Chapter X: Part I: Morphology and Physiology (6)

As the large branches penetrate into the wing, the balls (pelotons) of fine tracheal threads tend to unroll, and each of the new ramifications of the secondary tracheal system is accompanied in its course by a bundle of capillary tubes. This secondary system of wing-tracheæ, then, arises from the mother trachea at the end of the third stage, when we find already formed the chitinous tunic, which will persist through the fourth stage up to pupation. It differs from the tracheoles in not communicating with the air-passage; it possesses no spiral membrane at the origin, and takes no part in respiration.

Gonin thus sums up the nature of the two tracheal systems in the rudimentary wing, which he calls the provisional and permanent systems. “The first, appearing in the second stage of the larva, comprises all the capillary tubes, and arising from numerous branches passes off from the lateral trunk of the thorax before reaching the wing; the second is formed a little later by the direct ramification of the principal branch.

“These two systems are absolutely independent of each other within the wing. Their existence is simultaneous but not conjoint. One is functionally active after the third moult; the other waits the final transformation before becoming active.”

FIG. 145.—_A_, section of wing-bud of larva of _Pieris brassicæ_ of
stage I, in front of the invagination pit. _B_, section passing
through the invagination pit. _C_, section of same in stage II,
through the invagination pit;—_D_, behind it, making the bud appear
independent of the thoracic wall. _E_, wing-bud at the beginning of
the 3d larval stage, section passing almost through the pedicel or
hypodermic insertion, the traces of which appear at _hi_; _h_,
hypodermis; _t_ or _tr_, trachea; _i_, opening of invagination;
_ec_, embryonic cells; _l_, external layer or envelope; _in_,
internal wall of the wing; _ex_, external wall; _s_, cell of a
tactile hair; _tc_, capillary tubes; _c_, cavity of
invagination.—After Gonin.
]

=Evagination of the wing outside of the body.=—We have seen that the alary germs arise as invaginations of the hypodermis; we will now, with the aid of Gonin’s account, briefly describe, so far as is known, the mode of evagination of the wings. During the fourth and last stage of the caterpillar of Pieris, the wings grow very rapidly, and undergo important changes.

Six or seven days after the last larval moult the chitinous wall is formed, the wing remaining transparent. It grows rapidly and its lower edge extends near the legs. It is now much crumpled on the edge, owing to its rapid growth within the limits of its own segment. Partly from being somewhat retracted, and partly owing to the irregularity of its surface, the wing gradually separates from its envelope, and the cavity of invagination (Fig. 145, _c_) becomes more like a distinct or real space. The outer opening of the alary sac enlarges quite plainly, though without reaching the level of the edge of the wing.

This condition of things does not still exactly explain how the wing passes to the outside of the body. Gonin compares these conditions to those exhibited by a series of sections of the larva, made forty-eight hours later, on a caterpillar which had just spun its girdle of silk. At this time the wings have become entirely external, but, says Gonin, we do not see the why or the how. The partition of the sac has disappeared, and with it the cavity and the leaf of the envelope.

It appears probable that the partition has been destroyed, because
the space between the two teguments is strewn with numerous bits,
many of which adhere to the chitinous integument, while others are
scattered along the edges of the wings, in their folds, or between
the wings and the wall of the thorax.

Another series of sections showed that the exit of the fore wings
had been accomplished, while the hinder pair was undergoing the
process of eversion. In this case the partition showed signs of
degeneration: deformation of the nuclei, indistinct cellular limits,
pigmentation, granular leucocytes, and fatty globules.

After the destruction of the partition, what remains of the layer of
the envelope is destined to make a part of the thoracic wall and
undergoes for this purpose a superficial desquamation. The layer of
flattened cells is removed and replaced by a firmer epithelium like
that covering the other regions. It is this renewed hypodermis which
conceals the wing within, serves to separate it from the cavity of
the body, and gives the illusion of a complete change in its
situation. Other changes occur, all forming a complete regeneration,
but which does not accord with the description of Van Rees for the
Muscidæ. Finally, Gonin concludes that the débris scattered about
the wing comes from the two layers of the partition of the sac, from
the flattened hypodermis of the renewed envelope, from the chitinous
cuticle of the wing, and from the inner surface of the chitinous
integument.

He thinks that the metamorphosis of Pieris is intermediate between
the two types of Corethra and of Musca, established by Weismann, as
follows:

=Corethra.=—The wing is formed in a simple depression of the
hypodermic wall. No destruction.

=Pieris.=—The rudiment is concealed in a sac attached to the
hypodermis by a short pedicel. Destruction of the partition and its
replacement by a part of the thoracic wall by means of the imaginal
epithelium.

=Musca.=—The pedicel is represented by a cord of variable length,
whose cavity may be obliterated (Van Rees). The imaginal hypodermis
is substituted for the larval hypodermis, which has completely
disappeared, either by desquamation (Viallanes), or by histolytic
resorption (Van Rees).

=Extension of the wing; drawing out of the tracheoles.=—When it is disengaged from the cavity, the wing greatly elongates and the creases on its surface are smoothed out; the blood penetrates between the two walls, and the cellular fibres, before relaxed and sinuous, are now firmly extended.

Of the two tracheal systems, the large branches are sinuous, and they are rendered more distinct by the presence of a spiral membrane; but the two tunics are not separated as in the other tracheæ of the thorax; moreover, the mouth choked up with débris does not yet communicate with that of the principal trunk. The bundles of tracheoles on their part form straight lines, as if the folds of the organ had had no influence on them. As they have remained bound together, apart from the chitinous membrane of the tracheal trunk, they become drawn out with this membrane, at the time of exuviation, _i.e._ of pupation, and are drawn out of the neighboring spiracle.

