gnat, the grasshopper--these and other less familiar types seem
at first to have little in common. They differ in size, in form,
in color, in habits, and modes of life. Yet the researches of
entomologists, following the clew supplied by the illustrious
Savigny, have proved not only that while differing greatly in details
they are constructed on one common plan, but also that other groups,
as, for instance, Crustacea (lobsters, crabs, etc.) and Arachnida
(spiders and mites), can be shown to be fundamentally similar.
Thus, then, although it can be demonstrated that perfect insects,
however much they differ in appearance, are yet reducible to one
type, the fact becomes much more evident if we compare the larvæ. M.
Brauer and I have pointed out that two types of larvæ, which I have
proposed to call Campodea-form and Lindia-form, and which Packard has
named Leptiform and Eruciform, run through the principal groups of
insects.
[Illustration: Strange and Rare Fishes
1, Diodon; 2, Rhinobatus; 3, Tetrodon; 4, Galaxias; 5, Pterois; 6,
Ostracion; 7, Pelor; 8, Amia; 9, Haplochiton; 10, Callionymus; 11,
Cottus; 12, Malthe; 13, Blennius; 14, Pomacentrus; 15, Chromis; 16,
Scorpæna]
Let me say a word as to the general insect type. It may be described
shortly as consisting of animals possessing a head, with mouth-parts,
eyes, and antennæ; a many-segmented body, with three pairs of legs
on the segments immediately following the head; with, when mature,
either one or two pairs of wings, generally with caudal appendages.
Thus, then, we find in many of the principal groups of insects
that, greatly as they differ from one another in their mature
condition, when they leave the egg they more nearly resemble the
typical insect type, consisting of a head, a three-segmented thorax,
with three pairs of legs, and a many-jointed abdomen, often with
anal appendages. Now, is there any mature animal which answers to
this description? We need not have been surprised if this type,
through which it would appear that insects must have passed so
many ages since (for winged Neuroptera have been found in the
carboniferous strata), had long ago become extinct. Yet it is not
so. The interesting genus Campodea still lives; it inhabits damp
earth, and closely resembles the larva of Chloëon, constituting,
indeed, a type which occurs in many orders of insects. It is true
that the mouth-parts of Campodea do not resemble either the strongly
mandibulate form which prevails among the larvæ of Coleoptera,
Orthoptera, Neuroptera, Hymenoptera, Lepidoptera; or the suctorial
type of the Homoptera and Heteroptera. It is, however, not the less
interesting or significant on that account, since its mouth-parts are
intermediate between the mandibulate and haustellate types; a fact
which seems to me most suggestive.
It appears, then, that there are good grounds for considering that
the various types of insects are descended from ancestors more or
less resembling the genus Campodea, with a body divided into head,
thorax, and abdomen; the head provided with mouth-parts, eyes, and
one pair of antennæ; the thorax with three pairs of legs; and the
abdomen, in all probability, with caudal appendages.
If these views are correct, the genus Campodea must be regarded as a
form of remarkable interest, since it is the living representative of
a primeval type, from which not only the Collembola and Thysanura,
but the other great orders of insects have derived their origin.
Since, then, individual insects are certainly in many cases developed
from larvæ closely resembling the genus Campodea, why should it be
regarded as incredible that insects as a group have gone through
similar stages? That the ancestors of beetles under the influence of
varying external conditions, and in the lapse of geological ages,
should have undergone changes which the individual beetle passes
through under our own eyes and in the space of a few days, is surely
no wild or extravagant hypothesis. Again, other insects come from
vermiform larvæ much resembling the genus Lindia, and it has been
also repeatedly shown that in many particulars the embryo of the
more specialized forms resembles the full-grown representatives of
lower types. I conclude, therefore, that the Insecta generally are
descended from ancestors resembling the existing genus Campodea,
and that these again have arisen from others belonging to a type
represented more or less closely by the existing genus Lindia.
