_Cercopidæ_[516] are furnished with the claw-suckers; but the noisy
_Cicadæ_, as well as the heteropterous section, at least as far as
my examination of them has gone, have them not. De Geer has observed,
speaking of a small fly of this order (_Thrips Physapus_), that the
extremity of its feet is furnished with a transparent membranaceous
flexible process, like a bladder. He further says that, when the animal
fixes and presses this vesicle on the surface on which it walks, its
diameter is increased, and it sometimes appears concave, the concavity
being in proportion to the pressure; which made him suspect that it
acted like a cupping-glass, and so produced the adhesion[517]. This
circumstance affords another proof that the foot-cushions in the
_Orthoptera_ may act the same part; they appear to be vesicular; and
in numbers of specimens, after death, I have observed that they become
concave, particularly in _Acrida viridissima_.
In _Cimbex_, and others amongst the saw-fly tribes, the claw-sucker
is distinguished by this remarkable peculiarity, that its upper
surface is concave[518], so that before it is used it must be bent
inwards. Besides these, at the extremity of each tarsal joint these
animals are furnished with a spoon-shaped sucker, which seems
analogous to the cushions in the _Gryllina_, _Locustina_, &c.: and,
what is more remarkable, the two spurs (_calcaria_) at the apex of
the shanks have likewise each a minute one[519].--Various other
insects of this order have the claw-suckers. Amongst others the
common wasp (_Vespa vulgaris_) is by these enabled to walk up and
down our glass windows.
We learn from De Geer that several mites, to finish with the
_Aptera_, have something of this kind. Among these is the cheese-mite
(_Acarus Siro_): its four fore feet being terminated by a vesicle
with a long neck, to which it can give every kind of inflexion.
When it sets its foot down, it enlarges and inflates it; and when
it lifts it up, it contracts it so that the vesicle almost entirely
disappears. This vesicle is between two claws[520].--The itch
Acarus (_A. Scabiei_) is similarly circumstanced.--_Ixodes Ricinus_
and _Reduvius_ have also these vesicles--which are armed with two
claws--on all their feet[521].
I am next to consider those climbers that ascend and descend, and
probably maintain themselves in their station, by the assistance
of a _secretion_ which they have the power of producing. You will
immediately perceive that I am speaking of the numerous tribes of
spiders (_Araneidæ_), which, most of them, are endowed with this
faculty. Every body knows that these insects ascend and descend
by means of a thread that issues from them; but perhaps every one
has not remarked--when they wish to avoid a hand held out to catch
them, or any other obstacle--that they can sway this thread from the
perpendicular. When they move up or down, their legs are extended,
sometimes gathering in and sometimes guiding their thread[522]; but
when their motion is suspended, they are bent inwards. These animals,
although they have no suckers or other apparatus--except the hairs
of their legs and the three claws of their biarticulate tarsi, to
enable them to do it--can also walk against gravity, both in a
perpendicular and a prone position. Dr. Hulse, in Ray's _Letters_,
seems to have furnished a clue that will very well explain this. I
give it you in his own homely phrase. "They," spiders, "will often
fasten their threads in several places to the things they creep up;
the manner is by beating their bums or tails against them as they
creep along[523]." Fixing their anus by means of a web, the anterior
part of their body, when they are resting, we can readily conceive,
would be supported by the claws and hairs of their legs; and their
motion may be accomplished by alternately fixing one and then the
other. But you will remember I give you this merely as conjecture,
having never verified it by observation.
It may not be amiss to mention here another apterous insect that
reposes on perpendicular or prone surfaces, without either suckers
or any viscous secretion by which it can adhere to them. I mean the
long-legged or shepherd spiders (_Phalangium_). The tarsi of these
insects are setaceous and nearly as fine as a hair, consisting
sometimes of more than forty joints, those toward the extremity
being very minute, and scarcely discernible, and terminating in a
single claw. These tarsi, which resemble antennæ rather than feet,
are capable of every kind of inflexion, sometimes even of a spiral
one. These circumstances enable them to apply their feet to the
inequalities of the surface on which they repose, so that every joint
may in some measure become a point of support. Their eight legs
also, which diverge from their body like the spokes from the nave of
a wheel, give them equal hold of eight almost equidistant spaces,
which, doubtless, is a great stay to them.