FIG. 146.—Full-grown larva of _Pieris brassicæ_, opened along the
dorsal line: _d_, digestive canal; _s_, silk-gland; _g_, brain; _st
I_, prothoracic stigma; _st IV_, 1st abdominal stigma; _a_, _a′_,
germs (buds) of fore and hind wings; _p_, bud of prothoracic
segment;—those of the third pair are concealed under the
silk-glands; _I–III_, thoracic rings.—After Gonin.
]

“This is a very curious phenomenon, which can be verified
experimentally: if we cut off the wing, while sparing the larval
integument around the thoracic spiracles, we preserve the two
tracheal systems; the same operation performed after complete
removal of the larval skin does not give the secondary tracheal
system.” (Gonin.) Deceived by the appearance of the tracheoles while
still undeveloped, Landois and Pancritius, who have not mentioned
the drawing out of the capillaries of the larva, affirm that they
are destroyed by resorption in the chrysalis.

“The study of the tracheæ is closely connected with that of the
veins (nervures). It is well to guard against the error of Verson,
who mistakes for these last the large tracheal branches of the wing.
This confusion is easily explained; it proves that Verson had, with
us, recognized that the secondary system is, in the larva, exempt
from all respiratory function. Landois thought that the pupal period
was the time of formation of the veins. It seems to me probable that
they are derived from the sheath of the peritracheal spaces.”
(Gonin, pp. 30–33.)

FIG. 147.—Left anterior wing of a larva 3 days before pupation. The
posterior part is rolled up: _st_, prothoracic stigma; _tr. i._,
internal tracheal trunk; _tr. e._, _tr. e.′_, external tracheal
trunk; _p_, cavity of a thoracic leg, with the imaginal bud
_b_.—After Gonin.
]

The appearance of the wing-germs in the fully grown caterpillar, as revealed by simple dissection, is shown at Fig. 146; Fig. 147 represents a wing of a larva three days before pupation, with the germ of a thoracic leg.

FIG. 148.—Graber’s diagrams for explaining the origin and primary
invagination of the hypodermis to form the germs of the leg (_b_),
and wings (_f_, _A-C_), and afterwards their evagination _D_, so
that they lie on the outside of the body. _E_, stage _B_, showing
the hypodermal cavities (_f_) and stalks connecting the germs with
the hypodermis (_z_).—After Graber.
]

FIG. 149.—Section lengthwise through the left wing of mature larva in
_Pieris rapæ_: _t_, trachea; _hyp_, hypodermis; _c_, cuticula.—After
Mayer.
]

A. G. Mayer has examined the late development of the wings in _Pieris rapæ_. Fig. 149 represents a frontal section through the left wing of a mature larva and shows the rudiment of the wing, lying in its hypodermal pocket or peripodal cavity. How the trachea passes into the rudimentary wing, and eventually becomes divided into the branches, around which the main veins afterwards form, is seen in Figs. 144, 147, 159.

The histological condition of the wing at this time is represented by Fig. 151, the spindle-like hypodermal cells forming the two walls being separated by the ground-membrane of Semper.

“While in the pupa state,” says Mayer, “the wing-membrane is thrown into a very regular series of closely compressed folds, a single scale being inserted upon the crest of each fold. When the butterfly issues from the chrysalis, these folds in the pupal wings flatten out, and it is this flattening which causes the expansion of the wings.... It is evident that the wings after emergence undergo a great stretching and flattening. The mechanics of the operation appears to be as follows. The hæmolymph, or blood, within the wings is under considerable pressure, and this pressure would naturally tend to enlarge the freshly emerged wing into a balloon-shaped bag; but the hypodermal fibres (_h_) hold the upper and lower walls of the wing-membrane closely together, and so, instead of becoming a swollen bag, the wing becomes a thin flat one. And thus it is that the little thick corrugated sac-like wings of the freshly emerged insect become the large, thin, flat wings of the imago.... The area of the wing of the imago of _Danais plexippus_ is 8.6 times that of the pupa. Now, as the wing of the young pupa has about 60 times the area of the wing in the mature larva, it is evident that in passing from the larval state to maturity the area of the wings increases more than 500 times.”

FIG. 150.—Diagrammatic reproduction of Fig. 149 showing the wing-germ
in its peripodal cavity (_p_): _h’drm_, hypodermis; _tr_, trachea;
_cta_, cuticula; _a_, anterior end.—After Mayer.
]

FIG. 151.—Section of the wing-germ, the upper and lower sides
connected by spindle-like hypodermic cells (_h_), forming the rods
of the adult wing; _mbr_, ground-membrane of Semper.—After Mayer.
]

_f._ The primitive origin of the wings

Farther observations are needed to connect the mode of formation of the wings in the holometabolous insects with the more primitive mode of origin seen in the hemimetabolous orders, but the former mode is evidently inherited from the latter. Pancritius remarks that the development of the rudiments of the wing in a hypodermal cavity is in the holometabolic insects to be regarded as a later inherited character, the external conditions causing it being unknown.

Fritz Müller was the first to investigate the mode of development of the wings of the hemimetabolic insects, examining the young nymphs of Termites. He regards the wings as evaginations of the hypodermis, which externally appear as thoracic scale-like projections, into which enter rather late in nymphal life tracheæ which correspond to the veins which afterward arise.

FIG. 152.—Rudimentary wing of young nymph of Blatta, with the five
principal veins developed.
]

The primitive mode of origin of the wings may, therefore, be best understood by observing the early stages of those insects, such as the Orthoptera and Hemiptera, which have an incomplete metamorphosis. If the student will examine the nymphs of any locust in their successive stages, he will see that the wings arise as simple expansions downward and backward of the lateral edges of the meso- and metanotum. In the second nymphal stage this change begins to take place, but it does not become marked until the succeeding stage, when the indications of veins begin to appear, and the lobe-like expansion of the notum is plainly enough a rudimentary wing.

Graber[26] thus describes the mode of development of the wings in the nymph of the cockroach:

“If one is looking only at the exterior of the process, he will
perceive sooner or later on the sides of the meso- and metathorax
pouch-like sacs, which increase in extent with the dorsal integument
and at the same time are more and more separated from the body.
These wing-covers either keep the same position as in the
flat-bodied Blattidæ, or in insects with bodies more compressed the
first rudiments hang down over the sides of the thorax. As soon as
they have exceeded a certain length, these wing-covers are laid over
on the back. However, if we study the process of development of the
wings with a microscope, by means of sections made obliquely through
the thorax, the process appears still more simple. The chief force
of all evolution is and remains the power of growth in a definite
direction. In regard to the skin this growth is possible in insects
only in this way; namely, that the outer layer of cells is increased
by the folds which are forced into the superficial chitinous skin.
These folds naturally grow from one moult to another in proportion
to the multiplication of the cells, and are not smoothed out until
after the moulting, when the outer resistance is overcome.

