[Illustration:

FIG. 121.—_A_, track curves of two of the tibial spines of the left,

middle legs of a stag-beetle. Natural size. _B_, the same

enlarged; _fg_, the longitudinal axis of the trunk; _cd_ and _ab_,

the active curve passing inward,—_bc_ and _de_, the passive going

outward. _C_, two curves described by the left hind legs; in this

case, the curves are not inwards or backwards, but partly directly

inward (_b_), and in part obliquely forwards (_a_).

]

“The diversions and modifications in the course of the active step,

as furnished by the moving factor of the remaining legs, are already

clearly illustrated by the curves shown by the joints of the hind

tibia of a May-beetle (Fig. 120) and a stag-beetle (Fig. 121, _c_).

The actual faint line in this case does not run from the front

toward the back, as would correspond to the active leg-motion, but

either directly inward (Fig. 121, _cb_), or even somewhat to the

front. In the May-beetles, and even more in the running

garden-beetle, the curves of the hind legs present themselves as

screw-like lines (Fig. 122, _l_{3}_), while the scrawling of the

remaining members (_l_{1}_, _l_{2}_) is much simpler.

“Inasmuch as we now have a cursory knowledge of the movements made

by each individual leg for itself,—movements, however, which plainly

occur very differently according to the structure of these

appendages,—the question now is of the combined play, the total

effect of all the legs taken together, and therefore of the walk and

measure of the united work of the foot.

“In opposition to the caterpillars and many other crawling animals

which extend their legs in pairs and really swing them by the

worm-like mode of contraction of the dermo-muscular tube, the legs

of fully grown insects are moved in the contrary direction and in no

sense in pairs, but alternately—or, more strictly speaking, in a

diagonal direction.

“For an examination of the gait of insects, we choose, for obvious

reasons, those which have very long legs and which at the same time

are slow walkers.

“Insects may be called ‘double-three-footed,’ from the manner in

which they alternately place their legs. There are always three legs

set in motion at the same time, or nearly so, while in the meantime

the remaining legs support the body, after which they change places.

[Illustration:

FIG. 122.—The same by the left fore (_l_{1}_), middle (_l_{2}_), and

hind, leg (_l_{3}_) of a Carabus. Natural size.

]

[Illustration:

FIG. 123.-Tracks of a _Blaps mortisaga_ marked by the differently

painted tibial points: ●, tracks of fore, —○, middle, —/, hind

leg. Natural size.

]

[Illustration:

FIG. 124.—Tracks of _Necrophorus vespilio_. Natural size.

]

“To be more exact, it is usually thus: At first (Fig. 118) the left

fore leg (_L_{1}_) steps out, then follows the right middle leg

(_R_{2}_), and the left hind leg (_L_{3}_). Then while the left fore

leg begins to retract and thus make the backward movement, the right

fore leg is extended, whereupon the left middle leg and the right

hind leg are raised in the same order as the first three feet.”

Graber[23] painted the feet of beetles and let them run over paper,

and goes on to say:

“Let us first pursue the tracks of the Blaps, for example (Fig.

123). Let the insect begin its motion. The left fore leg stands at

_a_, the right middle leg at β, and the left hind leg at _c_. The

corresponding number of the other set of three feet at α, _b_, γ. At

the first step the three feet first mentioned advance to _a′_β′_c′_,

the second set on the other hand to α′_b′_γ′. Thereby the tracks

made by the successive steps fall quite, or almost quite, on each

other, as appear also in the tracks of a burying beetle (Fig. 124).

“As the fore legs are directed forward and the hind legs backward,

while the middle legs are placed obliquely, the reason of the more

marked impressions of the latter is evident.

“The highest testimony to the precise exactitude and accuracy of the

walking mechanism of insects is furnished by the fact that in most

insects, and particularly in those most fleet of foot, which,

whether they are running away or chasing their prey, must be able to

rely entirely upon their means of locomotion;—the fact, we say, that

whether they desire to move slowly or more quickly, the distances of

the steps, measured by the length as well as by the cross-direction,

hardly differ a hair’s breadth from one another, and this is also

the case when the tarsi are cut off and the insects are obliged to

run on the points of their heels (tibiæ).

“Thence, inasmuch as the trunk of insects is carried by two legs and

by one on each side alternately, it may surely be concluded _a

priori_ that when walking it is inclined now to the right and now to

the left, and that the track, too, which is left behind by a precise

point of the leg, can in no wise be a straight line; and in reality

this is not the case.

