as to whether _shard_[720] means wing-cases, dung, or a fragment of
earthenware, and whether _born_ should be spelled with or without
the _e_, it might have thrown some weight into the scale of those
who contend for the orthography adopted above, and that the meaning
of _shard_ in this place is dung, if they had been aware that the
beetle (_Geotrupes stercorarius_) is actually _born_ amongst dung,
and no where else; and that no beetle which makes a hum in flying can
with propriety be said, as Dr. Johnson has interpreted the epithet
in his Dictionary, "to be born amongst broken stones or pots."
That Shakespeare alluded to the Beetle, and not to the Cockchafer
(_Melolontha vulgaris_), seems clear from the fact of the former
being to be heard in all places almost every fine evening in the
summer, while the latter is common only in particular districts, and
at one period of the year. S.
[720] _Sharn_ is the common name of cow-dung in the North: therefore
Shakespeare probably wrote _sharn_-born. _Mr. MacLeay._
[721] De Geer, vii. 123.
[722] Id. ibid. 126.
[723] PLATE VI. FIG. 4, 5. 10, 11. 24-26.
[724] For a full description of this instrument see Reaum. i. 125,
&c. PLATE VI. FIG. 13.
[725] The mode, however, in which this is effected in all insects
furnished with a proboscis, can scarcely be by suction, strictly so
called, or the abstraction of air, since the air-vessels of insects
do not communicate with their mouths: it is more probably performed
in part by capillary attraction; and, as Lamarck has suggested,
(_Syst. des Anim. sans Vertèbres_, p. 193.) in part by a succession
of undulations and contractions of the sides of the organ.
[726] PLATE VI. FIG. 16-19.
[727] PLATE VII. FIG. 5.
[728] PLATE VII. FIG. 8. 10.
[729] _Obs. on the Animal Œconomy_, p. 221. Compare Reaum. ii. 167.
[730] Redi _de Insectis_, 39.
[731] _New Travels_, i. xxxix.
[732] _Phil. Trans._ 1740, p. 441. I confess, notwithstanding Mr.
Baker's general accuracy, that I suspect some mistake here.
[733] Leeuw. _Op._ ii. 363.
[734] Not having ever met with another specimen, I am unable to say
of what precise species of aphidivorous fly it is the larva, nor can
I find a figure of it, though it approaches near to one given by De
Geer (vi. _t._ 7. _f._ 1-3). Its shape is oblong-oval, length about
four lines, and colour pale red speckled with black. Each of the
seven or eight segments which compose the body projects on each side
into three serrated flat aculei or teeth; three or four similar but
smaller aculei arm the head: and two, much larger than the rest, the
anus, one on each side of the usual bifid protuberance which bears
the respiratory plates. A bifid tubercular elevation is also placed
in the middle of the back of each segment.
LETTER XIII.
_FOOD OF INSECTS CONTINUED._
STRATAGEMS EMPLOYED IN PROCURING IT.
The _stratagems_ of insects in obtaining their food are now to engage
our attention. I shall not dwell on those inartificial modes of
surprising their prey, of which examples may be found amongst almost
every order of insects, such as watching behind a leaf or other
object affording concealment until its approach; but shall proceed to
describe the various artifices of the race of spiders, of which there
are several hundred distinct species differing essentially from each
other both in characters and manners.
Many of these are constantly under our eyes; and were it not that we
are accustomed to neglect what is the subject of daily occurrence,
we should never behold a spider's web without astonishment. What,
if we had not witnessed it, would seem more incredible than that
any animal should spin threads; weave these threads into nets more
admirable than ever fowler or fisherman fabricated; suspend them with
the nicest judgement in the place most abounding in the wished-for
prey; and there concealed watch patiently its approach? In this case,
as in so many others, we neglect actions in minute animals, which
in the larger would excite our endless admiration. How would the
world crowd to see a fox which should spin ropes, weave them into
an accurately-meshed net, and extend this net between two trees for
the purpose of entangling a flight of birds? Or should we think we
had ever expressed sufficient wonder at seeing a fish which obtained
its prey by a similar contrivance? Yet there would, in reality, be
nothing more marvellous in their procedures than in those of spiders,
which, indeed, the minuteness of the agent renders more wonderful.
All spiders do not spin webs. A considerable number adopt other
means for catching insects. Of these I shall speak hereafter. At
present I shall endeavour to give you a clear idea of the operations
of the _weavers_, explaining successively the instruments by which
they spin--the mode of forming their nets, together with the various
descriptions of them--and the manner in which they entrap and secure
their prey.
The thread spun by spiders is in substance similar to the silk of
the silk-worm and other caterpillars, but of a much finer quality.