FIG. 153.—Partial metamorphosis of _Melanoplus femur-rubrum_,
showing the five nymph stages, and the gradual growth of the
wings, which are first visible externally in 3, 3_b_,
3_c_.—Emerton _del._
]

“As, however, the first wing-layers depend upon the wrinkling of the
general integument of the body through the increase in the upper
layer, the further growth of the wings depends in the later stages
upon the wrinkling of the epidermis of the wing-membrane even, which
fact we also observe under the microscope when the new wings drawn
forth from the old covers appear at first to be quite creased
together. These wing-like wrinkles in the skin are not empty
pouches, but contain tissues and organs within, which are connected
with the skin, as the fat of the body, the network of tracheæ,
muscles, etc. Alongside the tracheæ, running through the former
wing-pouches and accompanied by the nerves, there are canals through
which the blood flows in and out.

FIG. 154.—Stages in the growth of the wings of the nymph of _Termes
flavipes_: _A_, young; _a_, a wing enlarged. _B_, older nymph;
_b_, fore wing; _n_, a vein. _C_, wings more advanced;—_D_,
mature.
]

FIG. 155.—Wings of nymph of Psocus.
]

“After the last moult, however, when the supply of moisture is very
much reduced in the wing-pouches, which are contracted at the
bottom, their two layers become closely united, and afterward grow
into one single, solid wing-membrane.

“These thick-walled blood-tubes arising above and beneath the upper
and lower membrane of the wing are the veins of the wings; the
development of the creased wings in the pupa of butterflies is
exactly like that of cockroaches and bugs. The difference is only
that the folds of integument furnishing the wings with an ample
store of material for their construction reach in a relatively
shorter time, that is the space of time between two moults, the same
extent that they would otherwise attain only in the course of
several periods of growth in the ametabolous insects.”

FIG. 156.—Nymph of _Aphrophora permutata_, with enlarged view of the
wings and the veins: _pro_, pronotum; _sc_, mesoscutum; 1_ab_, 1st
abdominal segment.
]

Ignorant of Graber’s paper, we had arrived at the same result, after an examination of the early nymph-stages of the cockroach, as well as the locusts, Termites, and various Hemiptera. In all these forms it is plainly to be seen that the wings are simply expansions, either horizontal or partly vertical (where, as in locusts, etc., the body is compressed, and the meso- and metanota are rounded downwards), of the hinder and outer edge of the meso- and metanotum. As will be seen by reference to the accompanying figures, the wings are notal (tergal) outgrowths from the dorsal arch of the two hinder segments of the thorax. At first, as seen in the young pupal cockroach (Fig. 152) and locust (Fig. 153, also Figs. 154 and 156) the rudiments of the wings are continuous with the notum. Late in nymphal life a suture and a hinge-joint appear at the base of the wing, and thus there is some movement of the wing upon the notum; finally, the tracheæ are well developed in the wings, and numerous small sclerites are differentiated at the base of the wing, to which the special muscles of flight are attached, and thus the wings, after the last nymphal moult, have the power of flapping, and of sustaining the insect in the air; they thus become true organs of flight.

It is to be observed, then, that the wings in all hemimetabolous insects are outgrowths from the notum, and not from the flanks or pleurum of the thorax. There is, then, no structure in any other part of the body with which they are homologous.

FIG. 157.—Development of wings of Trichoptera: _A_, portion of
body-wall of young larva of Trichostegia; _ch_, cuticula, forming at
_r_ a projection into the hypodermis, _m_; _r_, and _d_, forming
thus the first rudiment of the wing. _B_, the parts in a larva of
nearly full size; _a_, _c_, _d_, _b_, the well-developed hypodermis
of the wing-germ separated into two parts by _r_, the penetrating
extension of the cuticula; _v_, mesoderm, _C_, wing-pad of another
Phryganeid freed from its case at its change to the pupa: _b_, _d_,
outer layer of the hypodermis (_m_) of the body-wall; _v_, inner
layer within nuclei.—After Dewitz, from Sharp.
]

The same may be said of the true Neuroptera, Trichoptera (Fig. 157), the Coleoptera, and the Diptera, Lepidoptera, and Hymenoptera. As we have observed in the house fly,[27] the wings are evidently outgrowths of the meso- and metanotum; we have also observed this to be most probably the case in the Lepidoptera, from observations on a Tortrix in different stages of metamorphosis. It is also the case with the Hymenoptera, as we have observed in bees and wasps;[28] and in these forms, and probably all Hymenoptera, the wings are outgrowths of the scutal region of the notum.

With these facts before us we may speculate as to the probable origin of the wings of insects. The views held by some are those of Gegenbaur, also adopted by Lubbock, and originally by myself.[29] According to Gegenbaur:

“The wings must be regarded as homologous with the lamellar tracheal
gills, for they do not only agree with them in origin, but also in
their connection with the body, and in structure. In being limited
to the second and third thoracic segments they point to a reduction
in the number of the tracheal gills. It is quite clear that we must
suppose that the wings did not arise as such, but were developed
from organs which had another function, such as the tracheal gills;
I mean to say that such a supposition is necessary, for we cannot
imagine that the wings functioned as such in the lower stages of
their development, and that they could have been developed by having
such a function.”