“A plainly marked regular curve, which approaches a sinuous line, as

seen in Fig. 125, is often obtained by painting many insects, for

example Trichodes, Meloë, etc., which, when running, either bring

the end of their hind body near to the ground or into contact with

it.

[Illustration:

FIG. 125.—Tracks of Trichodes; the middle sinuous line is made by

the tip of the abdomen. Natural size.

]

[Illustration:

FIG. 126.—Tracks of another insect which, in running, can only use

three legs (_r_{1}_, _l_{4}_, _r_{3}_) which become indicated

differently from normal conditions. Natural size.

]

[Illustration:

FIG. 127.—The same of an insect crossing over a surface inclined 30°

from the horizon, whereby the placing of the feet becomes changed.

Natural size.—This and Figs. 120–126 after Graber.

]

“The locomotive machine of insects may be called, to a certain

extent, a double set of three feet each, as most insects, and

particularly those provided with a broad trunk, are able to balance

themselves with one of these two sets of feet, and indeed when

walking, as well as when standing still, can move about even better

with one set of these feet than with four legs. In the latter case,

that is, if one cuts off a pair of legs from an insect, the trunk

can balance itself only with extreme difficulty, and there is

therefore little prospect that insects will ever become four-footed.

“But if one compels insects to run on three legs, he will thus make

the interesting discovery that to make up the deficiency they place

the remaining feet and bring them to the ground somewhat differently

than when the second set of feet is active. Figs. 124 and 126 may be

compared for this purpose. The former shows the footprints of a

burying beetle running with all six legs, the latter the track of

the same insect, which, however, has at its disposal only the right

fore leg, the left middle leg, and the right hind leg. One may

plainly see here that the track of the hind leg on the right side

(_r_{3}_) approaches the track of the middle leg on the left side,

and then further, that the _right fore leg_ (_r_{1}_) _steps out

more to the right to make up for the deficiency of the middle leg_.

“A similar adaptation of the position of the legs, which is entirely

dependent on the choice of the insect, may also be observed there,

if one compels insects which are not provided with corresponding

adhesive lobes to run away over crooked surfaces. Fig. 123 shows the

footprints of a Blaps when running upon a horizontal plane. Fig.

127, on the contrary, shows the tracks of the legs when going

diagonally over a gradually inclined surface. Here, also, the insect

holds on with his fore and middle legs (_r_{1}_, _r_{2}_) stretched

upward, whereby also the impressions on both sides come to lie

farther apart than in the normal mode of walking.

“It will not surprise the reader who is familiar with the gait of

crabs, to hear that many insects also understand the laudable art of

going backward, wherein the hind legs simply change places with the

fore legs.

“The jumping motion of insects may be best studied in grasshoppers.

When these insects are preparing for a jump, they stretch out the

upper thigh horizontally, clap the tibiæ together, and also retract

the foot-segment. After a slight pause for rest, during which they

are getting ready for the jump, they then jerk the tibiæ suddenly

backward and against the ground with all their strength by means of

the extensor muscles.”

The correctness of Graber’s views has been confirmed by Marey by

instantaneous photographs (Figs. 128, 129).

=Locomotion on smooth surfaces.=—How flies and other insects are able to

walk up, or run with the body inverted, on hard surfaces has been lately

discovered by Dewitz, Dahl, and others. All authors are agreed that this

power is due to the presence of the specialized empodium of each tarsus.

Dewitz confirmed the opinion of Blackwell, that a glutinous liquid is

exuded from the apices of the tenent hairs which fringe the empodium. By

fastening insects feet uppermost on the under side of a covering glass

which projects from a glass slide, the hairs which clothe the empodia of

the foot of a fly (_Musca erythrocephala_) may be seen to be tipped with

drops of transparent liquid. On the leg being drawn back from the glass,

a transparent thread is drawn out, and drops are found to be left on the

glass. In cases where these hairs are wanting, as in the Hemiptera, the

adhesive fluid exudes directly from pores in the foot. In the beetles

(_Telephorus dispar_) and other insects the tenent hairs on the foot end

in sharp points, below which are placed the openings of the canals. The

glands, Dewitz states, are chiefly flask-shaped and unicellular,

situated in the hypodermis of the chitinous coat; each gland opening

into one of the hairs (Fig. 108); they are each invested by a

structureless tunica propria, and contain granular protoplasm, a nucleus

placed at the inner side, and a vesicle, prolonged into a tube which,

traversing the neck of the gland, is attached to the root of the hair;

the vesicle receiving the secretion. Each gland is connected with a fine

nerve-twig, and secretion is probably voluntary. Among the tenent hairs

of the empodium are others which must be supplied with a nerve, forming

tactile hairs, as they each proceed from a unicellular ganglion (Fig.

