of an enemy: the two spears finely pointed, sharp-edged, and
saw-toothed, adapted for piercing, cutting, and tearing; the reversed
direction of the teeth gives the weapon a hold in the flesh, and
prevents it from being readily drawn out. Here is an elaborate
store of power for the jactation of the javelins, in the numerous
muscle-bands; here is a provision made for the precision of the
impulse; and finally, here is a polished sheath for the reception of
the weapons and their preservation when not in actual use. All this
is perfect; but something still was wanting to render the weapons
effective, and that something your experience has proved to be
supplied.
The mere intromission of these points, incomparably finer and
sharper than the finest needle that was ever polished in a Sheffield
workshop, would produce no result appreciable to our feelings; and
most surely would not be followed by the distressing agony attendant
on the sting of a bee. We must look for something more than we have
seen.
We need not be long in finding it. For here, at the base of the
sheath, into which it enters by a narrow neck, lies a transparent
pear-shaped bag, its surface covered all over, but especially toward
the neck, with small glands set transversely. It is rounded behind,
where it is entered by a very long and slender membraneous tube,
which, after many turns and windings, gradually thickening and
becoming more evidently glandular, terminates in a blind end.
This is the apparatus for preparing and ejecting a powerful poison.
The glandular end of the slender tube is the secreting organ: here
the venom is prepared; the remainder of the tube is a duct for
conveying it to the bag, a reservoir in which it is stored for the
moment of use. By means of the neck it is thrown into the groove at
the moment the sting is projected, the same muscles, probably, that
dart forward the weapon compressing the poison-bag and causing it to
pour forth its contents into the groove, whence it passes on between
the two spears into the wound which they have made.
A modification of this apparatus is found throughout a very extensive
order of insects--the Hymenoptera; but in the majority of cases it
is not connected with purposes of warfare. Wherever it occurs it
is always confined to the female sex, or (as in the case of some
social insects) to the neuters, which are undeveloped females. When
it is not accompanied by a poison-reservoir it is ancillary to the
deposition of the eggs, and is hence called an ovipositor, though
in many cases it performs a part much more extensive than the mere
placing of the ova.
A very wide field of observation, and one easily cultivated, is
presented by the organs of sense in the insect races, and in
particular by those curious jointed threads which proceed from
the front or sides of the head, and which are technically called
antennæ. These may sometimes be confounded with the palpi; for in
a carnivorous beetle, for instance, both palpi and antennæ are
formed of a number of oblong, polished hard joints, set end to
end, like beads on a necklace. And it is probable there may be as
much community in the function as in the form of these two sets of
appendages; that both are the seats of some very delicate perceptive
faculty allied to touch, but of which we can not, from ignorance,
speak very definitely. It is likely, indeed, that sensations of a
very variable character are perceived by them, according to their
form, the degree of their development, and the habits of the species.
It is not impossible, judging from the very great diversity which we
find in the form and structure of these and similar organs in this
immense class of beings, compared with the uniformity that prevails
in the organs of sense bestowed on ourselves and other vertebrate
animals, that a far wider sphere of perception is open to them than
to us. Perhaps conditions that are appreciable to us only by the aid
of the most delicate instruments of modern science may be appreciable
to their acute faculties, and may govern their instincts and actions.
Among such we may mention, conjecturally, the comparative moisture
or dryness of the atmosphere, delicate changes in its temperature,
in its density, the presence of gaseous exhalations, the proximity
of solid bodies indicated by subtile vibrations of the air, the
height above the earth at which flight is performed, measured
barometrically, the various electrical conditions of the atmosphere;
and perhaps many other physical diversities which can not be
classed under sight, sound smell, taste, or touch, and which may be
altogether unappreciable, and therefore altogether inconceivable, by
us. It is probable, however, that the antennæ are the organs in which
the sense of _hearing_ is specially seated.