Of course it may be argued that these facts have not really the
significance which they seem to me to possess. It may be said that
when Divine power created insects, they were created with these
remarkable developmental processes. By such arguments the conclusions
of geologists were long disputed. When God made the rocks, it was
tersely said, He made the fossils in them. No one, I suppose, would
now be found to maintain such a theory; and I believe the time will
come when it will be generally admitted that the structure of the
embryo, and its developmental changes, indicate as truly the course
of organic development in ancient times as the contents of rocks and
their sequence teach us the past history of the earth itself.
INSECTS: THEIR WINGS, STINGS, EARS, AND EYES
--PHILIP HENRY GOSSE
The most perfect fliers in existence are insects. The swallow and
the humming-bird are powerful on the wing, and rapid; but neither
these nor any other “winged fowl” can be compared with many of the
filmy-winged insects. The common house-fly, for example, will remain
for hours together floating in the air beneath the ceilings of our
dwelling-rooms, hovering and dancing from side to side, without
effort and without fatigue. It has been calculated that in its
ordinary flight the house-fly makes about 600 strokes with its wings
every second, and that it is carried through the air a distance of
five feet during that brief period. But, if alarmed, the velocity
can be increased six or sevenfold, as every one must have observed,
so as to carry the insect thirty or five-and-thirty feet in the
second. In the same space of time, observes Mr. Kirby, a race-horse
could clear only ninety feet, which is at the rate of more than a
mile a minute. Our little fly, in her swiftest flight, will in the
same space of time go more than the third of a mile. Now compare the
infinite difference of the size of the two animals (ten millions of
the fly would hardly counterpoise one racer), and how wonderful will
the velocity of this minute creature appear! Did the fly equal the
race-horse in size, and retain its present powers in the ratio of its
magnitude, it would traverse the globe with the rapidity of lightning.
Bees, again, are accomplished masters of aerial motion. The
humblebees, notwithstanding their heavy bodies, are the most powerful
fliers of this class. The same excellent entomologist tells us that
they “traverse the air in segments of a circle, the arc of which
is alternately to right and left. The rapidity of their flight
is so great that, could it be calculated, it would be found, the
size of the creature considered, far to exceed that of any bird,
as has been proved by the observations of a traveler in a railway
carriage proceeding at the rate of twenty miles an hour, which was
accompanied, though the wind was against them, for a considerable
distance by a humblebee (Bombus subinterruptus), not merely with the
same rapidity, but even greater, as it not infrequently flew to and
fro about the carriage, or described zigzag lines in its flight. The
aerial movements of the hive-bee are more distinct and leisurely.”
You have doubtless often admired the noble dragon-fly, with its four
ample and widespread wings of gauze, hawking in a green lane, or over
a pool in the noon of summer. It sails, or rather shoots with arrowy
fleetness hither and thither, now forward, now backward, now to the
right, now to the left, without turning its body, but simply by the
action of its powerful and elegant wings. Leeuwenhoek once saw an
insect of this tribe chased by a swallow in a menagerie a hundred
feet long. The dragon-fly shot along with such astonishing power of
wing, to the right, to the left, and in all directions, that this
bird of rapid flight and ready evolution was unable to overtake and
capture it, the insect eluding every attempt, and being in general
fully six feet in advance of the bird. A dragon-fly has been known
to fly on board a ship at sea, the nearest land being the coast of
Africa, five hundred miles distant, a fact highly illustrative of its
power of wing.
It is a point of interest to know the structure of the organs by
which such results are accomplished. Let us begin with the common
fly. Well, we will borrow one of his wings for the lesson, and,
putting it into the stage-forceps, we shall be able to turn it in any
direction for observation beneath the microscope.