As in them, it proceeds from reservoirs, into which it is secreted
in the form of a viscid gum: but in the mode of its extrication is
very dissimilar, issuing not from the mouth but the hinder part of
the abdomen. If you examine a spider, you will perceive in this part
four or six little teat-like protuberances or spinners. These are
the machinery through which, by a process more singular than that
of rope-spinning, the thread is drawn. Each spinner is furnished
with a multitude of tubes, so numerous and so exquisitely fine,
that a space often not much bigger than the pointed end of a pin,
is furnished, according to Reaumur[735], with a thousand of them.
From each of these tubes, consisting of two pieces, the last of
which terminates in a point infinitely fine, proceeds a thread of
inconceivable tenuity, which, immediately after issuing from it,
unites with all the other threads into one. Hence from each spinner
proceeds a compound thread; and these four threads, at the distance
of about one-tenth of an inch from the apex of the spinners, again
unite, and form the thread we are accustomed to see, which the spider
uses in forming its web. The threads, however, are not all of the
same thickness, for Leeuwenhoek observed that some of the tubes were
larger than others, and furnished a larger thread. Thus a spider's
thread, even spun by the smallest species, and when so fine that
it is almost imperceptible to our senses, is not, as we suppose, a
single line, but a rope composed of at least four thousand strands.
How astonishing! But to feel all the wonder of this fact we must
follow Leeuwenhoek in one of his calculations on the subject. This
renowned microscopic observer found by an accurate estimation that
the threads of the minutest spiders, some of which are not larger
than a grain of sand, are so fine that four millions of them would
not equal in thickness one of the hairs of his beard. Of such tenuity
it is utterly beyond the power of the imagination to conceive: the
very idea overwhelms our faculties, and humbles us under a sense of
their imperfection.--Of the probable accuracy of this calculation
you may any day in summer convince yourself, by taking one of the
large field spiders (_Epeira Diadema_), and after pressing its
abdomen against a leaf or other substance, so as to attach the
threads to the surface--the same preliminary step which the spider
adopts in spinning--drawing it gradually to a small distance. You
will plainly perceive that the proper thread of the spider is formed
of four smaller threads, and these again of threads so fine and
numerous, that there cannot be fewer than a thousand issue from each
spinner; and if you pursue your researches with the microscope, you
will find that precisely the same takes place in the minutest species
that spins.--You will inquire what can be the end of machinery so
complex? One probable reason is, that it was necessary for drying the
gum sufficiently to form a tenacious line, that an extensive surface
should be exposed to the air; which is admirably effected by dividing
it at its exit from the abdomen into such numerous threads. But the
chief cause, perhaps, is the occasion (hereafter to be adverted to)
which the spider sometimes has to employ its threads in their finer
and unconnected state before they unite to form a single one.--The
spider is gifted by her Creator with the power of closing the
orifices of the spinners at pleasure, and can thus, in dropping from
a height by her line, stop her progress at any point of her descent:
and, according to Lister[736], she is also able to retract her
threads within the abdomen; but this is doubted, and with apparent
reason, by De Geer[737].
The only other instruments employed by the spider in weaving are her
feet, with the claws of which she usually guides, or keeps separated
into two or more, the line from behind; and in many species these
are admirably adapted for the purpose, two of them being furnished
underneath with teeth like those of a comb, by means of which the
threads are kept asunder. But another instrument was wanting. The
spider in ascending the line by which she has dropped herself from an
eminence, winds up the superfluous cord into a ball. In performing
this the pectinated claws would not have been suitable. She is
therefore furnished with a _third_ claw between the other two[738],
and is thus provided for every occasion.
The situations in which spiders place their nets are as various as
their construction. Some prefer the open air, and suspend them in
the midst of shrubs or plants most frequented by flies and other
small insects, fixing them in a horizontal, a vertical, or an oblique
direction. Others select the corners of windows and of rooms, where
prey always abounds; while many establish themselves in stables and
neglected out-houses, and even in cellars and desolate places in
which one would scarcely expect a fly to be caught in a month. It is
with the operations of these last especially, that we are accustomed
to associate the ideas of neglect and desertion by man--associations
which both in painting and allegory have been often happily
applied. Hogarth, when he wished to produce a speaking picture of
neglected charity, clothed the poor's box in one of his pieces with
a spider's web: and the Jews, in one of the fables with which they
have disfigured the records of holy writ, have not less ingeniously
availed themselves of the same idea. They relate that the reason why
Saul did not discover David and his men in the cave of Adullam[739]
was, that God had sent a spider which had quickly woven a web across
the entrance of the cave in which they were concealed; which being
observed by Saul, he thought it useless to investigate further a spot
bearing such evident proofs of the absence of any human being[740].