The forms which are assumed by the antennæ of insects are very
diverse; and I can bring before you only a very small selection out
of the mass. One of the most simple forms is that found in many
beetles, as in this Carabus, for example. Here each antennæ is
composed of eleven joints, almost exactly alike and symmetrical, each
joint a horny body of apparently a long oval shape, polished on the
surface, but not smooth, because covered with minute depressed lines,
and clothed with shaggy hair. There is, however, a slight illusion
in the appearance: it seems as if the dividing point of the joints
were, as I have just said, at the termination of the oval, but when
we look closely we see that the summit of each oval is, as it were,
cut off by a line, and by comparing the basal joints with the others,
we see that this line is the real division, that the summit of the
oval really forms the bottom of the succeeding joint, and that the
constricted part is no articulation at all. The first, or basal joint
(called the scapus), and the second (called the pedicella), differ in
form from the rest, here but slightly, but often considerably. The
whole of the remaining joints are together termed the clavola.
There is a very extensive family of beetles known as Lamellicornes,
because the antennal joints are singularly flattened and applied one
over the other like the leaves of a book (lamella, a leaf).
But this structure is seen to still greater advantage in the much
larger cockchafer, so abundant in May in some seasons. The insect
widely expands them, evidently to receive impressions from the
atmosphere; when alarmed, they are closed and withdrawn beneath the
shield of the head, but on the first essay toward escape, or any kind
of forward movement, the leaves are widely opened, and then, after
an instant’s pause to test the perceptions on the sensorium, away it
travels.
But much more curious and beautiful are the antennæ of many moths,
which often resemble feathers, particularly in the group Bombycina,
of which the silkworm is an example; and in the male sex, which
displays this structure more than the female.
This is the antenna of a large and handsome and not at all uncommon
moth--the oak egger (Lasiocampa quercus). It consists of about
seventy joints, so nearly alike in size and outline that the whole
forms an almost straight rod, slightly tapering to the tip. Each
joint, however, sends forth two long straight branches, so disposed
that the pair make a very acute angle, and the whole double series
of seventy on each side form a deep narrow groove. These two series
of branches, being perfectly regular and symmetrical, impart to the
antennæ the aspect of exquisite feathers.
It is, however, when we examine the elements of this structure in
detail, using moderately high powers of enlargement, that we are
struck with the elaborateness of the workmanship bestowed upon them.
Each of the lateral branches is a straight rod, thick at its origin,
whence it tapers to a little beyond its middle, and then thickens
again to its tip. Here two horny spines project from it obliquely,
one much stouter than the other, at such an angle as nearly to touch
the tip of the succeeding branch.
Besides this, each branch is surrounded throughout its length
with a series of short stiff bristles, very close-set, projecting
horizontally (to the plane of the axis of the branch), and bent
upward at the end candelabrum-fashion. The mode in which they are
arranged is in a short spiral, which makes about forty-five whorls
or turns about the axis; at least in the branches which are situated
about the middle of the antennæ; for these diminish in length toward
the extremity, bringing the feather to a rather abrupt point.
The entire surface of the branch gleams under reflected light with
metallic hues, chiefly yellows and bronzy greens; which appear to
depend on very minute and closely applied scales that overlap each
other. The main stem of the feather--that is, the primary rod or
axis--is somewhat sparsely clothed with scales of another kind, thin,
oblong, flat plates, notched at the end, and very slightly attached
by means of a minute stem at the base--the common clothing scales of
the Lepidoptera.
We may acquire some glimpse of a notion why this remarkable
development of antennæ is bestowed upon the male sex of this moth
by an acquaintance with its habits. It has been long a practice
with entomologists, when they have reared a female moth from the
chrysalis, to avail themselves of the instincts of the species to
capture the male. This sex has an extraordinary power of discovering
the female at immense distances, and though perfectly concealed;
and will crowd toward her from all quarters, entering into houses,
beating at windows, and even descending chimneys, to come at the dear
object of their solicitude. Collectors call this mode of procuring
the male “sembling,” that is “assembling,” because the insects of
the sex assemble at one point. It can not be practiced with all
insects, nor even with all moths; those of this family, Bombycidæ,
are in general available; and of these, none is more celebrated for
the habit than the oak egger. The very individual whose antenna has
furnished us with this observation was taken in this way; for having
bred a female of this species, one evening I put her into a basket in
my parlor. One male, the same evening, came dashing into the kitchen;
but the next day, soon after noon, in the hot sunshine of August, no
fewer than four more males came rapidly in succession to the parlor
window, which was a little open, and, after beating about the panes a
few minutes, found their way in, and made straightway for the basket,
totally regardless of their own liberty.