FIG. 158.—Changes in external form of the young larva of _Calotermes
rugosus_, showing, in _A_ and _B_, the mode of origin of the
wing-pads: _A_, newly hatched, with 9 antennal joints, × 8. _B_,
older larva, with 10 joints, × 8. _C_, next stage, with 11 joints, ×
8. _D_, larva, with twelve joints; the position of the parts of the
alimentary canal are shown: _v_, crop; _m_, stomach; _b_, “paunch”;
_e_, intestine; _r_, heart, × 16⁄3.—After Fritz Müller, from Sharp.
]

If we examine the tracheal gills of the smaller dragon-fly (Agrion), or the May-flies, or Sialidæ, or Perlidæ, or Phryganeidæ, we see that they are developed in a very arbitrary way, either at the end of the abdomen, or on the sternum, or from the pleurum; moreover, in structure they invariably have but a single trachea, from which minute twigs branch out;[30] in the wings there are five or six main tracheæ, which give rise to the veins. Thus, in themselves, irrespective of their position, they are not the homologues of the gills. The latter are only developed in the aquatic representatives of the Neuroptera and Pseudoneuroptera, and are evidently adaptive, secondary, temporary organs, and are in no sense ancestral, primitive structures from which the wings were developed. There is no good reason to suppose that the aquatic Odonata or Ephemerids or Neuroptera were not descendants of terrestrial forms.

To these results we had arrived by a review of the above-mentioned facts, before meeting with Fritz Müller’s opinions, derived from a study of the development of the wings of Calotermes (Fig. 158). Müller[31] states that “(1) The wings of insects have not originated from ‘tracheal gills.’ The wing-shaped continuations of the youngest larvæ are in fact the only parts in which air tubes are completely wanting, while tracheæ are richly developed in all other parts of the body.[32] (2) The wings of insects have arisen from lateral continuations of the dorsal plates of the body-segments with which they are connected.”

Now, speculating on the primary origin of wings, we need not suppose
that they originated in any aquatic form, but in some ancestral land
insect related to existing cockroaches and Termes. We may imagine
that the tergites (or notum) of the two hinder segments of the
thorax grew out laterally in some leaping and running insect; that
the expansion became of use in aiding to support the body in its
longer leaps, somewhat as the lateral expansions of the body aid the
flying squirrel or certain lizards in supporting the body during
their leaps. By natural selection these structures would be
transmitted in an improved condition until they became flexible,
_i.e._ attached by a rude hinge-joint to the tergal plates of the
meso- and metathorax. Then by continued use and attempts at flight
they would grow larger, until they would become permanent organs,
though still rudimentary, as in many existing Orthoptera, such as
certain Blattariæ and Pezotettix. By this time a fold or hinge
having been established, small chitinous pieces enclosed in membrane
would appear, until we should have a hinge flexible enough to allow
the wing to be folded on the back, and also to have a flapping
motion. A stray tracheal twig would naturally press or grow into the
base of the new structure. After the trachea running towards the
base of the wing had begun to send off branches into the rudimentary
structure, the number and direction of the future veins would become
determined on simple mechanical principles. The rudimentary
structures beating the air would need to be strengthened on the
front or costal edge. Here, then, would be developed the larger
number of main veins, two or three close together, and parallel.
These would be the costal, subcostal, and median veins. They would
throw out branches to strengthen the costal edge, while the branches
sent out to the outer and hinder edges of the wings might be less
numerous and farther apart. The net-veined wings of Orthoptera and
Pseudoneuroptera, as compared with the wings of Hymenoptera, show
that the wings of net-veined insects were largely used for
respiration as well as for flight, while in beetles and bees the
leading function is flight, that of respiration being quite
subordinate. The blood would then supply the parts, and thus
respiration or aëration of the blood would be demanded. As soon as
such expansions would be of even slight use to the insect as
breathing organs, the question as to their permanency would be
settled. Organs so useful both for flight and aëration of the blood
would be still further developed, until they would become permanent
structures, genuine wings. They would thus be readily transmitted,
and being of more use in adult life during the season of
reproduction, they would be still further developed, and thus those
insects which could fly the best, _i.e._ which had the strongest
wings, would be most successful in the struggle for existence. Thus
also, not being so much needed in larval life before the
reproductive organs are developed, they would not be transmitted
except in a very rudimentary way, as perhaps masses of internal
indifferent cells (imaginal discs), to the larva, being the rather
destined to develop late in larval and in pupal life. Thus the
development of the wings and of the generative organs would go hand
in hand, and become organs of adult life.[33]

=The development and structure of the tracheæ and veins of the wing.=—The so-called veins (“nervures”) originate from fine tracheal twigs which pass into the imaginal discs. A single longitudinal trachea grows down into the wing-germ (Fig. 147), this branch arising through simple budding of the large body-trachea passing under the rudiment of the wing.

FIG. 159.—Germ of a hind wing detached from its insertion, and
examined in glycerine: _i_, pedicel of insertion to the hypodermis;
_tr_, trachea; _b_, semicircular pad; _e_, enveloping membrane; _c_,
bundle of capillary tracheoles; the large tracheæ of the wing not
visible; they follow the course of the bundles of tracheoles.—After
Gonin.
]

Gonin states that before the tracheæ reach the wing they divide into a great number of capillary tubes united into bundles and often tangled. This mass of tracheæ does not penetrate into the wing-germ by one of its free ends, but spreading over about a third of the surface of the wing, separates into a dozen bundles which spread out fan-like in the interior of the wing. (Fig. 159). These ramifications, as seen under the microscope, are very irregular; they form here and there knots and anastomoses. They end abruptly in tufts at a little distance from the edge of the wing. A raised semicircular ridge (_b_) surrounds the base of the wing, and within this the capillaries are formed, while on the other side they are covered by a cellular layer.

Landois, he says, noticed neither the pedicel of the insertion of
the wing (_i_) nor the ridge (_b_). Herold only states that the
tracheæ pass like roots into the wing. Landois believed that they
formed an integral part of it. Dewitz and Pancritius used sections
to determine their situation.

Fig. 160 will illustrate Landois’ views as to the origin of the tracheæ and veins. _A_ represents the germ of a hind wing attached to a trachea; _c_ the elongated cells, in which, as seen at _B_, _c_, a fine tangled tracheal thread (_t_) appears, seen to be magnified at _C_. The cell walls break down, and the threads become those which pass through the centre of the veins.