At first it seems a very thin, transparent membrane, of a shape
between triangular and oval, with a few fine black lines running
through it, and along one edge. But on bringing a greater magnifying
power to bear on it, we see that the clear surface is covered with
minute short stiff hairs, each of which has an expanded base. And
still further, by delicate focusing, we find that there are two sets
of these hairs, which come into view alternately, those of one row
projecting upward toward our eye, those of the other downward. They
are placed on both the upper and under surface, and are in fact
appendages of two distinct membranes, applied to each other. There
is some reason to believe that these hairs are delicate organs of
touch communicating impressions through the skin to a sensitive layer
beneath; at least such seems their function on the body, and we may
judge from analogy that it is not different here.
The black lines are elastic, horny tubes, over which the membranes
are spread and stretched, like the silk of an umbrella by its ribs.
The upper membrane is firmly attached to the tubes (which are called
nervures); the lower has but a slight adhesion, and is easily
stripped from them. The nervures originate in the body, and diverge
like a fan to various points of the tip, and to the upper and lower
edges; some of them, however, terminate in the substance of the
wing without reaching the edge, and some send off cross branches by
which two are connected together. They generally maintain the same
thickness throughout, but there are enlargements where the branches
join the main trunks. These nervures are hollow, and are, during
life, filled with a subtile fluid, which is supplied from the vessels
of the body. They contain also ramifications of the exquisite spiral
air-vessels.
In this wing of the bee all of these structures may be seen to
greater advantage. Unlike the fly, which has but a single pair of
wings, the bee has two pairs, of which the fore pair is the larger
and more horny, the hinder pair seeming to be, as it were, cut out of
the hinder and inner side of the fore ones. The two edges--the hinder
edge of the fore pair and the front edge of the hind pair--then
correspond, but it is necessary that, during flight, when the wings
are expanded, the two wings on each side should _maintain_ this
relative position, neither overlapping the other, but together
presenting one broad surface, wherewith to beat the air. There must
be, therefore, some contrivance for locking together the two edges
in question, which yet shall be capable of being unlocked at the
pleasure of the animal; for the wings during repose slide over one
another. This contrivance is furnished by a series of hairs or spines
running along the front edge of the hindwing; they are bent up into
strong semicircular hooks, arching outward, looking, under a high
power, like the hooks on a butcher’s stall. On the other hand, the
margin of the forewing is strengthened, and is turned over with a
shallow doubling, so as to make a groove into which the hooks catch;
and thus, while the forewings are expanded, the hooks of the other
pair are firmly locked in their doubled edge, while, as soon as
flight ceases, and the wings are relaxed, there is no hindrance to
the sliding of the front over the hind pair.
The wings of many insects are interesting on account of the organs
with which they are clothed. A familiar example is furnished by the
common gnat. There is the same general structure as before--two clear
elastic membranes stretched over slender horny tubular nervures, and
studded on both surfaces with short spine-like hairs, which in this
case, however, are excessively numerous and minute. But along the
nervures, and along other lines which run (generally) parallel with
the front margin, and also along the whole margin, there are set long
leaf-like scales of very curious appearance and structure.
There are, however, other insects which display these or similar
appendages in far greater profusion, and in much variety of form and
appearance. In the fissures of cliffs that border the seashore may
often be found some wingless but active insects, which are endowed
with the power of leaping in great perfection. From their hinder
extremity being furnished with long projecting bristles, they are
sometimes called bristle-tails, but naturalists designate the genus
Machilis. If you can get one sufficiently still to examine it, you
will be delighted with the lustre of its clothing, which appears
dusted all over with a metallic powder of rich colors--red, brown,
orange, and yellow, foiled by dull lead-gray in places.
If you touch one of these nimble leapers, though ever so slightly,
you will see the result on your finger-ends, for they will be found
covered with a thin stratum of the finest dust, which displays the
colored metallic reflection seen on the insect. By touching one with
a plate of glass, instead of your finger, you will get the same
dust to adhere to this transparent medium, by applying which to
the microscope you may at once discern the marvelous nature of the
raiment with which the little creature is bedecked.
