that, during the Summer season, they constantly dread the approach of
evening, that being the time when these blood-thirsty marauders sally
forth in great numbers, pursue them wherever they go, and exempt no
part of the face, hands, or even the legs from their depredations; the
consequences of which are, violent, though happily only local and
temporary inflammation, attended with insupportable itching, succeeded
by tumors very similar to those occasioned by a scald; when these have
discharged the pellucid fluid they contain, the symptoms subside.
Instances have been known in the vicinity of London, where for several
days the eyes of the sufferers have been closed, the nose and lips
violently swelled, the fingers of both hands so affected as to prevent
their motion, and the legs equally affected. It is remarkable, that in
general those who thus suffer are not conscious of the moment when
they receive the injury, but are soon made sensible of it by the
effect it produces. The approach of the enemy is, however, always
known by the singing or humming noise they make; the peculiar note of
which, though rendered very familiar by daily repetition, is never
esteemed sufficiently musical to render it pleasant or agreeable to
the destined victims. Amongst the variety of remedies which have been
recommended for the cure of this temporary evil, Barbut mentions the
immediate application of volatile alkali, or scratching the part newly
stung, and washing it with cold water; he likewise asserts, that
rubbing the part at night with fuller’s earth and water abates the
inflammation. As preventives are certainly more acceptable than
curatives, I wish I were enabled to recommend such in the present
case: in one instance, the application of vinegar every evening before
sun-set produced a happy effect; possibly washing the parts exposed
with extract of saturn properly diluted might prove effectual.
In the Philosophical Transactions for the year 1767, is an account of
uncommonly numerous swarms of gnats which made their appearance at
Oxford, during the months of July, August, and September of the
preceding year. So many myriads sometimes occupied the same part of
the atmosphere in contiguous bodies, that they resembled a very black
cloud, greatly darkened the air, and almost totally interrupted the
solar rays. The repeated bites of these malignant insects were so
severe, that the legs, arms, heads, and other parts of many persons
were swelled to an enormous size. The colour of the parts was red and
fiery, perfectly similar to that of some of the most alarming
inflammations. Some of these gnats had their bodies greatly distended
by the uncommon quantities of blood which they had imbibed.
In short, there is no species of insects more troublesome to mankind
than the gnat; others give more pain with their stings, but it is only
when they are attacked, or by accident, that we are stung by them; but
the gnats thirst for our blood, and follow us in whole companies to
attack us. In marshy places of this country the limbs of the
inhabitants are kept swelled during the whole season. In warmer
climates, particularly the West Indies, they are, under the
denomination of musquetoes, still more formidable.
Hooke, in his Micrographia, pleads in justification of these terrible
little insects, that they do not wound the skin and suck the blood out
of enmity or revenge, but through mere necessity, and to satisfy their
hunger:--it may be so; and on this account we cannot annex the
criminality to them which appertains to such of the highest rank in
the scale of the animal creation, who, though not urged by the same
powerful motive, pursue a somewhat similar conduct; but those who have
experienced their assaults, will scarcely admit this plea as a
sufficient apology, or feel themselves amicably disposed towards them;
as, from whatever cause their attacks may proceed, the effect is so
very unpleasant, as almost to justify the sufferers in addressing them
in the language of the frogs in the fable to the boys, “Consider, I
beseech ye, that though this may be sport to you, it is death to us,”
and ejaculating a wish, that they might be enabled to gratify their
rapacious appetites by some other means. EDIT.
OF THE PROBOSCIS OF THE TABANUS OR OX-FLY.
