Abounding everywhere, the Nemocera live, some on the blood of man and
animals, some on small insects, and others on the juices of fragrant
flowers. From [Greek: nêma], thread; [Greek: keras], horn.
In all climates, in every latitude, in the fields and woods, even in
our dwellings, they may be seen fluttering and plundering. The
Nemocera are divided into two families, that of the _Culicidæ_, of
which the gnat (_Culex_), which has a long, thin trunk, and a sucker
provided with six bristles, is a member; and that of the _Tipulidæ_,
which have a short thick trunk, and a sucker having two bristles.
[Illustration: Figs. 19 and 20.--The Gnat (_Culex pipiens_).]
We will begin our examination with the Gnat (_Culex pipiens_), of
which Réaumur, in his "Mémoires pour servir à l'Histoire des
Insectes," has given such a curious and complete history. "The gnat is
our declared enemy," says Réaumur, in the introduction to his memoir,
"and a very troublesome enemy it is. However, it is well to make its
acquaintance, for if we pay a little attention we shall be forced to
admire it, and even to admire the instrument with which it wounds us.
Besides which, throughout the whole course of its life it offers most
interesting matter of investigation to those who are curious to know
the wonders of Nature. During a period in its life the observer,
forgetting that it will at some time annoy him, feels the greatest
interest in its life-history."
As this is the case, let us explain the history of these insects,
which excite so much interest. The illustrious naturalist we have just
mentioned will be our guide.
The body of the gnat is long and cylindrical. When in a state of
repose one of its wings is crossed over the other. They present a
charming appearance when seen through a microscope, their nervures, as
well as their edges, being completely covered with scales, shaped like
oblong plates and finely striated longitudinally. These scales are
also found on all the segments of the body.
[Illustration: Fig. 21.--Antennæ of Gnat, magnified.]
[Illustration: Fig. 22.--Head of Gnat, magnified.]
The antennæ of the gnat, particularly those of the male, have a fine
feathery appearance (Fig. 21).
Their eyes, covered with network, are so large that they cover nearly
the whole of the head. Some have eyes of a brilliant green colour, but
looked at in certain lights they appear red. Fig. 22 shows the head of
the gnat with its two eyes, its antennæ, and trunk.
The instrument which the gnat employs for puncturing the skin, and
which is called the trunk (Fig. 23), is well worthy of our attention.
That which is generally seen is only the case of those instruments
which are intended to pierce our skin and suck our blood, and in which
they are held, as lancets and other instruments are held in a
surgeon's case. The case (Fig. 24) is cylindrical, covered with
scales, and terminates in a small knob. Split from end to end that it
may open, it contains a perfect bundle of stings. Réaumur tried to
observe, by allowing himself to be stung by gnats, what took place
during the attack. He forgot, in watching the operations of the
insect, the slight pain caused by the wound, soliciting it as a
favour, his only regret being not to obtain it when he wished.
Réaumur observed that the compound sting, which is about a line in
length, enters the skin to the depth of about three-quarters of a
line, and that during that time the case bends into a bow, until the
two ends meet. He noticed besides, that the trunk-case of certain
gnats was even more complicated than that which we have described. But
we will not dwell any longer on this point.
[Illustration: Figs. 23 and 24.--Trunk of Gnat, magnified.]
[Illustration: Figs. 25, 26, 27.--Lancets of the Gnat.]
Let us now try to give an idea of the construction and composition of
this sting, which, after piercing the skin, draws our blood.
According to Réaumur, the sting of the gnat is composed of five parts.
He acknowledges, however, that it is very difficult to be certain of
the exact number of these parts, on account of the way in which they
are united, and of their form. At the present day we know that there
are six. Réaumur, as also Leuwenhoek, thought he saw two in the form
of a sword blade with three edges. These have the points reversed, and
are serrated on the convex side of the bend (Fig. 25). To form an idea
of the shape of the other points, the reader should look at Figs. 26
and 27. He will then see that the gnat's sting is a sword in
miniature.
The prick made by so fine a point as that of the sting of the gnat
ought not to cause any pain. "The point of the finest needle," says
Réaumur, "compared to the sting of the gnat, is the same as the point
of a sword compared to that of the needle." How is it then that so
small a wound does not heal at once? How is it that small bumps arise
on the part that is stung? The fact is, that it is not only a wound,
but it has been imbued with an irritating liquid.
This liquid may be seen to exude, under different circumstances, from
the trunk of the gnat, like a drop of very clear water.