It must be manifest to you that some extraordinary sense is bestowed
upon these moths, or else some ordinary and well-known sense in
extraordinary development. It may be smell; it may be hearing; but
neither odor nor sound, perceptible by our dull faculties, is given
forth by the females; the emanation is far too subtile to produce any
vibrations on our sensorium, and yet sufficiently potent, and widely
diffused, to call these males from their distant retreats in the
hedges and woods.
The male gnat presents in its antennæ a pair of plumes of equal
beauty, but of a totally different character. The pattern here is one
of exceeding lightness and grace.
In the tribe of two-winged insects, which we term, _par excellence_,
flies (Muscadæ), the antennæ are of peculiar structure. The common
house-fly shall give us a good example. Here, in front of the head,
is a shell-like concavity, divided into two by a central ridge. Just
at the summit of this projection are the two antennæ, originating
close together, and diverging as they proceed. Each antenna consists
of three joints, of which the first is very minute, the second is
a reversed cone, and the third, which is large, thick, and ovate,
is bent abruptly downward immediately in front of the concavity.
From the upper part of this third joint projects obliquely a stiff
bristle or style, which tapers to a fine point. It is densely hairy
throughout; and is more beset with longer hairs on two opposite
sides, which decrease regularly in length from the base, making a
wide and pointed plume.
Such are a few examples of what are presumed to be the _ears_ of
insects; let us now turn our attention to their _eyes_. And we can
scarcely select a more brilliant, or a larger example, than is
presented by this fine dragon-fly (Æshna), which I just now caught
as it was hawking to and fro in my garden. How gorgeously beautiful
are these two great hemispheres that almost compose the head, each
shining with a soft satiny lustre of azure hue, surrounded by
olive-green, and marked with undefined black spots, which change
their place as you move the insect round!
Each of these hemispheres is a compound eye. I put the insect in the
stage-forceps, and bring a low power to bear upon it with reflected
light. You see an infinite number of hexagons, of the most accurate
symmetry and regularity of arrangement. Into those which are in the
centre of the field of view, the eye can penetrate far down, and you
perceive that they are tubes; of those which recede from the centre,
you discern more and more of the sides; while, by delicate adjustment
of the focus, you can see that each tube is not open, but is covered
with a convex arch of some glassy medium polished and transparent
as crystal. There are, according to the computations of accurate
naturalists, not fewer than 24,000 of these convex lenses in the two
eyes of such a large species of dragon-fly as this. Every one of
these 24,000 bodies represents a perfect eye; every one is furnished
with all the apparatus and combinations requisite for distinct
vision; and there is no doubt that the dragon-fly looks through
them all. In order to explain this, I must enter into a little
technical explanation of the anatomy of the organs, as they have been
demonstrated by careful dissection.
The glassy convex plate or facet in front of each hexagon is a
cornea, or corneule, as it has been called. Behind each cornea,
instead of a crystalline lens, there descends a slender transparent
pyramid, whose base is the cornea, and whose apex points toward the
interior, where it is received and embraced by a translucent cup,
answering to the vitreous humor. This, in its turn, is surrounded by
another cup, formed by the expansion of a nervous filament arising
from the ganglion on the extremity of the optic nerve, a short
distance from the brain. Each lens-like pyramid, with its vitreous
cup and nervous filament, is completely surrounded and isolated by
a coat (the choroid) of dark pigment, except that there is a minute
orifice or pupil behind the cornea, where the rays of light enter
the pyramid, and one at the apex of the latter, where they reach the
fibres of the optic nerve.
Each cornea is a lens with a perfect magnifying power. The focus of
each cornea has been ascertained by similar experiments to be exactly
equal to the length of the pyramid behind it, so that the image
produced by the rays of light proceeding from any external object,
and refracted by the convex cornea, will fall accurately upon the
sensitive termination of the optic nerve-filament placed there to
receive it.