The next species of locomotion exhibited by perfect insects is
_flying_. I am not certain whether under this head I ought to
introduce the sailing of spiders in the air; but as there is no other
under which it can be more properly arranged, I shall treat of it
here. I shall therefore divide flying insects into those that fly
without wings, and those that fly with them.
I dare say you are anxious to be told how any animals can fly
_without wings_, and wish me to begin with them. As an observer of
nature, you have often, without doubt, been astonished by that sight
occasionally noticed in fine days in the autumn, of webs--commonly
called gossamer webs--covering the earth and floating in the air; and
have frequently asked yourself--What are these gossamer webs? Your
question has from old times much excited the attention of learned
naturalists. It was an old and strange notion that these webs were
composed of dew burned by the sun.
"...... The fine nets which oft we woven see
Of scorched dew,"
says Spenser. Another, fellow to it, and equally absurd, was that
adopted by a learned man and good natural philosopher, and one of
the first fellows of the Royal Society, Robert Hooke, the author of
_Micrographia_. "Much resembling a cobweb," says he, "or a confused
lock of these cylinders, is a certain white substance which, after a
fogg, may be observed to fly up and down the air: catching several
of these, and examining them with my microscope, I found them to
be much of the same form, looking most like to a flake of worsted
prepared to be spun; though by what means they should be generated
or produced is not easily imagined: they were of the same weight, or
very little heavier than the air; _and 'tis not unlikely, but that
those great white clouds, that appear all the summer time, may be
of the same substance_[524]." So liable are even the wisest men to
error when, leaving fact and experiment, they follow the guidance of
fancy. Some French naturalists have supposed that these _fils de la
Vierge_, as they are called in France, are composed of the cottony
matter in which the eggs of the Coccus of the vine (_C. Vitis_) are
enveloped[525]. In a country abounding in vineyards this supposition
would not be absurd; but in one like Britain, in which the vine is
confined to the fruit-garden, and the Coccus seldom seen out of the
conservatory, it will not at all account for the phænomenon. What
will you say, if I tell you that these webs (at least many of them)
are air-balloons--and that the aëronauts are not
"Lovers who may bestride the gossamer
That idles in the wanton summer air,
And yet not fall"--
but _spiders_, who long before Montgolfier, nay, ever since the
creation, have been in the habit of sailing through the fields of
ether in these air-light chariots! This seems to have been suspected
long ago by Henry Moore, who says,
"As light and thin as cobwebs that do fly
In the blew air, caus'd by the autumnal sun,
That boils the dew that on the earth doth lie,
May seem this whitish rag then is the scum;
_Unless that wiser men make't the field-spider's loom_[526]."