Réaumur sometimes saw this liquid even in the trunk itself. "There is
nothing better," he observes, "to prevent the bad effects of gnat
bites than at once to dilute the liquid they have left in the wound
with water. However small this wound may be, it will not be difficult
for water to be introduced. By rubbing, it will be at once enlarged,
and there is nothing to do but to wash it. I have sometimes found this
remedy answer very well."
The gnat is not always in the form of a winged insect, greedy for our
blood. There is a period during which they leave us in repose. This is
the larva period. It is in water, and in stagnant water in particular,
that the larva of the insect which occupies our attention is to be
found. It resembles a worm, and may be found in ponds from the month
of May until the commencement of winter.
[Illustration: Fig. 28.--Larva of the Gnat.]
If we desire to follow the larva of the gnat from the beginning, we
have only to keep a bucket of water in the open air. After a few days
this water will be observed to be full of the larvæ of the gnat (Fig.
28). They are very small, and come to the surface of the water to
breathe; for which purpose they extend the opening of a pipe, A, which
is attached to the last segment of the body, a little above the
surface. They are, consequently, obliged to hold their heads down. By
the side of the breathing-tube is another tube, B, shorter and thicker
than the former, nearly perpendicular to the body, its orifice being
the exterior termination of the digestive tube. At the anus it is
fringed with long hairs, having the appearance, when in the water, of
a funnel. At the end of the same tube, and inside the hair funnel, are
four thin, oval, transparent, scaly blades, having the appearance of
fins. They are placed in pairs, of which one emanates from the right
side, the other from the left.
These four blades or fins have the power of separating from each
other. Each segment of the abdomen has on both sides a tuft of hair,
and the thorax has three. The head is round and flat, and is provided
with two simple brown eyes. Round the mouth are several wattles,
furnished with hair, of which two of crescent-like form are the most
conspicuous. These tufts move with great quickness, causing small
currents of liquid to flow into the mouth, by means of which the
necessary food, microscopic insects and particles of vegetable and
earthy matter, is brought to the larva.
They change their skin several times during their continuance in this
state. This latter fact has been remarked by Dom Allou, a learned
Carthusian, "whose pleasure," says Réaumur, "consisted in admiring the
works of the Almighty, when not occupied in singing his praises." We
think it will be interesting to repeat the few lines which accompany
the mention made by Réaumur of this worthy Carthusian. They appear to
us to be well worth reading, even at the present day.
"If the pious monks who composed so many societies, possessed, like
Dom Allou, the love of observing insects, we might hope that the most
essential facts in the history of those little creatures would soon be
made known to us. What enjoyment more worthy of the calling they have
chosen could these pious men pursue than that which would place before
their eyes the marvellous creations of an Almighty Power? Even their
leisure would then incline them to adore that Power, and would furnish
them the means to make others do so who are occupied by too serious or
too frivolous employments."
After having changed its skin three times in a fortnight or three
weeks, the larva of the gnat throws off its covering for a fourth
time, and is no longer in the larva state. It is changed both in shape
and condition. Instead of being oblong, its body is shortened,
rounded, and bent in such a way that the tail is applied to the under
part of the head. This is the case when the animal is in repose; but
it is able to move and swim, and then, by bending its body and
straightening it again, propels itself through the water.
[Illustration: Fig. 29.--Pupa of the Gnat.]
In this new condition, that is to say, in the pupa state (Fig. 29), it
does not eat. It no longer possesses digestive organs, but it is
necessary, even more than before its metamorphosis, that it should
breathe atmospheric air. Besides, the organs of respiration are
greatly changed. During the time the insect was in the larva state, it
was through the long tube fixed to the posterior part that it received
or expelled the air; but in casting its skin it loses the tube, two
appendages resembling an ass's ears being for the pupa what the tube
was for the larva, the opening of these ears being held above the
surface of the water. From this pupa the perfect insect will emerge;
it is developed little by little, and the principal members may be
distinguished under the transparent membranous skin which envelopes
it.
When the insect is about to change from the pupa state, it lies on the
surface of the water, straightening the hind part of its body, and
extending itself on the surface of the water, above which the thorax
is raised. Before it has been a moment in this position, its skin
splits between the two breathing trumpets, the split increasing very
rapidly in length and breadth.
"It leaves uncovered," says Réaumur, "a portion of the thorax of the
gnat, easily to be recognised by the freshness of its colour, which is
green, and different from the skin in which it was before enveloped.