The rays which pass through the several pyramids are prevented from
mingling with each other by the isolating sheath of dark pigment; and
no rays except those which pass along the axis of each pyramid can
reach the optic nerve; all the rest being absorbed in the pigment of
the sides. Hence it is evident that as no two corneæ on the rounded
surface of the compound eye can have the same axis, no two can
transmit a ray of light from the very same point of any object looked
at; while, as each of the composite eyes is immovable, except as the
whole head moves, the combined action of the whole 24,000 lenses
can present to the sensorium but the idea of a single, undistorted,
unconfused object, probably on somewhat of the same principle by
which the convergence of the rays of light entering our two eyes
gives us but a single stereoscopic picture.
The soft blue color of this dragon-fly’s eyes--as also the rich
golden reflections seen on the eyes of other insects, as the
whameflies, and many other Diptera--is not produced by the pigment
which I have alluded to, but is a prismatic reflection from the
corneæ.
You would suppose that, having 24,000 eyes, the dragon-fly was pretty
well furnished with organs of vision and surely would need no more;
but you would be mistaken. It has three other eyes of quite another
character.
If you look at the commissure or line of junction of the two compound
eyes on the summit of the head, you will see just in front of the
point where they separate and their front outlines diverge a minute
crescent-shaped cushion of a pale-green color, at each angle of which
is a minute antenna. Close to the base of each antenna there is set,
in the black skin of the head that divides the green crescent from
the compound eyes, a globose polished knob of crystal-like substance,
much like the “bull’s-eyes” or hemispheres of solid glass that are
set in a ship’s deck to enlighten the side-cabins. On the front side
of the crescentic cushion there is a third similar glassy sphere, but
much larger than the two lateral ones. What are these three spherules?
They are eyes, in no important respect differing from the individuals
which compose the compound masses except that they are isolated. The
shining glassy hemisphere is a cornea of hard transparent substance,
behind which is situated a spherical lens, lodged in a kind of cup
formed by an expansion of the optic nerve, and which is surrounded
by a colored pigment-layer. You may study these simple eyes, or
stemmata, as they are called, in many other insects, though they are
not so universally present as the compound eyes. On the forehead of
the honey-bee they are well seen, as three black shining globules,
placed, as in the dragon-fly, in a triangle.
FAIRY FLIES
--FRED. ENOCK
If it were possible to obtain a reply from all living naturalists
as to what first attracted their attention to insect life, I
venture to think that seventy-five per cent or more of the replies
would be: “The first sight of a living butterfly.” How many of us
(no matter what our specialty may now be) can look back to that
time when, perhaps, a tortoise-shell flaunted its beauty before
our youthful eyes, and we were drawn to it and fascinated by its
gorgeous color, as it delicately sipped the nectar from a dandelion
or thistle, gently opening and shutting its wings, spreading them as
wide as possible so that every part should be seen! The colors and
markings flashed before our enraptured gaze, and while we were held
captive by its beauty, another still more beautiful butterfly--the
peacock--sailed past and alighted close to the first, riveting our
attention by the marvelously lovely “eyes” on its wings; and again
another--this time a red admiral--in full sail bore down upon us,
opened fire, and we surrendered, swearing allegiance for evermore
to Atalanta and all her crew. Few boys could stand still and not be
affected or influenced by such beauty. Such then has been, and will
be, the foundation of our naturalists--“butterfly hunters” first,
specialists later on.
As we are briefly running through the Hymenoptera our difficulties
seem to increase, for with the next division, the Chalcididæ, we
hardly know what to do, or to whom we can turn for assistance in
naming these brilliantly spangled green and gold colored flies, whose
“name is legion.” The laborers in this field are indeed few, so much
so that there is not a “specialist” even at the Natural History
Museum, South Kensington.