Plate XVI. Fig. 1. is a microscopic view of the proboscis of a tabanus,
with which it pierces the skins of horses and oxen, and nourishes itself
with their blood; Fig. 2. the same of the natural size. The singular and
compound structure, together with the wonderful form and exquisite
beauty of this apparatus, discovers such a view of the wisdom, power,
and greatness of its infinite composer, as must strike with admiration
every contemplative observer, and lead him to reflect on the weakness,
impotence, and nothingness of all human mechanism, when compared with
the immense skill and inimitable finishing displayed in the subject
before us. The whole of this formidable apparatus is composed of six
parts, exclusive of the two guards or feelers a a, all of which are
inclosed in a fleshy case, which in the figure is totally removed, as it
contained nothing remarkably different from that of other insects with
two wings. The guards or feelers a a, are of a spungy or fleshy
substance, and are grey, covered with short hairs or villi; they are
united to the head by a little joint of the same texture, which in this
view of the object could not be shewn. These guards are a defence to the
other parts of the apparatus, as they are laid upon it side by side,
whenever the animal stings, and by that means preserve it from external
injury. The two lancets b b and B, evidently open the wound, and are of
a delicate and tender structure, formed like the dissecting knife of the
anatomist, with a sharp point and slender edge, but gradually increasing
to the back. The two instruments, c c and C, appear as if intended to
enlarge the wound, by irritating the parts round it; to accomplish
which, they are jagged or serrated; they may also serve, from their hard
and horny texture, to defend the tube e E, which is of a softer nature
and tubular to admit the blood, and convey it to the stomach; this
delicate part is inclosed in a case d D, which entirely covers it.
These parts are drawn separately at B, C, D, E. De Geer observes, that
it is only the female that sucks the blood of animals; and Reaumur
declares, that having made one disgorge itself, the blood it threw up,
appeared to him to be more than the whole body of the insect could have
contained.
Many other instances of the variety and curious fabrication of this
little organ in different insects, may be found in the works of Reaumur
and De Geer; enough has been said to shew that its mechanism not only
eludes the human eye, but far surpasses every work of man; I shall
therefore proceed, in the next place, to notice
THE ANTENNÆ OF INSECTS.
The antennæ are fine slender horns consisting of several articulations,
moveable in various directions, and constituting one of the
discriminating characteristics of insects. They are beautiful in form,
and of a very delicate structure, so finely articulated, and so minutely
jointed, as to be instantaneously moveable in every direction. They are
situated on the fore part of the head.
The shape, the length, the number, and kind of articulations, not only
vary in different species, but the antennæ of the male generally differ
from those of the female. The greater number of insects have only two
antennæ, but the oniscus, the pagurus, and astacus have four. Regular
rows of minute holes are said to have been discovered in the antennæ.
Several insects cover their eyes with them while they sleep.
We are far from being certain of the use of this organ; some writers
have conjectured that they were the organs of smell and hearing, others
have supposed them appropriated to a delicate species of feeling,
sensible to the least motion or disturbance in the circumambient fluid
in which they move.[53] The following observations throw some light on
this obscure subject. When a wingless insect is placed at the end of a
twig, or in any other situation where it meets with a vacuity, it moves
the antennæ backward and forward, elevates and depresses them from side
to side, and will not advance further lest it should fall. Place a stick
or any other substance near the antennæ, and the insect immediately
applies them to this new object, seems to examine whether it be
sufficient to support its weight, and then proceeds on its journey. From
these observations it would appear that the antennæ assist the insect in
judging of the vicinity of objects, and probably enable them to walk
with safety in the dark.
[53] Some have thought them intended to defend the eyes, but though
this might seem probable in regard to the short plumose ones, it can
never hold good in those that are slender and smooth, which can be of
no such service. Others have thought them made for wiping and cleaning
the eyes, but for this purpose they are totally unfit; the fore legs
of the insect are much better calculated for this use by the hairs or
fibrilla with which they are covered. Possibly they may be the organs
of smelling, since we evidently find that many insects possess this
sense in a very exquisite degree, and yet we see no external organs
except these to serve that purpose. EDIT.
That these observations are not, however conclusive, appears from an
experiment of a very ingenious naturalist: being desirous of
ascertaining the nature and use of the antennæ and proboscis of a
butterfly, he gently approached one that was flying about in search of
food; he observed that it turned the antennæ about every way, till
coming within scent of a flower, it kept them fixedly bent toward that
object, directing its course by their guidance, till it arrived at the
flower; there they appeared to act as an organ of smell, and that the
minute holes with which it is furnished assisted in promoting this
operation. When the creature had reached the flower, it hovered over it
as with rapture, poising itself quietly upon its wing, like a kite or
hawk in the air; it then dropped suddenly, till it was on a level with
the flower, when it began to agitate its wings briskly and to unroll its
spiral trunk, thrusting it to the bottom of the flower; in a little time
the trunk was rolled up, and again in a moment unrolled; these
operations it repeated till the flower yielded no more juices, the
butterfly then sought for and alighted on another.[54]
[54] After all, this subject must for the present remain undecided.