108, _n″_). The secretion is forced out of the gland by the contraction

of the protoplasm, Dewitz having seen the secretion driven out from the

internal vesicle into its neck.

[Illustration:

FIG. 128.—The walk of an orthopterous insect: series to be followed

from right to left.—After Marey.

]

[Illustration:

FIG. 129.—Beetle walking: series to be followed from left to

right.—After Marey.

]

[Illustration:

FIG. 130.—_A_, end of an adhesive hair of a weevil (Eupolus): _i′_,

canal: _i‴_, its external opening at the end of the hair. _B_, end

of a similar hair of Telephorus with drops of the secretion.—After

Dewitz.

]

In the spherical last tarsal joint of Orthoptera (Fig. 109), which

is without these tenent hairs, nearly all the cells of the

hypodermis are converted into unicellular glands, each of which

sends out a long, fine, chitinous tubule, which is connected with

its fellows by very fine hairs and is continuous with the chitinous

coat of the foot and opens through it. The sole of the foot is

elastic and adapts itself to minute inequalities of surfaces, while

the anterior of each tarsal joint is almost entirely occupied by an

enlargement of the trachea, which acts on the elastic sole like an

air chamber, rendering it tense and at the same time pliant. Dewitz

adds that the apparatus situated on the front legs of the male of

_Stenobothrus sibiricus_ (Fig. 131) must have the function of

causing the legs to adhere closely to the female by the excretion of

an adhesive material. The hairs of the anterior tarsi of male Carabi

also appear to possess the power of adhesion. In the house-fly the

empodia seem to be only called into action when the insect has to

walk on vertical smooth surfaces, as at other times they hang

loosely down.

Burmeister observed the use of a glutinous secretion for walking in

dipterous larvæ, and Dewitz found that the larva of a Musca used for

this purpose a liquid ejected from the mouth. The larvæ of another

fly (_Leucopis puncticornis_) perform their loop-like walk by

emitting a fluid from both mouth and anus. A Cecidomyia larva is

able to leap by fixing its anterior end by means of an adhesive

fluid. The larva of the leaf-beetle, Galeruca, moves by drawing up

its hinder end, fixing it thus, and carrying the anterior part of

the body forward with its feet until fully extended, when it breaks

the glutinous adhesion. The abdominal legs of some saw-fly larvæ

have the same power.

Dahl could not detect in the foot of the hornet (_Vespa crabro_) any

space which could be considered as a vacuum.

[Illustration:

FIG. 131.—_Stenobothrus sibiricus_ pairing: _A_. the ♂, fore tarsus

(_t_) greatly enlarged; _ar_, arolia; _p_, pulvillus.—After

Pagenstecher.

]

Simmermacher states that in most cases of climbing beetles the tubular

tenent hairs pour out a secretion (Figs. 133, 134), “and it is probable

that we have here to do with the phenomena not of actual attachment by,

as it were, gluing, but of adhesion; the orifice of the tubes is divided

obliquely, and the tubes are, at this point, extremely delicate and

flexible, so as to adhere by their lower surface; in this adhesion they

are aided by the secreted fluid.” In the case of the Diptera he does not

accept the theory by which the movement of the fly along smooth surfaces

is ascribed to an alternate fixation and separation, but believes in a

process of adhesion, aided by a secretion, as in many Coleoptera. (In

the Cerambycidæ there is no secretion, and the tubules are merely

sucking organs, like those observed in the male Silphidæ.) “The

attaching lobes, closely beset with chitinous hairs, are enabled, in

consequence of the pressure of the foot, to completely lie along any

smooth surface; this expels the air beneath the lobes, which are then

acted on by the pressure of the outer air.” (Journ. Roy. Micr. Soc.,

1884, p. 736.) Another writer (Rombouts) thinks this power is due to

capillary adhesion.

[Illustration:

FIG. 132.—Fore leg of ♂ Dyticus, under side, with sucker, formed of 3

enlarged tarsal joints: with a small cupule highly magnified. ×

120.—After Miall.