Let us go back to one of those “neglected families” which have
received but small attention. One reason for this want of attention
is, no doubt, because of the extreme smallness of the members of
this family, the largest being not more than one-twentieth of an
inch long, whereas the smallest is less than one-eighty-fifth of an
inch from head to tail. These insect atoms have been classed among
the Chalcididæ by Haliday--the originator of the Mymaridæ--who
first noticed them in 1833. Since that date Westwood has placed
them among the Proctotrupidæ; and now Ashmead--author of American
Proctotripidæ--has decided in favor of Haliday’s arrangement, and in
this I fully concur.
The fairy flies are, without doubt, among the many wonderful
parasitic Hymenoptera, the most admirable in their exquisite
structure, as well as in their habits and economy. All the species
are egg parasites, and each species has its peculiar taste, selecting
with unerring instinct the right kind of egg--generally that of an
injurious insect--in which the female lays one of its own eggs, which
in due time hatches or develops into an active maggot. This maggot
feeds upon the contained fluids, and finds sufficient nutriment
to bring it to full size, when it assumes the pupal stage. The
fly, being matured, bites out a round piece of the eggshell large
enough to allow it to escape. The most noticeable character in the
fairy flies is the transverse line across the face a little above
the insertion of the antennæ. The wings are devoid of all wing
nerves, for the sub-costal is so short and stumpy that the wing
looks perfectly free. Both the upper and under surfaces of the wings
are covered with minute hairs, and the margins of both wings are
surrounded by long hair-like ciliæ.
Owing to the kindness of the authorities of Dublin Museum, I have
been permitted to make a most exhaustive and critical examination of
Haliday’s type collection of British Mymaridæ, and though the hand
of time and those of others have materially interfered with their
original arrangement, the result has been most satisfactory to me.
Although one or two of the most interesting types are absent, I have
been enabled to re-establish Haliday’s genus Panthus, which certain
compilers had, for no apparent reason, ignored. I found two specimens
still bearing the old labels, and after long and critical microscopic
examination I saw that both were distinct from any others. I applied
for and obtained permission to remove the carded specimens, and
remount them in Canada balsam. This was successfully done, and on
making a photo-micrograph the peculiar generic characters were
brought out most distinctly. Some naturalists appear to imagine that
a pocket lens will be sufficient to identify these species, but in
an insect which is but a fiftieth or an eightieth of an inch long
it is of absolute importance that every joint and detail should be
examined under the microscope, and the relative proportion of each
joint of the antennæ be compared with closely allied species--just
as they are with all large Hymenoptera. The curve, too, of the
delicate wings is a most important feature which must not be hurried
over. This point leads me to speak of the immense superiority of
photo-micrographs over drawings of these fairy flies. No matter how
exact an enthusiastic naturalist may be in his endeavors, it is
a physical impossibility for any one to follow out and reproduce
the exact curve of these microscopic wings; and as the flies when
properly prepared and “set” in Canada balsam lend themselves
peculiarly to photo-micrography, it is wise to take advantage of
this. The details can afterward be drawn to a much larger scale. It
is my intention to do this with every species which I have collected
during the past twenty-five years, and of which I have a very large
number, far exceeding those known to science. The illustrations
accompanying this short article will, I think, show what exceedingly
good subjects the fairy flies are for photo-micrographs.
Since Haliday’s arrangement of the British Mymaridæ, elucidated
by Francis Walker, no new species have been recorded, the genera
numbering eleven, the species, thirty-five, as follows, viz.:
_Ooctonus_, four species
_Gonatocerus_, five species
_Alaptus_, two species
_Litus_, one species
_Eustochus_, one species
_Mymar_, one species
_Cosmocoma_, eight species
_Caraphractus_, one species
_Anaphes_, seven species
_Anagrus_, four species
_Camptoptera_, one species
Of these I have found representatives of all, and a few notes may not
be uninteresting to those who desire to search for these fairy flies.
Ooctonus. Of this genus I have not found very many specimens. It is
thick-set, and least like a fairy. Unfortunately, Haliday did not
publish any detailed account of his captures, but from the fact of
his naming one (plentifully represented in his collection) Ooctonus
hemipterus, it is just possible it may be parasitic in eggs of bugs.