FIG. 160.—Origin of the wings and their veins.—After Landois.
]

FIG. 161.-Section of the “rib” of a vein: _c_, cord; _b_, twig.—After
Schaeffer.
]

=The wing-rods.=—Semper discovered in transverse sections of the wings, what he called _Flügelrippen_; one such rib accompanying the trachea in each vein. He did not discover its origin, and his description of it is said to be somewhat erroneous. Schaeffer has recently examined the structure, remarking: “I have surely observed the connection of this cellular tube with the tracheæ. It is found in the base of the wing where the lumen of the tracheæ is much widened. I only describe the fully formed rib (_rippe_). In a cross-section it forms a usually cylindrical tube which is covered by a very thin chitinous intima which bears delicate twigs (Fig. 161). These twigs are analogous to the thickened ridge of the tracheal intima. I can see no connection between the branches of the different twigs. Through the ribs (_rippen_) extend a central cord (_c_) which shows in longitudinal section a clear longitudinal streaking. Semper regarded it as a nerve. But the connection of the tube with the trachea contradicts this view. I can only regard the cord as a separation-product of the cells of the walls.”

FIG. 162.—Parts of a vein of the cockroach, showing the nerve (_n_) by
the side of the trachea (_tr_); _c_, blood-corpuscles.—After
Moseley.
]

=Other histological elements.=—These are the blood-lymph, corpuscles, blood-building masses, and nerves. Schaeffer states that in the immature pupal wings we find besides the large tracheæ, which are more or less branched, and in the wing-veins at a later period, blood-corpuscles which are more or less gorged with nutritive material, and also the “balls of granules” of Weismann, which are perhaps the “single fat-body cells” detected by Semper. Schaeffer also states that into the hypodermal fold of the rudiments of the wings pass peculiar formations of the fat-body and tracheal system, and connected with the fat-body are masses of small cells which by Schaeffer are regarded as blood-building masses.

Fine nerves have also been detected within the veins, Moseley stating that a nerve-fibre accompanies the trachea in all the larger veins in the insects he has examined (Fig. 162), while it is present in Melolontha, where the trachea is absent.

LITERATURE ON THE WINGS

=Jurine, L.= Nouvelle méthode de classer les Hyménoptères et les
Dipterès. Genève, 1807, 4º pp. 319, 14 Pls.

—— Observations sur les ailes des Hyménoptères. (Mém. acad. Turin,
1820, xxiv, pp. 177–214.)

=Latreille, P. A.= De la formation des ailes des Insectes. (Mém. sur
divers sujets de l’histoire naturelle des Insectes, etc. Paris,
1819. Fasc. 8.)

—— De quelques appendices particuliers du thorax de divers Insectes.
(Mém. du Mus. d’Hist. nat., 1821, vii, pp. 1–21, 354–363.)

=Chabrier, J.= Essai sur le vol des insectes. (Mém. du Mus. d’Hist.
nat., 1820, vi, pp. 410–476; 1821, vii, pp. 297–372; 1822, viii, pp.
47–99, 349–403.) Separate, pp. 328, 13 Pls.

=Burmeister, Hermann.= Handbuch der entomologie, i, 1832, pp. 96–106,
263–267, 494–505.

—— Untersuchungen über die Flügeltypen der Coleopteren. (Abhandl. d.
naturf. Ges. Halle, 1854, ii, pp. 125–140, 1 Taf.)

=Romand, B. E. de.= Tableau de l’aile supérieure des Hyménoptères, 1
Pl. Paris, 1839. (Revue Zool., ii, pp. 339; Bericht von Erichson für
1839, pp. 54–56.)

=Lefebure, A.= Communication verbale sur la ptérologie des
Lépidoptères. (Annal. Soc. Ent. France, 1842, i, pp. 5–35, 3 Pls.
Also Revue Zool. Paris, 1842, pp. 52–58, 1 Pl.)

=Deschamps, B.= Recherches microscopiques sur l’organisation des
élytres des Coléoptères. (Ann. sc. nat., sér. 3, iii, 1845, pp.
354–363.)

=Heer, Oswald.= Die Insektenfauna der Tertiärgebilde von Oeningen und
Radaboj., 1847, 1. Teil, pp. 75–94.

=Newman, E.= Memorandum on the wing-rays of insects. (Trans. Ent. Soc.
London, ser. 2, iii, 1855, pp. 225–231.)

=Westwood, J. O.= Notes on the wing-veins of insects. (Trans. Ent.
Soc. London, ser. 2, iv, 1857, pp. 60–64.)

=Loew, H.= Die Schwinger der Dipteren. (Berlin, Entom. Zeitschr.,
1858, pp. 225–230.)

=Saussure, H. de.= Études sur l’aile des Orthoptères. (Ann. scienc.
nat., 5 sér. x, p. 161.)

=Schiner, J. R.= Ueber das Flügelgeäder der Dipteren. (Verhdl. k. k.
Zool.-bot., Ges. Wien, 1864, pp. 193–200, 1 Taf.)

=Hagen, H. A.= Ueber rationelle Benennung des Geäders in den Flügeln
der Insekten. (Stettin. Ent. Zeitung, 1870, xxxi, pp. 316–320, 1
Taf.)

—— Kurze Bemerkungen über das Flügelgeäder der Insekten. (Wiener
Entom. Zeit., 1886, v, pp. 311, 312.)

=Plateau, F.= Qu’est-ce que l’aile d’un insecte? (Stett. Ent. Zeit.
Jahrg. 32, 1871, pp. 33–42, 1 Taf. Journal d. Zool., ii, 1873, pp.
126–137.)

=Moseley, H. N.= On the circulation in the wing of _Blatta orientalis_
and other insects, etc. (Quart. Journ. Micr. Sc. 1871, xi, pp.
389–395, 1 Pl.)

=Roger, Otto.= Das Flügelgeäder der Kafer. Erlangen, 1875, 90 p.

=Rade, E.= Die westfalischen Donacien und ihre nachsten Verwandten. 3
Taf. (Vierter Jahresber. d. Westfal Prov.-Vereins f. Wiss. u. Kunst,
1876, pp. 52–87; Flügel, pp. 61–68.)

=Katter, F.= Ueber Inseckten, speziell Schmetterlingsflügel. (Entom.
Nachr., iv, 1878, pp. 279–281, 293–298, 304–309, 321–323.)

=Hofmann, Georg v.= Ueber die morphologische Deutung der
Insektenflügel. (Jahresber. d. akad.-naturwiss. Vereins, Graz, v
Jahrg., 1879, pp. 63–68.)