Where he also alludes to the old opinion of scorched dew. But the
first naturalists who made this discovery appear to have been
Dr. Hulse and Dr. Martin Lister--the former first observing that
spiders shoot their webs into the air; and the latter, besides this,
that they were carried upon them in that element[527]. This last
gentleman, in fine serene weather in September, had noticed these
webs falling from the heavens, and in them discovered more than once
a spider, which he named the _bird_. On another occasion, whilst he
was watching the proceedings of a common spider, the animal suddenly
turning upon its back and elevating its anus, darted forth a long
thread, and vaulting from the place on which it stood, was carried
upwards to a considerable height. Numerous observations afterwards
confirmed this extraordinary fact; and he further discovered, that
while they fly in this manner, they pull in their long thread with
their fore feet, so as to form it into a ball--or, as we may call it,
air-balloon--of flake. The height to which spiders will thus ascend
he affirms is prodigious. One day in the autumn, when the air was
full of webs, he mounted to the top of the highest steeple of York
minster, from whence he could discern the floating webs still very
high above him. Some spiders that fell and were entangled upon the
pinnacles he took. They were of a kind that never enter houses, and
therefore could not be supposed to have taken their flight from the
steeple[528]. It appears from his observations, that this faculty
is not confined to one species of spider, but is common to several,
though only in their young or half-grown state[529]; whence we may
infer, that when full-grown their bodies are too heavy to be thus
conveyed. One spider he noticed that at one time contented itself
with ejaculating a single thread, while at others it darted out
several, like so many shining rays at the tail of a comet. Of these,
in Cambridgeshire in October, he once saw an incredible number
sailing in the air[530]. Speaking of his _Ar. subfuscus minutissimis
oculis_, &c. he says, "Certainly this is an excellent rope-dancer,
and is wonderfully delighted with darting its threads: nor is it
only carried in the air, like the preceding ones; but it effects
itself its ascent and sailing: for, by means of its legs closely
applied to each other, it as it were balances itself, and promotes
and directs its course no otherwise than as if nature had furnished
it with wings or oars[531]." A later but equally gifted observer of
nature, Mr. White, confirms Dr. Lister's account. "Every day in fine
weather in autumn," says he, "do I see these spiders shooting out
their webs, and mounting aloft: they will go off from the finger,
if you will take them into your hand. Last summer one alighted on
my book as I was reading in the parlour; and running to the top of
the page and shooting out a web, took its departure from thence.
But what I most wondered at was, that it went off with considerable
velocity in a place where no air was stirring; and I am sure that
I did not assist it with my breath. So that these little crawlers
seem to have while mounting some locomotive power without the use
of wings, and move faster than the air in the air itself[532]." A
writer in the last number of Thomson's _Annals of Philosophy_[533],
under the signature of Carolan, has given some curious observations
on the mode in which some geometric spiders shoot and direct their
threads, and fly upon them; by which it appears, that as they dart
them out they guide them as if by magic, emitting at the same time
a stream of air, as he supposes, or possibly some subtile electric
fluid. One which was running upon his hand, dropped by its thread
about six inches from the point of his finger, when it immediately
emitted a pretty long line at a right angle with that by which it
was suspended. This thread, though at first horizontal, quickly rose
upwards, carrying the spider along with it. When it had ascended as
far above his finger as it had dropped before below it, it let out
the thread by which it had been attached to it, and continued flying
smoothly upwards till it nearly reached the roof of the room, when it
veered on one side and alighted on the wall. In flying, its motion
was smoother and quicker than when a spider runs along its thread.
He observes, that as the line lengthens behind them, the tendency of
spiders to rise increases.--I have myself more than once observed
these creatures take their flight, and find the following memorandum
with respect to their mode of proceeding. "The spider first extends
its thighs, shanks, and feet, into a right line, and then elevating
its abdomen till it becomes vertical, shoots its thread into the
air, and flies off from its station." It is not often, however, that
an observer can be gratified with this interesting sight, since
these animals are soon alarmed. I have frequently noticed them--for
at the times when these webs are floating in the air they are very
numerous--on the vertical angle of a post, or pale, or one of the
uprights of a gate, with the end of their abdomen pointing upwards,
as if to shoot their thread previously to flying off; when, upon
my approaching to take a nearer view, they have lowered it again,
and persisted in disappointing my wish to see them mount aloft. The
rapidity with which the spider vanishes from the sight upon this
occasion and darts into the air, is a problem of no easy solution.