Indeed, the bodies of insects are throughout formed of parts so
different from ours, that we can probably conceive no more idea of the
use of some of their organs, than a man born blind or deaf can of the
senses of vision or hearing. They may have senses different from ours,
and these may be the organs of them. EDIT.
The differences in the form, &c. of the antennæ are characterized by
naturalists under the following names:
Setaceæ; are those that, like a bristle, grow gradually taper towards
the point or extremity, as in many of the phalenæ. Filiformes;
thread-shaped, and of an uniform thickness. Moniliformes; these are
filiform like the preceding, and of a regular thickness, but consist of
a series of round knobs, like a necklace of beads, as in the chrysomela.
Clavatæ; formed like a club, increasing gradually from the base to the
extremity, as in the papilio, butterfly. Capitatæ; these are also formed
like a club, but the last articulation is larger than the rest,
finishing with a kind of capital or head. Fissiles; these are like the
former, only that the capitulum or head is divided longitudinally into
three or four parts or laminæ, as in the scarabæi. Perfoliatæ; are also
capitated, but have the capitulum divided horizontally, and the laminæ
connected by a kind of thread passing through their center, as in the
dermestes and dytiscus. Pectinatæ; so called from their similitude to a
comb, though they more properly resemble a feather, as in the phalenæ
and elateres; this is most obvious in the male. Aristatæ; such as have
a lateral hair, which is either naked, or furnished with smaller hairs,
as in the fly.
Besides the foregoing terms, the antennæ are called breviores, or short,
when they are shorter than the body; mediocres, or middling, when they
are of the same length; and longiores, when they are longer.
Near the mouth there is also a species of small filiform articulated
antennæ, called the palpi, or feelers; they are generally four in
number, sometimes six; they are placed under and at the sides of the
mouth, which situation, together with their size, sufficiently
distinguish them from the antennæ; they are in continual motion, the
animal thrusting them in every matter, as a hog would its nose, when in
search of food. Some have supposed them to be a kind of hand to assist
in holding the food when it is near the mouth.
OF THE EYES OF INSECTS.
The structure of the eye has always been considered as a wonderful piece
of mechanism; the admirable manner in which those of the human species
are formed, and the nature of vision, are speculations which cannot but
excite the attention of every inquisitive mind. The eyes of insects,
though they differ considerably in their construction from those of
other animals, are no less objects of our admiration. Indeed, among the
exterior parts of insects, none are more worthy of minute investigation,
and very few persons are to be found, who can be insensible to the
beauties of this organ when exhibited under the microscope, as that
instrument alone points out to us the prodigious art employed in their
organization, and evidently shews how many wonders escape the unassisted
sight.
The construction of the eye in insects is not only distinct from that of
other animals, but also differs in different species. They vary in
number, situation, connection, and figure. In other creatures the eyes
are moveable, and two in number, one on each side of the head: in
insects, the genus of cancri excepted, the eyes are fixed; they have no
eye-brows, but the outer coating is hard and transparent.
The greater part of insects have two eyes; in the monoculus they
approach so near to each other, as to appear like one; the gyrinus has
four eyes, the scorpion six, the spider eight, and the scolopendra
three.
Of the eyes of insects, some have them single, that is, placed at a
small distance from each other; while others are furnished with an
indefinite number, all placed in one common case or socket; the latter
are generally termed the reticulated eyes.
OF THE RETICULATED EYES OF INSECTS.
The microscope does not disclose greater wonders, when it exhibits to us
millions of animals invisible to the naked eye, where we should suppose
nothing living existed, than when it discovers to us hidden beauties in
those, which, though they are large enough to be seen by our natural
eye, yet in their several minute parts are no ways discernible, but by
the assistance of glasses.