=Kolbe, H. J.= Das Flügelgeäder der Psociden und seine systematische
Bedeutung. (Stettin. Entom. Zeitung, 1880, pp. 179–186, 1 Taf.)

—— Die Zwischenraume zwischen den Punktstreifen der
punktiertgestreiften Flügeldecken der Coleoptera als rudimentare
Rippen aufgefasst. (Jahresber. zool. Sektion d. Westfal. Prov.-Ver.
f. Wiss. u. Kunst. Münster, 1886, pp. 57–59, 1 Taf.)

=Lee, A. Bolles.= Les balanciers des Diptères, leurs organes
sensifères et leurs histologie. (Recueil Zool. Suisse, i, 1885, pp.
363–392, 1 Pl.)

=Poppius, Alfred.= Ueber das Flügelgeäder der finnischen
Dendrometriden. 1 Taf. (Berl. Entom. Zeitschr., 1888, pp. 17–28.)

=Comstock, J. H.= On the homologies of the wing-veins of insects.
(American Naturalist, xxi, 1887, pp. 932–934.)

=Brauer, F.= Ansichten über die paläozoischen Insekten und deren
Deutung. (Annal. d. k. k. naturhist. Mus. Wien, Bd. i, 1886, pp.
86–126, 2 Taf.)

=Brauer, F., und J. Redtenbacher.= Ein Beitrag zur Entwicklung des
Flügelgeäders der Insekten. (Zool. Anz. 1888, pp. 443–447.)

=Redtenbacher, J.= Vergleichende Studien über das Flügelgeäder der
Insekten. 12 Taf. (Annalen d. k. k. naturhist. Hofmuseums zu Wien,
1886, i, pp. 153–231.)

=Schoch, G.= Miscellanea entomologica. I. Das Geäder des
Insektenflügels; II. Prolegomena zur Fauna dipterorum Helvetiae,
Wissenschaftl. (Beilage z. Programm d. Kantonsschule Zurich, 1889,
4º, 40 p.)

=Bondsdorff, A.= von. Ueber die Ableitung der Skulpturverhältnisse bei
den Deckflügeln der Coleopteren. (Zool. Anz., 1890, xiii Jahrg., pp.
342–346.)

=Spuler, Arnold.= Zur Phylogenie und Ontogenie des Flügelgeäders der
Schmetterlinge. (Zeitschr. wissens. Zool., liii, 597–646, 2 Taf.,
1892.)

Also the writings of Adolph, Bugnion, Calvert, Comstock, Diez, Giraud,
Gonin, Graber, Kellogg, Packard, Pratt, Scudder, Walsh.

_g._ Mechanism of flight

=Marey’s views on the flight of insects.=—As we owe more to Marey than to any one else for what exact knowledge we have of the theory of flight of insects, the following account is condensed from his work entitled “Movement.” The exceedingly complicated movements of the wings would lead us, he says, to suppose that there exists in insects a very complex set of muscles of flight, but in reality, he claims, there are only the two elevator and depressor muscles of each wing.[34] And Marey says that when we examine more closely the mechanical conditions of the flight of insects, we see that an upward and downward motion given by the muscles is sufficient to produce all these successive acts, so well coordinated with each other; the resistance of the air effecting all the other movements. He also refers to the experiments of Giraud which prove that the insect needs for flight a rigid main-rib and a flexible membrane.

FIG. 163.—The two upper lines are produced by the contacts of a
drone’s wing on a smoked cylinder. In the middle are recorded the
vibrations of a tuning-fork (250 vibrations per second) for
comparison with the frequency of the wing movements. Below are seen
the movements of the wing of a bee.—After Marey.
]

If we take off the wing of an insect, and holding it by the small joint which connects it with the thorax, expose it to a current of air, we see that the plane of the wing is inclined more and more as it is subjected to a more powerful impulse of the wind. The anterior nervure resists, but the membranous portion which is prolonged behind bends on account of its greater pliancy.

The wings of insects may be regarded simply as vibrating wires, and hence the frequency of their movements can be calculated by the note produced. Their movements can be recorded directly on a revolving cylinder, previously blackened with smoke, the slightest touch of the tip of the wing removing the black and exposing the white paper beneath; Fig. 163 was obtained in this way. By this method it was calculated that in the common fly the wings made 330 strokes per second, the bee 190, the Macroglossus 72, the dragon-fly 28, and the butterfly (_Pieris rapæ_) 9. Thus the smaller the species, the more rapid are the movements of the wings.

FIG. 164.—Appearance of a wasp flying in the sun: the extremity of the
wing is gilded.—After Marey.
]

The path or trajectory made by the tip of the wing is like a figure 8. Marey obtained this by fastening a spangle of gold-leaf to the extremity of a wasp’s wing. The insect was then seized with a pair of forceps and held in the sun in front of a dark background, the luminous trajectory shaping itself in the form of a lemniscate (Fig. 164).

To determine with accuracy the direction taken by the wing at
different stages of the trajectory, a small piece of capillary glass
tubing was blackened in the smoke of a candle, so that the slightest
touch on the glass was sufficient to remove the black coating and
show the direction of movement in each limb of the lemniscate. This
experiment was arranged as shown in Fig. 165. Different points on
the path of movement were tested by the smoked rod, and from the
track along which the black had been removed the direction of
movement was deduced. This direction is represented in the figure by
means of arrows.

FIG. 165.—Experiment to test the direction of movement of an insect’s
wing: _a_, _a′_, _b_, _b′_, different positions of the smoked rod.
]

=Theory of insect flight.=—“The theory of insect flight,” says Marey, “may be completely explained from the preceding experiments. The wing, in its to-and-fro movement, is bent in various directions by the resistance of the air. Its action is always that of an inclined plane striking against a fluid and utilizing that part of the resistance which is favorable to its onward progression.

“This mechanism is the same as that of a waterman’s scull, which as it moves backwards and forwards is obliquely inclined in opposite directions, each time communicating an impulse to the boat.”

The mechanism in the case of the insect’s wing is far simpler, however, than in the process of sculling, since “the flexible membrane which constitutes the anterior part of the wing presents a rigid border, which enables the wing to incline itself at the most favorable angle.”