Can the length of web that they dart forth counterpoise the weight
of their bodies; or have they any organ analogous to the natatory
vesicles of fishes[534], which contributes at their will to render
them buoyant in the air? Or do they rapidly ascend their threads
in their usual way, and gather them up, till having collected them
into a mass of sufficient magnitude, they give themselves to the
air, and are carried here and there in these chariots? I must here
give you Mr. White's very curious account of a shower of these webs
that he witnessed. On the 21st of September 1741, intent upon field
diversions, he rose before day-break; but on going out, he found
the whole face of the country covered with a thick coat of cobweb,
drenched with dew, as if two or three setting-nets had been drawn one
over the other. When his dogs attempted to hunt, their eyes were so
blinded and hoodwinked that they were obliged to lie down and scrape
themselves. This appearance was followed by a most lovely day. About
nine A. M. a shower of these webs (formed not of single floating
threads, but of perfect flakes, some near an inch broad, and five or
six long,) was observed falling from very elevated regions, which
continued without interruption during the whole of the day;--and
they fell with a velocity which showed that they were considerably
heavier than the atmosphere. When the most elevated station in the
country where this was observed was ascended, the webs were still to
be seen descending from above, and twinkling like stars in the sun,
so as to draw the attention of the most incurious. The flakes of the
web on this occasion hung so thick upon the hedges and trees, that
baskets-full might have been collected. No one doubts, he observes,
but that these webs are the production of small spiders, which swarm
in the fields in fine weather in autumn, and have a power of shooting
out webs from their tails, so as to render themselves buoyant and
lighter than the air[535]. In Germany these flights of gossamer
appear so constantly in autumn, that they are there metaphorically
called "_Der fliegender Sommer_" (the flying or departing summer);
and authors speak of the web as often hanging in flakes like wool on
every hedge and bush throughout extensive districts.
Here we may inquire--Why is the ground in these serene days covered
so thickly by these webs, and what becomes of them? What occasions
the spiders to mount into the air, and do the same species form
both the terrestrial and aërial gossamer?--And what causes the
webs at last to fall to the earth? I fear I cannot to all these
queries return a fully satisfactory answer; but I will do the best
I can. At first one would conclude from analogy, that the object
of the gossamer which early in the morning is spread over stubbles
and fallows--and sometimes so thickly as to make them appear as
if covered with a carpet, or rather overflown by a sea, of gauze,
presenting, when studded with dew-drops, as I have often witnessed, a
most enchanting spectacle--is to entrap the flies and other insects
as they rise into the air from their nocturnal station of repose, to
take their diurnal flights. But Dr. Strack's observations render this
very doubtful; for he kept many of the spiders that produce these
webs in a large glass upon turf, where they spun as when at liberty,
and he could never observe them attempt to catch or eat--even when
entangled in their webs--the flies and gnats with which he supplied
them; though they greedily sucked water when sprinkled upon the
turf, and remained lively for two months without other food[536]. As
the single threads shot by other spiders are usually their bridges,
this perhaps may be the object of the webs in question: and thus
the animals may be conveyed from furrow to furrow or straw to straw
less circuitously, and with less labour, than if they had travelled
over the ground. As these creatures seem so thirsty, may we not
conjecture that the drops of dew, with which they are always as it
were strung, are a secondary object with them? So prodigious are
their numbers, that sometimes every stalk of straw in the stubbles,
and every clod and stone in the fallows, swarms with them. Dr. Strack
assures us that twenty or thirty often sit upon a single straw, and
that he collected about 2000 in half an hour, and could have easily
doubled the number had he wished it: he remarks, that the cause of
their escaping the notice of other observers, is their falling to the
ground upon the least alarm.
As to what becomes of this immense carpeting of web there are
different opinions. Mr. White conjectures that these threads, when
first shot, might be entangled in the rising dew, and so drawn up,
spiders and all, by a brisk evaporation, into the region where the
clouds are formed[537]. But this seems almost as inadmissible as
that of Hooke, before related. An ingenious and observant friend,
thinking the numbers of the flying spiders not sufficient to produce
the whole of the phenomenon in question, is of opinion that an
equinoctial gale, sweeping along the fallows and stubbles coated with
the gossamer, must bring many single threads into contact, which,
adhering together, may gradually collect into flakes; and that being
at length detached by the violence of the wind, they are carried
along with it: and as it is known that such winds often convey even
sand and earth to great heights, he deems it highly probable that so
light a substance may be transported to so great an elevation, as not
to fall to the earth for some days after, when the weather has become
serene, or to descend upon ships at sea, as has sometimes happened.