"As soon is the split is enlarged--and to do so sufficiently is the
work of a moment--the fore part of the perfect insect is not long in
showing itself; and soon afterwards the head appears, rising above the
edges of the opening. But this moment, and those which follow, until
the gnat has entirely left its covering, are most critical, and when
it is exposed to fearful danger. This insect, which lately lived in
the water, is suddenly in a position in which it has nothing to fear
so much as water. If it were upset on the water, and the water were to
touch its thorax or body, it would be fatal. This is the way in which
it acts in this critical position--As soon as it has got out its head
and thorax, it lifts them as high as it is able above the opening
through which they had emerged, and then draws the posterior part of
its body through the same opening; or rather that part pushes itself
forward by contracting a little and then lengthening again, the
roughness of the covering from which it desires to extricate itself
serving as an assistance.
"A larger portion of the gnat is thus uncovered, and at the same time
the head is advanced farther towards the anterior end of the covering;
but as it advances in this direction, it rises more and more, the
anterior and posterior ends of the sheath thus becoming quite empty.
The sheath then becomes a sort of boat, into which the water does not
enter; and it would be fatal if it did. The water could not find a
passage to the farther end, and the edges of the anterior end could
not be submerged until the other was considerably sunk. The gnat
itself is the mast of its little boat. Large boats, which pass under
bridges, have masts which can be lowered; as soon as the boat has
passed the bridge the mast is hoisted up by degrees, until it is
perpendicular. The gnat rises thus until it becomes the mast of its
own little boat, and a vertical mast also. It is difficult to imagine
how it is able to put itself in such a singular, though for it
necessary, position, and also how it can keep it. The fore part of the
boat is much more loaded than the other, but it is also much broader.
Any one who observes how deep the fore part of the boat is, and how
near the edges of its sides are to the water, forgets for the time
being that the gnat is an insect that he would willingly destroy at
other times. One feels uneasy for its fate; and the more so if the
wind happens to rise, particularly if it disturbs the surface of the
water. But one sees with pleasure that there is air enough to carry
the gnat along quickly; it is carried from side to side; it makes
different voyages in the bucket in which it is borne. Though it
is only a sort of boat--or rather mast, because its wings and legs
are fixed close to its body, it is perhaps, in proportion to the
size of its boat, a larger sail than one would dare to put on a
real vessel--one cannot help fearing that the little boat will
capsize. * * * As soon as the boat is capsized, as soon as the gnat
is laid on the surface of the water, there is no chance left for it.
I have sometimes seen the water covered with gnats which had perished
thus as soon as they were born. It is, however, still more extraordinary
that the gnat is able to finish its operations. Happily they do not last
long; all dangers may be passed over in a minute.
[Illustration: Fig. 30.--Gnats emerging.]
"The gnat, after raising itself perpendicularly, draws its two front
legs from the sheath, and brings them forward. It then draws out the
two next. It now no longer tries to maintain its uneasy position, but
leans towards the water; gets near it, and places its feet upon it;
the water is sufficiently firm and solid support for them, and is able
to bear them, although burdened with the insect's body. As soon as the
insect is thus on the water it is in safety; its wings are unfolded
and dried, which is done sooner than it takes to tell it, at length
the gnat is in a position to use them, and it is soon seen to fly
away, particularly if one tries to catch it."
[Illustration: Fig. 31.--Eggs of the Gnat, magnified.]
One more word about the gnat, whose life is full of such interesting
details.
The reader will perhaps not feel much pleasure in learning that the
fecundity of these insects is extraordinary. Many generations are born
in a single year, each generation requiring only three weeks or a
month to arrive at a condition to bring forth a new generation. Thus,
the number of gnats which comes into existence in the course of a
year is something fearful. Only a few days after the pupæ in a bucket
are transformed into gnats, eggs which have been left by the females
may be observed on the surface of the water in little clusters.
Many species of gnats, known as mosquitoes, are to be found in
America. All travellers speak of the sufferings endured by a stranger
in that country from the bites of these insects. One can only preserve
oneself from these cruel enemies during sleep by hanging gauze, called
a mosquito curtain, round the bed. Mosquito curtains are not only
necessary in America; during the hot season, in Spain, throughout the
whole of Italy, and a part of the south of France, it is necessary to
hang these curtains round the bed, if one wishes to obtain any sleep;
it is also a necessary precaution not to have a light in one's
bedchamber, as the sight of it at once attracts these dangerous
companions, whose buzzing and stinging prevent any possibility of
repose during the whole night. Such is our advice to people who travel
in the above-mentioned countries.