“The muscles only maintain the to-and-fro movement, the resistance of the air does the rest, namely, effects those changes in surface obliquity which determine the formation of an 8–shaped trajectory by the extremity of the wing.”

FIG. 166.—Bee flying about in the chamber of the apparatus.—After
Marey.
]

Lendenfeld has applied photography to determine the position of the
wings of a dragon-fly, and Marey has carried chronophotography
farther to indicate the normal trajectory of the wing, and to show
the position in flight. Fig. 166 shows a bee in various phases of
flight. “The insect sometimes assumes almost a horizontal position,
in which case the lower part of its body is much nearer the
object-glass than is its head, and yet both extremities are equally
well defined in the photograph. The successive images are separated
by an interval of 1⁄20 of a second (a long time when compared to the
total time occupied by a complete wing movement, _i.e._ 1⁄190 of a
second). And hence it is useless to attempt to gain a knowledge of
the successive phases of movement by examining the successive
photographs of a consecutive series representing an insect in
flight. Nevertheless an examination of isolated images affords
information of extreme interest with regard to the mechanism of
flight.

“We have seen that owing to the resistance of the air the expanse of
wing is distorted in various directions by atmospheric resistance.
Now, as the oscillations during flight are executed in a horizontal
plane, the obliquity of the wing-surface ought to diminish the
apparent breadth of the wing. This appearance can be seen in Fig.
167. There is here a comparison between two Tipulæ: the one in the
act of flight, the other perfectly motionless and resting against
the glass window.

FIG. 167.—Illustration to show two Tipulæ, one of them remaining
motionless on the glass, and the other moving its limbs in
different directions, and setting its body at various
inclinations: the illustration only represents a small part of a
long series.—After Marey.
]

“The motionless insect maintains its wings in a position of vertical
extension; the plane is therefore at right angles to the axis of the
object-glass. The breadth of the wing can be seen in its entirety;
the nervures can be counted, and the rounding off of the extremities
of the wings is perfectly obvious. On the other hand, the flying
insect moves its wings in a horizontal direction, and owing to the
resistance of the air the expanse of the wings is obliquely
disposed, and only the projection of its surface can be seen in the
photograph. This is why the extremity of the wings appears as if it
were pointed, while the other parts look much narrower than normal.
The extent of the obliquity can be measured from the apparent
alteration in width, for the projection of this plane with the
vertical is the sine of the angle. From this it may be gathered that
the right wing (Fig. 168, third image) was inclined at an angle of
about 50° with the vertical, say 40° with the horizontal. This
inclination necessarily varies at different points of the trajectory
and must augment with the rapidity of movement; the obliquity
reaching its maximum in those portions of the wings which move with
the greatest velocity, namely, towards the extremities. The result
is that the wing becomes twisted at certain periods of the
movement.” (See the fourth image in Fig. 168.) The position of the
balancers seems to vary according to that of the wings. (Marey’s
Movement, pp. 253–257.)

FIG. 168.—Tipula in the act of flying, showing the various attitudes
of the wings and the position of the balancers.
]

=Graber’s views as to the mechanism of the wings, flight, etc.=—Although in reality insects possess but four wings, nature, says Graber, evidently endeavors to make them dipteral. This end is attained in a twofold manner. In the butterflies, bees, and cicadas, the four wings never act independently of each other, as two individual pairs, but they are always joined to a single flying plate by means of peculiar hooks, rows of claws, grooved clamps, and similar contrivances proceeding from the modified edges of the wings; indeed, this connection is usually carried so far that the hind wings are entirely taken in tow by the front, and consequently possess a relatively weak mechanism of motion. The other mode of wing reduction consists in the fact that one pair is thrown entirely out of employment. We observe this for instance in bugs, beetles, grasshoppers, etc.

In the meantime, then, we may not trust to appearances. As their
development indeed teaches us, the wings as well as the additional
members must be regarded as actual evaginations of the common
sockets of the body, and in order especially to refute the prevalent
opinion that these wing-membranes are void of sensation, it should
be remembered that Leydig has proved the existence, as well as one
can be convinced by experiment, of a nerve-end apparatus in certain
basal or radical veins of the wing-membrane, which is very extensive
and complicated, and therefore indicates the performance of an
important function, perhaps of a kind of balancing sense, and also
that these same insect wings, with their delicate membrane, are very
easily affected by different outside agents, as, for instance,
warmth, currents of air, etc.

Usually in their inactive or passive state the wings are held off horizontally from the body during flight, and are laid upon the back again when the insect alights; but an exception occurs in most butterflies and Neuroptera, among which the wing-joint allows only one movement round the oblique and long axis of the wings. From this cause, too, the insects just mentioned can unfold their wings suddenly.

FIG. 169.—Anterior part of a Cicada for demonstrating the mechanism of
the articulation of the fore wing: _a_, articular head; _b_,
articular pan, frog, or cotyla; _g_, elastic band; _c_, _d_, _e_,
system of elastic rods; _r__{1}, _r__{2}, 1st and 2d abdominal
segments. _HF_, hind wings.—After Graber.
]

The transition of the wings from the active to the resting condition seems to be by way of a purely passive process, which, therefore, usually gives no trouble to the insect. The wing being extended by the tractive power of the muscles, flies back, when this ceases, to its former or resting posture by means of its natural elasticity, like a spiral spring disturbed from its balance. The structure of this spring joint is very different, however.

It usually consists (Fig. 169) of two parts. The wing can move
itself up and down in a vertical plane by means of the forward
joint, and at the same time can rotate somewhat round its long axis,
because the chitinous part mentioned above is ground off after the
fashion of a mandrel.

The hinder joint, at a greater distance from the body, virtually
consists of a rounded piece (_a_) capitate towards the outside, and
of a prettily hollowed socket (_b_) formed by the union of the thick
ribs of the hind wings, which slides round the head joint when the
wings snap back upon the back. The mechanism which causes this
turning is, however, of a somewhat complicated nature. The most
instrumental part of it is the powerful elastic band (_g_) which is
stretched over from the hinder edge of the mesothorax (_R_{2}_)
towards that of the wings. This membrane is extended by the
expansion of the wings, and draws them towards the body as soon as
the contraction of the muscles relaxes. This closing band of the
wings is assisted by a leverage system consisting of three little
chitinous rods (_c_, _d_, _e_), which at its joining presses inwards
on the body on one side, and on the hinder edge and head-joint of
the wing on the other.