The dust is now seen to be composed of myriads of thin scales,
mostly regular and symmetrical in their forms, though varying
exceedingly among themselves in this respect. Some are heart-shaped,
some shovel-shaped, some round, oval, elliptical, half round, half
elliptical, long and narrow, sometimes irregular and unequal, and of
various other indescribable outlines.
The beautiful and extensive order called Lepidoptera or scale-winged,
_par excellence_, including the gay tribes of butterflies and moths,
presents us with many exceedingly interesting varieties in these
singular coverings.
Here are specimens from the pretty little white five-plume moth
(Pterophorus), so common in meadows in summer. The general shape of
the scales from the body and wings is that of a willow-leaf, some
singly pointed, but more cut at the tip into two, three, or four
notches. Those from the legs are longer and slenderer in proportion;
and among the others from the wings there are some which take the
form of hairs, which send forth one or more branches from one side,
that form a very acute angle with the main stem. The scales proper
are all marked with longitudinal lines, very minute and close, but
they mostly bear a central band, and sometimes a marginal one on each
side, of spots set in sinuous lines like the bands on a mackerel’s
back; these are probably composed of pigment-granules.
The hairs with which the bodies of moths are invested are essentially
of the same character as the scales which clothe their wings. Here
are examples from the glowing sides of the abdomen of that richly
colored insect, the cream-spot tiger-moth (Arctia villica). You
see they are simple scales, drawn out to an inordinate length and
great tenuity; each has its quill-like footstalk, and we may trace
on some of them the ribs and transverse dotting, while here we see
all intermediate stages between the slenderest hair and the broadly
ovate, bluntly pointed scales from the wings.
You are familiar, of course, with the brilliant little blue butterfly
(Polyommatus Alexis) which dances and glitters in the sunshine on
waste places in June. Among the scales of ordinary form which clothe
the lovely little wings will occur one here and there of a different
shape from the rest. Here you may see one; it is much smaller than
the average; the footstalk is very long, and the shape of the entire
scale is that of a battledoor. The ribs are rather few and coarse,
and they have this peculiarity, that each rib swells at intervals
into rounded dilatations, each of which has a minute black point in
its centre. In some of these battledoor scales there is, near the
lower part of the expansion, a crescent of minute pigment-grains.
Scales taken from the brilliant changeable blue-green patch in
the hindwing of Papilio Paris, a fine Indian butterfly, have an
interesting appearance. They are simply pear-shaped in outline, with
few longitudinal ribs set far apart, and numerous strongly marked
corrugations running across between them. That these are really
elevations of the surface is well seen in some scales, even with
transmitted light, and a high power; for the slopes of the wrinkles
that face the light display the lustrous emerald reflection proper to
the wing, while the transmitted color of the whole scale is a rich
transparent red.
The dimensions of the scales do not bear any certain proportion to
the size of the insect which is clothed with them; those from the
broad wings of the noble Saturnia Atlas, for example, eight or nine
inches in expanse, being exceeded in size by some from those of the
little British muslin moth, an inch wide.
The little beetles which we are familiar with under the name of
weevils, characterized by their long slender snouts, at the end of
which they carry curiously folding antennæ, and which constitute
the family Curculionidæ, are in many cases clothed with scales, to
which they owe their colors and patterns. Several of British species
display a green or silvery lustre, which under the microscope is seen
to be produced by oval scales. But these are eclipsed by the splendor
of many tropical species, especially that well-known one from South
America which is called the diamond beetle, and scientifically
Entimus imperialis, from its unparalleled magnificence.