The _Tipulidæ_ have a narrow, elongated abdomen, and long and slight
limbs. The head is round, and the eyes, which are compound, are,
especially in the males, very large. The wings, which are long and
narrow, are sometimes held wide apart, sometimes horizontally, and
sometimes bent so as to form, as it were, a roof. The balancers are
naked and elongated; the abdomen long, cylindrical, and often
terminating in a club in the male, and in a point in the female. The
antennæ, which are longer than the head, are generally composed of
from fourteen to sixteen joints, and are sometimes in the form of a
comb or saw, sometimes furnished with hair, in form of plumes,
bunches, or in a whorl. The larvæ live on plants, in the fields, in
gardens, and sometimes in woods. The perfect insects, at first sight,
resemble gnats, but are without a trunk, or rather their trunk is
extremely short, terminating in two large lips, and the sucker is
composed of two fibres only.[12] The larger species of _Tipulæ_, which
are commonly known as "Daddy Longlegs," &c., and in France as
"_Tailleurs_" and "_Couturières_," are found in fields at the end of
September and commencement of October.
[12] The genus _Cecidomyia_, which belongs to this family,
presents the most extraordinary instance of agamo-genesis--or
reproduction without fertilisation by another individual--at
present known among insects. Until lately it was almost an
axiom with naturalists that no insect was capable of
reproduction until it had attained its adult or perfect state.
Several Continental observers, some of them without any
knowledge of the others' discoveries, have found that the
_larvæ_ of some of the species of this genus reproduce larvæ
resembling themselves in every respect; and what is still more
strange, these larvæ live in a free state within the parent
larvæ, feeding upon its tissues, and causing its ultimate
destruction.
A very interesting article on this subject will be found in the
_Popular Science Review_ for the 1st April, 1868. The larvæ of
a species (_Cecidomyia tritici_) frequently causes much injury
to the wheat.--ED.
"Although they sometimes fly a considerable distance," says Réaumur,
"when the sun is bright and hot, they generally do not go far; often,
indeed, only along the ground, or rather the top of the grass.
Sometimes they only use their wings to keep them above the level of
the herbage, and to take them along. Their legs, particularly the hind
ones, are disproportionately large. They are three times the length of
the body, and are to these insects what stilts are to the peasants of
marshy and inundated countries, enabling them to pass with ease over
the higher blades of grass."
One of the smaller species has been termed _culiciformis_, on account
of its resemblance to the gnat. The smaller are more active than the
larger species which we have mentioned. Not only do they fly more
rapidly, but there are some kinds which are continually on the wing.
In all seasons, even during the winter, at certain hours of the day,
clouds of small insects are seen in the air, which are taken for
gnats: they are _Tipulæ_. Their flight is worthy of attention; they
generally only rise and fall in the same vertical line. All these
flies come from larvæ, which resemble very elongated worms, having
scaly heads, generally furnished with two very small conical antennæ,
and certain other organs, for the purpose of obtaining food. Their
bodies are jointed without limbs, but nevertheless provided with
appendages which supply their place. The larvæ of the various species
are of very different habits. Some are aquatic, as that of _Tipula
culiciformis_, a small species which is very numerous in stagnant
waters.
It is necessary to say a few words about these worm-like larvæ, which
are extremely common. They are of a brilliant red colour, and inhabit
little oblong bent masses of earth, thickly pierced with holes. Each
hole allows a worm to extend its head, and the foremost part of its
body, out of the cell, which is made of light spongy matters, remains
of decayed leaves, &c. These larvæ are transformed into pupæ, in
the cell in which they have lived, during the larvæ state, losing by
this metamorphosis the scaly coverings of the head and of all the
exterior parts. They pass into the pupa state, and have the thorax
provided with dainty plumes, which probably assist in the action of
respiration. This pupa is very active and quick in its movements in
the water. When the moment comes for its last metamorphosis, it throws
off its feathery covering in much the same manner as the gnat.
Fig. 32 represents _Tipula oleracea_ in the different stages of larva,
pupa, and perfect insect.
[Illustration: Fig. 32.--Daddy Longlegs (_Tipula oleracea_).]
Other species of small _Tipulæ_ have aquatic larvæ very similar to
those which we have described. Réaumur remarked that each of these
worms is lodged in a thick mass, convex at the top, formed of a
transparent and adhesive white jelly. The larvæ of the larger _Tipulæ_
are not aquatic, but are of different habits, and live under the
ground; all soil which is not frequently turned is suitable to them,
but they are to be found especially in low damp meadows.