The most incurious observer must have remarked the great difference
which exists in the construction of spiders' webs. Those which we
most commonly see in houses are of a woven texture similar to fine
gauze, and are appropriately termed webs; while those most frequently
met with in the fields are composed of a series of concentric circles
united by _radii_ diverging from the centre, the threads being remote
from each other. These last, which in their simple state, or still
more when studded with dew drops, you must have a thousand times
admired, are with greater propriety termed _nets_; and the insects
which form them proceeding on geometrical principles may be called
_geometricians_, while the former can aspire only to the humbler
denomination of _weavers_. I shall endeavour to describe the process
followed in the construction of both, beginning with the latter.
The weaving spider which is found in houses, having selected some
corner for the site of her web, and determined its extent, presses
her spinners against one of the walls, and thus glues to it one end
of her thread. She then walks along the wall to the opposite side,
and there in like manner fastens the other end. This thread, which
is to form the outer margin or selvage of her web, and requires
strength, she triples or quadruples by a repetition of the operation
just described; and from it she draws other threads in various
directions, the interstices of which she fills up by running from
one to the other, and connecting them by new threads until the whole
has assumed the gauze-like texture which we see. Books of natural
history, all copying from one another, have described these kinds
of web as fabricated of a regular warp and woof, or of parallel
longitudinal lines crossed at right angles by transverse ones glued
to them at the points of intersection. This, however, is clearly
erroneous, as you will see by the slightest examination of a web of
this kind, in which no such regularity of texture can be discovered.
The webs just described present merely a simple horizontal surface,
but others more frequently seen in outhouses and amongst bushes
possess a very artificial appendage. Besides the main web, the spider
carries up from its edges and surface a number of single threads
often to the height of many feet, joining and crossing each other
in various directions. Across these lines, which may be compared to
the tackling of a ship, flies seem unable to avoid directing their
flight. The certain consequence is, that in striking against these
ropes they become slightly entangled, and, in their endeavours to
disengage themselves, rarely escape being precipitated into the net
spread underneath for their reception, where their doom is inevitable.
But the net is still incomplete. It is necessary that our hunter
should conceal her grim visage from the game for which she lies in
wait. She does not therefore station herself upon the surface of
her net, but in a small silken apartment constructed below it, and
completely hidden from view. "In this corner," to use the quaint
translation of Pliny by Philemon Holland, Doctor in Physic[741],
"with what subtiltie doth she retire making semblance as though she
meant nothing less than that she doth, and as if she went about some
other business! nay, how close lieth she, that it is impossible to
see whether any one be within or no!" But thus removed to a distance
from her net and entirely out of sight of it, how is she to know when
her prey is entrapped? For this difficulty our ingenious weaver has
provided. She has taken care to spin several threads from the edge
of the net to that of her hole, which at once inform her by their
vibrations of the capture of a fly, and serve as a bridge on which in
an instant she can run to secure it.
You will readily conceive that the _geometrical_ spiders, in forming
their concentric circled nets, follow a process very different from
that just described, than which indeed it is in many respects more
curious. As the net is usually fixed in a perpendicular or somewhat
oblique direction, in an opening between the leaves of some shrub or
plant, it is obvious that round its whole extent will be required lines
to which can be attached those ends of the _radii_ that are furthest
from the centre. Accordingly the construction of these exterior lines
is the spider's first operation. She seems careless about the shape
of the area which they inclose, well aware that she can as readily
inscribe a circle in a triangle as in a square, and in this respect
she is guided by the distance or proximity of the points to which she
can attach them. She spares no pains, however, to strengthen and keep
them in a proper degree of tension. With the former view she composes
each line of five or six or even more threads glued together; and
with the latter she fixes to them from different points a numerous
and intricate apparatus of smaller threads. Having thus completed the
foundations of her snare[742], she proceeds to fill up the outline.
Attaching a thread to one of the main lines, she walks along it,
guiding it with one of her hind feet that it may not touch in any part
and be prematurely glued, and crosses over to the opposite side, where
by applying her spinners she firmly fixes it. To the middle of this
diagonal thread, which is to form the centre of her net, she fixes a
second, which in like manner she conveys and fastens to another part of
the lines encircling the area. Her work now proceeds rapidly. During
the preliminary operations she sometimes rests, as though her plan
required meditation. But no sooner are the marginal lines of her net
firmly stretched, and two or three radii spun from its centre, than
she continues her labour so quickly and unremittingly that the eye can
scarcely follow her progress. The radii to the number of about twenty,
giving the net the appearance of a wheel, are speedily finished. She
then proceeds to the centre, quickly turns herself round, and pulls
each thread with her feet to ascertain its strength, breaking any one
that seems defective and replacing it by another. Next, she glues
immediately round the centre five or six small concentric circles,
distant about half a line from each other, and then four or five larger
ones, each separated by a space of half an inch or more. These last
serve as a sort of temporary scaffolding to walk over, and to keep
the radii properly stretched while she glues to them the concentric
circles that are to remain, which she now proceeds to construct.