A piece of one of the wing-cases of this beetle is gummed to the
slide now upon the stage. We look at it by reflected light with a
magnifying power of 130 diameters. We see a black ground, on which
are strewn a profusion of what look like precious stones blazing in
the most gorgeous lustre. Topazes, sapphires, amethysts, rubies,
emeralds seem here sown broadcast; and yet not wholly without
regularity, for there are broad bands of the deep black surface,
where there are no gems, and, though at considerable diversity
of angle, they do all point with more or less precision in one
direction, viz., that of the bands. These gems are flat transparent
scales, very regularly oval in form, for one end is rather more
pointed than the other; there is no appearance of a footstalk, and
by what means they adhere I know not; they are evidently attached in
some manner by the smaller extremity to the velvety black surface of
the wing-case. The gorgeous colors seem dependent in some measure
on the reflection of light from their polished surface, and to vary
according to the angle at which it is reflected. Green, yellow,
and orange hues predominate; crimson, violet, and blue are rare,
except upon the long and narrow scales that border the suture of the
wing-cases, where these colors are the chief reflected.
If you have ever thought on the subject, you have probably taken
for granted that the various sounds produced by insects are voices
uttered by their mouths. But it is not so. No insect has anything
approaching to a voice. Vocal sounds are produced by the emission of
air from the lungs variously modified by the organs of the mouth. But
no insect breathes through its mouth; no air is expelled thence in
a single species; it is a biting, or piercing, or sucking organ; an
organ for the taking of food, or an organ for offence or defence; but
never an organ of sound. The wings are in most cases the immediate
causes of insect sounds.
There is a pretty little beetle (Clytus), not uncommon in summer in
gardens, remarkable for the brilliant gamboge-yellow lines across
its dark wing-cases, which makes a curious squeaking sound when you
take it in your hand. You think it is crying; but if you carefully
examine it with a lens while the noise is uttered, you will perceive
that the cause is the grating of the thorax against the front part of
the two wing-cases. Several other beetles produce similar sounds when
alarmed, by rubbing the other end of the wing-sheaths with the tip of
the abdomen. Many of those genera which feed on ordure and carrion do
this.
But the noisiest of all insects are those of the classes Orthoptera
and Homoptera, the crickets and grasshoppers, and the treehoppers.
The locusts and grasshoppers, it appears, make use of their hindlegs
in producing their crink. If you look at the grasshopper’s leg,
you will see that the thigh is marked with a number of transverse
overlapping angular plates, and that the shank carries a series of
short horny points along each side. The insect when it crinks brings
the shank up to its thigh, and rubs both to and fro against the
wing-sheaths, doing this by turns with the right and left legs, which
causes the regular breaks in the sound.
In this case we may without hesitation conclude that the friction
of the thigh-plates and shank-points on the rough edges of the
wing-cases produces the musical vibration of the tense membrane, as
rubbing a wet glass with the finger will yield a loud musical note.
The most elaborate contrivance for the production of sounds among
the insect races, however, is found among the Cicadæ, celebrated in
classical poetry as the very impersonations of song and eloquence.
Probably at some period of your life you have been stung by a bee
or wasp. I shall take it for granted that you have, and that having
tested the potency of these warlike insects’ weapons with one sense,
you have a curiosity to examine them with another. The microscope
shall aid your vision to investigate the morbific implement.
This is the sting of the honey-bee. It consists of a dark brown horny
sheath, bulbous at the base, but suddenly diminishing, and then
tapering to a fine point. This sheath is split entirely along the
inferior edge, and by pressure with a needle I have been enabled to
project the two lancets, which commonly lie within the sheath. These
are two slender filaments of the like brown horny substance, of which
the centre is tubular, and carries a fluid, in which bubbles are
visible. The extremity of each displays a beautiful mechanism, for it
is thinned away into two thin blade-edges, of which one remains keen
and knife-like, while the opposite edge is cut into several saw teeth
pointing backward.
The lancets do not appear to be united with the sheath in any part,
but simply to lie in its groove; their basal portions pass out into
the body behind the sheath, where you see a number of muscle-bands
crowded around them: these, acting in various directions, and being
inserted into the lancets at various points, exercise a complete
control over their movements, projecting or retracting them at their
will. But each lancet has a singular projection from its back,
which appears to act in some way as a guide to its motion, probably
preventing it from slipping aside when darted forth, for the bulbous
part of the sheath, in which these projections work, seems formed
expressly to receive them.