Thus we readily discern those protuberances on the heads of insects,
which are formed by a congeries of eyes; we can even perceive that they
consist of a number of lines crossing each other with great regularity
and exactness at some little distance, like the meshes of a net. By this
we know that they are reticulated substances; but in what manner they
are so, can only be shewn by the microscope.
The eyes of the libellula, on account of their size, are peculiarly well
adapted for microscopical examination; and, by the assistance of the
instrument, you will find that they are divided into a number of
hexagonal cells, each of which forms a complete eye. The external parts
of these eyes are so perfectly smooth, and so well polished, that, when
viewed as opake objects, they will, like so many mirrors, reflect the
images of all the surrounding objects. The figure of a candle may be
seen on their surface multiplied almost to infinity, shifting its beam
to each eye, according to the motion given to it by the hands of the
observer. Other creatures are obliged to turn their eyes towards the
object, but insects have eyes directed thereto, on whatsoever side it
may appear: they more than realize the wonderful accounts of fabulous
history: poets gave to Argus an hundred eyes; insects are furnished with
thousands, having the benefit of vision on every side with the utmost
ease and speed, though without any motion of the eye or flexion of the
neck.
Each of these protuberances, in its natural state, is a body cut into a
number of faces; like an artificial multiplying glass; but with this
superiority in the workmanship, that as there, every face is plane,
here, every one is convex, immensely more numerous, and contained in a
much smaller space. If one of these protuberant substances be nicely
taken from the head of the insect, washed clean, and placed before the
microscope, its structure is elegantly seen, and it becomes an object
worthy of the highest admiration. You will find that each of the eyes is
an hexagon, varying in its size according to its situation in the head,
and that each of them is a distinct convex lens, and has the same effect
in forming the image of an object placed before it. Of this you will be
convinced, by turning the mirror of the microscope so as to bring the
picture of some well-defined object under the eye; thus, turn it towards
a house, and in the eye of the insect you will perceive the house
diminished to a box, but multiplied into a city; turn it towards a
soldier, and you will have an army of pigmies performing every motion at
the same instant of time; again, turn the mirror towards a candle, and
you will have a beautiful and resplendent blaze from multitudes of
regular flames.
Hooke, Catalan, &c. have shewn that these small eyes are furnished with
every requisite of vision, and that each of them has the use, the power,
and properties of an eye. But we must have recourse to the works of
Swammerdam for a full account of the astonishing organization of the
eyes of insects. Among other things, he has shewn, that under each facet
there is a pyramid of fibres broad at the base, and growing smaller as
it proceeds inwards; the pyramid has the same number of sides as the
eye, and there are as many hexagonal pyramids, as there are small facets
or eyes in the insect. An innumerable number of pulmonary tubes ascend
these fibres, terminating in a white fibrous convex membrane; under
these membranes there is another, still more delicate and transparent;
beneath this, a second species of fibres is transversely applied, like
so many beams to support the pyramids that are laid upon them. Still we
cannot determine with certainty, how these numerous inlets to sight
operate for the service of the animal; they may increase the field of
view, augment the intensity of light, and be productive of advantages of
which we can have no conception.
Hooke computed 14000 of these facets in the two eyes of a drone;
Leeuwenhoek reckoned 6036 in the two eyes of a silkworm, when in its fly
state; in the eyes of the libellula he reckoned 12544 hexangular
lenses.
Swammerdam covered the reticulated eyes of certain insects with black
paint; in this state they flew at random, and seemed to be deprived of
their strength; when they settled, they did not avoid the hand that was
going to take hold of them. Reaumur made similar experiments on the eyes
of bees, which concurred with those of Swammerdam.
Some ephemera flies have four reticulated eyes, two of which are placed
as in the common fly; the other two are placed, one beside the other,
upon the upper part of the head, and have the appearance of a kind of
mushroom, the head extended somewhat beyond the stalk. The first pair
are of a brown colour, those of the mushroom form are of a very
beautiful citron colour.
In some of the fly class, these reticulated eyes are little inferior in
colour and brilliance to the brightest gem. The colour varies in
different species; in some you find it green, in others red, &c. some
have a most elegant changeable colour thrown over them, partly purple,
partly green, and partly of that brassy hue, which is seen on the backs
of some of our beetles, and which is not equalled by any other
production of art or nature.