Placing herself at the circumference, and fastening her thread to the
end of one of the radii, she walks up that one, towards the centre, to
such a distance as to draw the thread from her body of a sufficient
length to reach to the next. Then stepping across and conducting the
thread with one of her hind feet, she glues it with her spinners to the
point in the adjoining radius to which it is to be fixed. This process
she repeats until she has filled up nearly the whole space from the
circumference to the centre with concentric circles distant from each
other about two lines. She always, however, leaves a vacant interval
around the smallest first spun circles that are nearest to the centre,
but for what end I am unable to conjecture. Lastly, she runs to the
centre and bites away the small cotton-like tuft that united all the
radii, which being now held together by the circular threads have
thus probably their elasticity increased; and in the circular opening
resulting from this procedure she takes her station and watches for her
prey.
In the above description, which is from my own observations, I
have supposed the spider to fix the first and main line of her
net to points from one of which she could readily climb to the
other, dragging it after her; and many of these nets are placed in
situations where this is very practicable. They are frequently,
however, stretched in places where it is quite impossible for the
spider thus to convey her main line--between the branches of lofty
trees having no connection with each other; between two distinct and
elevated buildings; and even between plants growing in water. Here
then a difficulty occurs. How does the spider contrive to extend her
main line, which is often many feet in length, across inaccessible
openings of this description?
With the view of deciding this question, to which I could find no very
satisfactory answer in books, I made an experiment, for the idea of
which I am indebted to a similar one recorded by Mr. Knight[743], who
informs us that if a spider be placed upon an upright stick having
its bottom immersed in water, it will, after trying in vain all other
modes of escape, dart out numerous fine threads so light as to float in
the air, some one of which attaching itself to a neighbouring object
furnishes a bridge for its escape. It was clear that if this mode is
pursued by the geometric spiders, it would go considerably towards
furnishing a solution of the difficulty in question. I accordingly
placed the large field spider (_Epeira Diadema_) upon a stick about a
foot long, set upright in a vessel containing water. After fastening
its thread (as all spiders do before they move) at the top of the
stick, it crept down the side until it felt the water with its fore
feet, which seem to serve as antennæ: it then immediately swung itself
from the stick (which was slightly bent) and climbed up by the thread
to the top. This it repeated perhaps a score times, sometimes creeping
down a different part of the stick, but more frequently down the very
side it had so often traversed in vain. Wearied with this sameness in
its operations, I left the room for some hours. On my return I was
surprised to find my prisoner escaped, and not a little pleased to
discover, on further examination, a thread extended from the top of
the stick to a cabinet seven or eight inches distant, which thread had
doubtless served as its bridge. Eager to witness the process by which
the line was constructed, I replaced the spider in its former position.
After frequently creeping down and mounting up again as before, at
length it let itself drop from the top of the stick, not as before
by a single thread, but by _two_, each distant from the other about
the twelfth of an inch, guided as usual by one of its hind feet, and
one apparently smaller than the other. When it had suffered itself to
descend nearly to the surface of the water, it stopped short, and,
by some means which I could not distinctly see, broke off close to
the spinners the smallest thread, which still adhering by the other
end to the top of the stick floated in the air, and was so light as
to be carried about by the slightest breath. On approaching a pencil
to the loose end of this line, it did not adhere from mere contact.
I therefore twisted it once or twice round the pencil, and then drew
it tight. The spider, which had previously climbed to the top of the
stick, immediately pulled at it with one of its feet, and, finding it
sufficiently tense, crept along it, strengthening it as it proceeded by
another thread, and thus reached the pencil[744].
That this therefore is one mode by which the geometric spiders
convey the main line of their nets between distant objects, there
can be no doubt, but that it is the _only_ one is not so clear. If
the position of the main line be thus determined by the accidental
influence of the wind, we might expect to see these nets arranged
with great irregularity, and crossing each other in every direction;
yet it is the fact, that however closely crowded they may be, they
constantly appear to be placed not by accident but design, commonly
running parallel with each other at right angles with the points
of support, and never interfering. Another objection too presents
itself. From the experiment related, it is clear that the main line
of the net can never be longer than the height of the object from
which the spider dropped in forming it. But it is no uncommon thing
to see nets in which these lines are a yard or two long, fastened to
twigs of grass not a foot in height, and yet separated by obstacles
effectually precluding the possibility of the spiders having
_dragged_ the lines from one to the other. Here therefore some other
process must have been used.