We must, however, lay great stress on a few more kinds of wing
support.

FIG. 170.—Mesothoracic skeleton of a stag beetle: _schi_, scutellum,
on each side of which is the articulation of the fore wing (_V_),
consisting of two small styliform processes (_v_, _h_) of the base
of the wing; _za_, tooth which fits into the cavity of the
wing-lock (_gr_); _l_, edge of the right wing, passing into the
corresponding groove (_fa_) of the left; _Di_, diaphragm for the
attachment of the tergal muscle of the metasternum; _Di_{1}_ (not
explained by author); _Ka_, acetabulum of the coxa (_Hü_); _Se_,
chitinous process for the attachment of the coxal muscle; _Fe_,
femur; _Sch_, tibia; _B_{2}_, sternum.—After Graber.
]

The wing-cases of beetles at their return from flight are joined
together like the shells of a mussel on the inside as well as to the
wedge-shaped plate (Fig. 170, _schi_) between their bases. There is
even a kind of clasp at hand for this purpose. The base of the wing,
that is, bears a pair of tooth-like projections (_za_), which fit
into the corresponding hollows of the little plate.

The commissure arising from the joining of the inner edges is
characteristic. Usually the wings on both sides interlock by means
of a groove, as in stag-beetles, but sometimes even, as in Chlamys,
after the manner of two cog-wheels, so that we have here also an
imitation of the two most prevalent methods which the cabinet-maker
uses in joining boards together.

The act of folding the broad hind wings among beetles is not less
significant than the arrangement of the fore wing. If we forcibly
spread out the former in a beetle which has just been killed and
then leave it to its own resources again, we observe the following
result: According to its peculiar mode of joining, the costal vein
on the fore edge approaches the mid or discoidal vein of the basal
half as well as the distal half of the wing, whence arises a
longitudinal fold which curves in underneath. Then the distal half
snaps under like the blade of a pocket knife and lies on the plane
of the costal edge of the wing, while it also draws after it the
neighboring wing-area. The soft hinder-edge portion turns in
simultaneously when this wing-area remains fixed to the body while
the costal portion is moving towards the middle line of the body.

The wing-membranes of almost all insects have, moreover, the
capability of folding themselves somewhat, and this power of
extending or contracting the wing-membrane at will is of great
importance in flight.

Yes, but how is the folded wing spread out again? The fact may be
shown more simply and easily than one might suppose, and may be most
plainly demonstrated even to a larger public by making an artificial
wing exactly after the pattern of the natural one, in which bits of
whalebone may take the place of veins and a piece of india rubber
the membrane spread out between them. The reader will be patient
while we just explain to him the act of unfolding of the membranous
wing of the beetle. The actual impulse for this unfolding is due to
the flexor muscles which pull on, and at the same time somewhat
raise the vein on the costal edge. By this means the membranous fold
lying directly behind the costal vein is first spread out. But since
this fold is connected with the longitudinal fold of the distal end
of the wing which closes like a blade, the wing-area last mentioned
which is attached to the middle fold of the wing by the elastic
spring-like diagonal vein becomes stretched out. The hinder rayed
portion adjacent to the body is, on the other hand, simply drawn
along when the wing stands off from the body.

In order to properly grasp the mechanism of the insect wing we must
again examine its mode of articulation to the body somewhat more
accurately.

FIG. 171.—Longitudinal section through a Tipula: _a_, mouth; _an_,
antenna; _k_{3}_, maxillary palpus; _ol_, labrum; _oG_, brain;
_uG_, subœsophageal ganglion; _BG_, thoracic ganglion; _schl_,
œsophagus; _mD_, digestive canal; _Ov_, ovary; _vF_, fore wing;
_sch_, halter; _lm_, longitudinal—_b-r_, lateral muscles.—After
Graber.
]

If we select the halteres of a garden gnat (Tipula) at the moment of
extension, we shall find them to be formed almost exactly after the
pattern of our oars, since the oblong oar-blade passes into a
longitudinal handle. The pedicel of the balancer is formed by the
thick longitudinal primary veins of the wing-membrane. This pedicel
(Fig. 171) is implanted in the side of the thorax in such a manner
that the wing may be compared to the top of a ninepin. One may
think, and on the whole it is actually the fact, that the stiff
pedicel of the wing is inserted in the thoracic wall, and that a
short portion of it (Fig. 172), projects into the cavity of the
thorax. It is true there is no actual hole to be found in the
thoracic wall, as the intermediate space between the base or pedicel
of the wing and the aperture in the thorax is lined with a thin
yielding membrane, on which the wing is suspended as on an
axle-tree. According to this, therefore, the insect wing, as well as
any other appendage of arthropods, acts as a lever with two arms.
The reader can then conjecture what may be the further mechanism of
the wing machine. We only need now two muscles diametrically opposed
to each other and seizing on the power arm of the wing, one of which
pulls down the short wing arm, thereby raising the oar, while the
other pulls up the power arm. And indeed the raising of the wing
follows in the manner indicated, since a muscle (_hi_) is attached
to the end of the wing-handle (_a_) which projects freely into the
breast cavity by the contraction of which the power arm is drawn
down.

FIG. 172.—Scheme of the flying apparatus of an insect: _mnl_,
thoracic walls; _ab_, wings; _c_, pivot; _d_, point of insertion
of the depressor muscle of the wing (_kd_);—_a_, that of the
elevator of the wing (_ai_); _rs_, muscle for expanding,—_ml_, for
contracting, the walls of the thorax.—After Graber.
]

FIG. 173.—Muscles of the fore wing of a dragon-fly (_an_, _ax_),
exposed by removing the thoracic walls: _h_{1}_, _h_{2}_,
elevators,—_s_{1}_-_s_{5}_, depressors, of the wings (_s_{1}_,
_s_{2}_, rotators).—After Graber.
]