Fig. 3. Plate XVI. is a representation of a small part of the cornea of
a libellula, as seen by the microscope; the sides of the hexagons in
some positions of the light, appear of a fine gold colour, and divided
into three parallel borders. Fig. 4. the same object of its natural
size.
Fig. 5. Plate XVI. represents a small portion of the cornea of a
lobster; here each of the eyes are small squares, not hexagons; a
conformation which admits a smaller number in the same surface; so great
a number was not necessary in this instance, as the eyes of the lobster
are moveable. Fig. 6. the same of its natural size.
OF THE EYES OF THE MONOCULUS POLYPHEMUS.
The monoculus polyphemus, or king crab, has four eyes, two large and two
small ones; the large eyes are formed of a great number of transparent
amber-like cones, the small ones of a single cone,
“The internal surface of the large eyes, examined with the microscope,
is found to be thick set with a great number of small transparent cones,
of an amber colour, the bases of which stand downward, and their points
upward next the eye of the observer. The cones in general have an
oblique direction, except some in the middle of the cornea, about thirty
in number, the direction of which is perpendicular. The center of every
cone being the most transparent part, and that through which the light
passes, on that account the perpendicular or central cones always appear
beautifully illuminated at their points. In a word, they are all so
disposed, as that a certain number of them receive the light from
whatever point it may issue, and transmit it to the immediate organ of
sight, which we may reasonably suppose is placed underneath them. The
cones are not all of the same length; those on the edges of the cornea
are the longest, from whence they gradually diminish as they approach
the center, where they are not above half the length of those on the
edges.
“The structure of the small eyes being less elaborate, their internal
appearance, when placed in the microscope, will be described in a few
words. They consist of an oval transparent horny plate, of an amber
colour, in the center of which stands a single cone, through which and
the oval plate the light passes.”[55]
[55] See Mr. André’s paper with a plate, in the Phil. Trans. for 1782,
page 440.
OF THE EYES OF A SPIDER.
Though the form of this insect is naturally disgusting, yet the eyes
make a beautiful object for the microscope. They have generally eight;
two on the top of the head, that look directly upwards; two in the
front, a little below the foregoing, to discover what passes before it;
on each side a couple more, whereof one points sideways forward, the
other sideways backward; so that the spider can nearly see all around.
These eyes are immoveable, and seem to be formed of a hard transparent
horny substance. A portion of each sphere projects externally beyond the
socket, the largest part is sunk within it. There is round each eye a
circular transparent membrane. Mr. Baker placed the eye of a spider over
a pin-hole made through a piece of card, and then applied it as a lens
to examine objects; he found it magnified the objects greatly, but that
it did not exhibit them distinctly; this he however attributed to the
length of time the spider had been dead whose eye he used. The number of
eyes is not the same in all species of the spider.
OF THE STEMMATA.
It might be imagined, that as every fly has two reticulated eyes, they
could not have occasion for more; but so it has not appeared to that
GREAT BEING who formed them, for many are furnished besides with other
eyes, differing in form and construction from those that are
reticulated.
These were first noticed by M. de la Hire; they are three lucid
protuberances placed on the back part of the head of many insects: their
surface is glossy, of an hemispheric figure, and a coal black colour.
They are transparent, and disposed in a triangular form; by modern
naturalists they are termed stemmata.
Reaumur made experiments on these eyes, similar to those he had made on
the reticulated ones, and found that when the stemmata were covered with
dark varnish, the insects flew but to a small distance, and always at
random.
No insect is, I believe, found with both kind of eyes, unless in its
perfect state: there are many species which are not furnished with
stemmata, gnats and tipulæ are without them.
We are apt to suppose that nature has lavished all her bounty upon her
larger creatures, and left her minims of existence, as Shakspeare
phrases it, unfinished; with what different ideas must those be
impressed, who find the apparatus for vision in these small creatures so
various and so wonderful in their structure, and who must perceive so
much design and order manifested in the position, construction, and
number of these delicate and useful organs.
OF THE BODY OF INSECTS.
The trunk or body of the insect is situated between the head and
abdomen. Naturalists divide it into three parts; the thorax, scutellum,
and sternum.