiously hollowed at the end to re-
ceive the terminal joint, which forms a short claw, and
which falls down upon the former. The mandibles, too,
are not pincers, but consist each of a thick joint cut off
obliquely behind, like the nib of a pen, while the other
extremity is blunt and broad, and bears a strong curved
claw; the lip is oval, and cleft in the middle, and is
wedged in between the bases of the first pair of legs.



WHEEL-BEARERS 255.



CHAPTER XIV

WHEEL-BEAKERS

I MUST now introduce you to a class of animals pe-
culiarly microscopic, since without our marvel -show-
ing instrument they are wholly beyond the sphere of
human cognizance. Yet they have been ever since its
invention favorite objects with the microscopist; and I
am free to confess that, among all the classes of ani-
mated beings, this of the Rotifer a, has been my own
special delight. Their numerous and varied forms, often
of remarkable symmetry and elegance, their swiftly-re-
volving wheels, their vigorous and sprightly motions,
their curious habits and instincts, their complex organ-
ization, and the ease and correctness with which this is
discerned through their tissues, which have the transpar-
ent brilliance of the purest crystal all combine to im-
part a charm to the Wheel-bearers, which makes the ob-
server hail their appearance in his drops of water with
pleasure, and linger over them with unwearied delight.
The peculiarity which specially characterizes them is
the presence of certain organs called cilia, and their ar-
rangement in such a manner that their motion gives to
the observer the impression that two toothed wheels are
placed on the front of the animal, which are in rapid
revolution on their axes. This was believed to be the
real fact by the earlier microscopists, though they were



256 EVENINGS AT THE MICROSCOPE

utterly unable to conceive how such a movement could
consist with parts maintaining an organic connection be-
tween themselves. It is, however, an optical illusion,
depending on the nature of ciliary movement, which
therefore I must first endeavor to explain to you.

Cilia are organs which play a very important part as
instruments of locomotion, as well as of other functions,
in all the lower forms of animals, and in the early stages
of some of the higher forms. They are found also char-
acterizing the lowest forms of vegetable life, giving to
them the means of spontaneous locomotion, which ren-
ders them liable to be mistaken for animals. They con-
sist of prolongations of the fleshy tissue into long and
very delicate hairs, which are endowed with a special
faculty of motion. This consists of a bending down in
a given direction to a certain extent of flexure, followed
by a rapid resuming of the perpendicular; which is, how-
ever, immediately succeeded by like bendings and straight-
enings in alternate gradation. The simplest condition of
this movement is that in which a single cilium only
exists, by whose successive lash-like beats upon the sur-
rounding water the animal is rowed along as a boat
through the sea. But far more commonly cilia are ar-
ranged in rows, or in many series of rows, in which case
the bending and straightening of the individual cilia do
not occur otherwise than in strict and orderly relation to
each other. For instance, one cilium in a given row be-
gins to bend, the one next to it then begins, then the
third, then the fourth, and so on all precisely in the
same direction, all in precisely the same time, all with
precisely the same force, and all to precisely the same
extent. It follows that, before the first has completed its



WHEEIs-BEARERS

beat and resumed the erect position, three or four others
are in various degrees of flexion, regularly graduated;
and that if the eye could look laterally at such a row
of cilia suddenly arrested and fixed as they were, we
should see their tips tracing a wavy line instead of a
straight one. Moreover, since the bending of any cilium
brings its tip nearer to its successor than it was before,
and this approximation increases the further the flexure
proceeds, it follows that at the bottom of each wave the
tips of the cilia overlap their successors, while the spaces
perpendicularly above their bases are left more open by
the removal of their points.

Hence, in microscopical observation of ciliated ani-
mals, though the individual cilia are too minute to be
discerned while still, we can readily discern the increased
density (and therefore opacity) of the bottom of a wave,
contrasted with the increased openness (and therefore
clearness) of the summit. So that the optical effect is
that of an alternate succession of dark and light spots
blending into each other.

But as no cilium in the series is for two successive
moments in the same degree of flexure, and as both it
and all its predecessors and successors are ever urging
on their perfectly timed and regulated course, the waves
are never fixed, but always gliding on with a swift but
beautifully even rapidity. And as it is with the waves,
so it is with their optical effect upon the eye; the black
and white spots, or rather the black spots with blank in-
tervals, appear to be constantly chasing each other in
ceaseless race.

You are then prepared to take a peep at this beautiful
Brachionus pala. A cup of elegant form, swelling at the



258 EVENINGS AT THE MICROSCOPE

sides and narrowing a little at the mouth, has one side
of its rim furnished with four spines, the middle pair of
which are very slender, sharp, and needle-like; the other
side of the rim is undulated, but not toothed. The bottom
of the cup terminates in two broad blunted points when
seen directly in front; but a lateral view considerably
modifies the whole form. Then you see that the back of
the cup is much more swollen, the belly-edge being nearly
straight, and that this latter descends much lower than
the dorsal line, the bottom being as it were cut away
obliquely and slightly hollowed. Between the two bot-
tom points, there is a round opening, for an object which
we shall see presently. Such is the shell, or lorica, as it
is technically called, which is of a rather stiff, elastic con-
sistence, of a horny (chitinous) texture, and of the most
glassy transparency, permitting us to trace every vessel,
every organ, and every function of the animal within the
shell with perfect distinctness. The little creature is of
unwonted dimensions in its class, for it is one thirty-sixth
of an inch in length. Hence it is just visible as a white
speck moving in the water, to the unassisted eye, while a
pocket lens reveals its beautiful form.

Within this translucent shell you see a confused mass
of moving viscera, a multitude of irregular sacs and bands,
lying over each other, whose crowding, changing, and
vanishing lines distract the attention, and prevent you
from making out anything definitely. But a waved out-
line of hyaline flesh is protruding from the rim of the
shell, and now, having reached beyond the level of the
spine-points, it rapidly unfolds into three broad flattish
lobes, and in an instant each of the two lateral ones is
crowned by a wheel of dark points in rapid horizontal



WHEEL-BEARERS 259

revolution. Is not this a charming sight? Round and
round go the wheels, forming two perfect crowns, which
rotate with uninterrupted and unceasing course, smooth
and regular, which we can compare with nothing else
than the crown wheel of a watch, if allowed to run down.

Now these are examples of ciliary action. Though at
first it is almost impossible to persuade one's self that
there is not an actual rotation of parts, yet this- is only
an illusion, as I have already explained. The waves
alone move, the cilia themselves retaining their position
unchanged except that they alternately bend and erect
themselves. It may assist your idea of this motion to
advert to a field of corn, over which a smart breeze is
blowing. You see that waves chase each other across the
field; but your reason, and indeed your observation, tells
you that this appearance is produced entirely by the al-



Online LibraryPhilip Henry GosseEvenings at the microscope (Volume 1) → online text (page 17 of 32)