This, which is in part adopted from the German authors, is certainly
a much more reasonable supposition than the other; but some facts
seem to militate against it: for, in the first place, though gossamer
often occurs upon the ground when there is none in the air, yet the
reverse of this has never been observed; for gossamer in the air, as
in the instance recorded by Mr. White, is always preceded by gossamer
on the ground. Now, since the weather is constantly calm and serene
when these showers appear, it cannot be the wind that carries the web
from the ground into the air. Again, it is stated that these showers
take place after _several_ calm days[538]: now, if the web was raised
by the wind into the air, it would begin to fall as soon as the wind
ceased. Whence I am inclined to think that the cause assigned by Dr.
Lister is the real source of the whole phenomenon. Though ordinary
observers have overlooked them, he noticed these spiders in the air
in such prodigious numbers, that he deemed them sufficient to produce
the effect. I shall not, however, decide positively; but, having
stated the different opinions, leave you to your own judgement.
The next query is, What occasions the spiders to mount their chariots
and seek the clouds? Is it in pursuit of their food? Insects, in the
fine warm days in which this phenomenon occurs, probably take higher
flights than usual, and seek the upper regions of the atmosphere; and
that the spiders catch them there, appears by the exuviæ of gnats and
flies, which are often found in the falling webs[539]. Yet one would
suppose that insects would fly high at all times in the summer in
serene warm weather. Perhaps the flight of some particular species
constituting a favourite food of our little charioteers--the gnats,
for instance, which we have seen sometimes rise in clouds into the
air[540]--may at these times take place; or the species of spiders
that are most given to these excursions, may not abound in their
young state--when only they can fly--at other seasons of the year.
Whether the same species that cover the earth with their webs produce
those that fill the air, is to be our next inquiry. Did the appearance
of the one always succeed that of the other, this might be reasonably
concluded:--but the former, as I lately observed to you, often occurs
without being followed by the latter. Yet, since it should seem that
the aërial gossamer, though it does not always follow it, is always
preceded by the terrestrial, this warrants a conjecture that they may
be synonymous. Two German authors, Bechstein[541] and Strack[542],
have described the spider that produces gossamer in Germany under the
name of _Aranea obtextrix_[542]. But it is not clear, unless they have
described it at different ages, when spiders often greatly change their
appearance, that they mean the same species. The former describes his
as of the size of a small pin's head, with its eight eyes disposed in
a circle, having a black-brown body and light-yellow legs: while Dr.
Strack represents his _A. obtextrix_ as more than two lines in length;
eyes four in a square, and two on each side touching each other;
thorax deep brown with paler streaks; abdomen below dull white, above
dark copper brown, with a dentated white spot running longitudinally
down the middle. The first of these, if distinct, as I suspect they
are, agrees very well with the young of one which Lister observed
as remarkable for taking aërial flights[543]; and which I have most
usually seen so engaged. The other may possibly be that before noticed,
which he found in such infinite numbers in Cambridgeshire[544]. If
this conjecture be correct, it will prove that the same species first
produce the gossamer that covers the ground, and then, shooting other
threads, mount upon them into the air.
My last query was, What causes these webs ultimately to fall to the
earth? Mr. White's observation will I think furnish the best answer.
"If the spiders have the power of coiling up their webs in the air,
as Dr. Lister affirms, then when they become heavier than the air
they will fall[545]." The more expanded the web, the lighter and more
buoyant, and the more condensed, the heavier it must be.
I trust you will allow from this mass of evidence, that the English
_Arachnologists_--may I coin this term?--were correct in their account
of this singular phenomenon; and think, with me, that Swammerdam (who
however admits that spiders sail on their webs), and after him De
Geer, were rather hasty when they stigmatized the discovery that these
animals shoot their webs into the air, and so take flight, as a strange
and unfounded opinion[546]. The fact, though so well authenticated,
is indeed strange and wonderful, and affords another proof of the
extraordinary powers, unparalleled in the higher orders of animals,
with which the Creator has gifted the insect world. Were indeed man and
the larger animals, with their present propensities, similarly endowed,
the whole creation would soon go to ruin. But these almost miraculous
powers in the hands of these little beings only tend to keep it in
order and beauty. Adorable is that Wisdom, Power, and Goodness, that
has distinguished these next to nothings by such peculiar endowments
for our preservation as if given to the strong and mighty would work
our destruction.