Réaumur saw large districts of grassy swamps in Poictou, which, in
certain years, furnished very little grass for the cattle, on account
of the ravages caused by these larvæ. They had also much injured the
harvest in the same districts during those years.
These larvæ appear to require no other food than vegetable mould.
Their excrements are, in fact, according to Réaumur, nothing else
than dried earth, from which the stomach and intestines of the insect
have withdrawn all nourishing matter.
Old trees have often hollow cavities occasioned by the decay of the
trunk. When these cavities are old, their lower parts are full of a
sort of mould, which is in fact half-decayed wood. It is there that
the _Tipulæ_ often lay their eggs. Réaumur frequently found the larvæ
in the trunks of elms or willows, and also in the fleshy parts of
certain kinds of mushrooms. He carefully observed the habits of one,
which lived under the covering of a mushroom, the Oak agaric
(_Agaricus quercinus_). This larva is round, grey, and resembles an
earth-worm. It does not walk, but crawls; and the places where it
stops, or which it passes over, are covered with a sort of brilliant
slime, like that left by the snail or slug.
M. Guérin-Méneville has published some very interesting remarks on the
migrations of the larvæ of a particular kind of _Tipula_, known by the
name of _Sciara_. We will borrow from that entomologist the following
curious details, which will initiate us into one of the most wonderful
phenomena in the whole history of insects. These small larvæ are
without feet, hardly five lines in length, and about the third of a
line in diameter. They are composed of thirteen segments, and have
small black heads.
In some years, during the month of July, may be found on the borders
of forests in Norway and Hanover, immense trains of these larvæ,
formed by the union of an innumerable quantity fixed to each other by
a sticky substance. These collections of larvæ resemble some sort of
strange animal of serpent-like form, several feet long, one or two
inches in thickness, and formed by the union of an immense number,
which cling to each other by thousands, and move on together. The
whole society advances thus with one accord, leaving a track after it
on the ground, as a material indication of its presence.
These strange collections of living creatures form societies,
sometimes only a few yards long; but at other times it happens that
they form bands from ten to twelve yards in length, of the breadth of
a hand and the thickness of a thumb. M. Guérin-Méneville observed
columns as many as thirty yards in length. These troops advance as
slowly as a snail, and in a certain direction. If they encounter an
obstacle--as a stone, for instance--they cross over it, turn round it,
or else divide into two sections, which reunite after the obstacle is
passed. If a portion of the column be removed so as to divide it into
two parts, it is quickly reunited, as the hindmost portion soon joins
that which precedes it. Lastly, if the posterior part of this living
ribbon be brought into contact with the anterior, a circle is formed,
which turns round and round on the same ground for a long time,
sometimes even for a whole day, before breaking, and continuing to
advance. They are never met with in bad weather, but only when the sun
is warm.
The curious and astonishing phenomenon of an assembly of larvæ without
feet, advancing with an equal movement resulting from the individual
motion of thousands of little worms, was remarked for the first time,
in 1603, by Gaspard Schwenefelt. This naturalist says that the
inhabitants of Siberia consider this phenomenon as an indication of a
bad harvest if they go towards the mountains; whereas, if they descend
towards the plains, it is the sign of a good one. In 1715 Jonas Ramus
mentioned the same phenomenon, recalling a superstition attached to it
by the peasants of Norway. This writer informs us that the peasants of
that country, on meeting one of these moving columns, throw down their
belts or waistcoats on the ground before it. If the _orme-drag_ (that
is the name given to the moving column) crosses over this obstacle, it
is a good sign; but on the other hand, if the column turns round the
obstacle, instead of crossing it, some mischief may be expected.
The same animals were observed in 1845 at Birkenmore, near Hefeld, by
M. Rande, Royal Inspector of the Forests of Hanover.
M. Guérin-Méneville is of opinion that these larvæ, which exist in
great numbers in certain districts, sometimes devour all the nutritive
substances contained in the ground. After having done so, they are
obliged to come out of it, in order to seek at a distance a place
where they will find food, or perhaps only a suitable place to undergo
their metamorphosis. It is then that this singular journey commences.
As regards the uniting of these myriads of individuals into columns,
M. Guérin-Méneville thinks that it can be explained by the necessity
these insects feel for mutual protection against the drying effect of
the atmosphere when they are forced to leave the ground. United into
masses, and moistened by the glutinous matter which connects them,
they can leave their former place of abode without danger; if each
were by itself, they would soon perish. Here, as in other cases, union
is strength; and the strength of these larvæ lies in this protecting
moisture. However it may be explained, the migration of these troops
of insects are among the most astonishing phenomena of Nature.