]

The action of the pulvillus and claws when at rest or in use by the

honey-bee is well shown by Cheshire (Fig. 135, _B_). In ascending a

rough surface, “the points of the claws catch (as at _B_) and the

pulvillus is saved from any contact, but if the surface be smooth, so

that the claws get no grip, they slide back and are drawn beneath the

foot (as at _A_), which change of position applies the pulvillus, so

that it immediately clings. It is the character of the surface, then,

and not the will of the bee, that determines whether claw or pulvillus

shall be used in sustaining it. But another contrivance, equally

beautiful, remains to be noticed. The pulvillus is carried folded in the

middle (as at _C_, Fig. 105), but opens out when applied to a surface;

for it has at its upper part an elastic and curved rod (_cr_, Figs. 105

and 135), which straightens as the pulvillus is pressed down; _C_ and

_D_, Fig. 135, making this clear. The flattened-out pulvillus thus holds

strongly while pulled, by the weight of the bee, along the surface, to

which it adheres, but comes up at once if lifted and rolled off from its

opposite sides, just as we should pull a wet postage stamp from an

envelope. The bee, then, is held securely till it attempts to lift the

leg, when it is freed at once; and, by this exquisite yet simple plan,

it can fix and release each foot at least twenty times per second.”

(Bees and Bee-keeping, p. 127.)

[Illustration:

FIG. 133.—Cross-section through a tarsal joint of fore leg of Dyticus,

♂, showing the stalked chitinous suckers (_s_), with a marginal

bristle on each side: _t_, trachea; _a_, an isolated tubule or

sucker of Loricera,—_b_, of Chlænius,—_c_, of Cicindela; _d_, two

views of one of _Necrophorus germanicus_, ♂.

]

[Illustration:

FIG. 134.—Section through the tarsus of a Staphylinid beetle; the

glandular or tenent hairs arising from chitinous processes. _A_,

section through the tarsal joint of the pine weevil, _Hylobius

abietis_, showing the crowded, bulbous, glandular, or tenent hairs

arising from unicellular glands.—This and Fig. 133 after

Simmermacher.

]

Ockler divides the normal two-clawed foot into three subtypes: (1) with

an unpaired median empodium; (2) with two outer lateral adhesive lobes;

(3) with two adhesive lobes below the claws; the latter is the chief

type and forms either a climbing or a clasping foot. The amount of

movement possessed by the claws is limited, and what there is, is

effected by means of an elastic membrane and the extensor plate (Fig.

110). The “extensor sole” which is always present in insects with an

unpaired median fixing or adhesive organ (empodium) is to be regarded as

a modification of the extensor seta. The extensor plate is peculiar to

an insect’s foot. Ockler states that the so-called “pressure plate” of

Dahl is only a movably articulated, skeletal, supporting plate for the

median fixing lobule.

[Illustration:

FIG. 135.—Honey-bee’s foot in the act of climbing, showing the

automatic action of the pulvillus, × 30: _A_, position of foot in

climbing on a slippery surface, or glass; _pv_, pulvillus; _fh_,

tactile hairs; _un_, unguis; _t_, last tarsal joint. _B_, position

of foot in climbing rough surface. _C_, section of pulvillus just

touching flat surface; _cr_, curved rod. _D_, the same applied to

the surface.—After Cheshire.

]

=Climbing.=—In certain respects the power of climbing supplies the want

of wings, and even exists often in house-flies among which there is

shown a many-sided motion that is quite unheard of in other groups of

insects.

The best climbers are obviously those insects which live on trees and

bushes, as, for example, longicorn beetles and grasshoppers. These may

be accurately called the monkeys of the insect kind, even if their

movements take place less gracefully, and indeed rather stiffly and

woodenly. We already know what are the proper climbing organs; that is,

the sharp easily movable claws on the foot. With the help of these claws

certain insects, May-beetles for example, can hang upon one another like

a chain; indeed, bees and ants in this manner bind themselves together

into living garlands and bridges. There are still added to the chitinous

hooks flaps and balls of a sticky nature, by help of which likewise the

insects glue themselves together. To facilitate the spanning of still

thicker twigs, the climbing foot of insects has a greater movability

even than when it only serves as a sole. (Graber.)

=The mode of swimming of insects.=—To study the swimming movements of

insects, let us examine a Dyticus. It will appear, as Graber states, to

be wonderfully adapted to its element.

“The body resembles a boat. There is nowhere a projecting point or a

sharp corner which would offer unnecessary resistance to motion;

bulging out in the middle and pointed at the end, it cuts through

the resistance of the water like a wedge. The movable parts, the

oars, seem to be as well fitted for their purpose as the burden to

be moved by them. That the hind legs must bear the brunt of this

follows from their position exactly in the middle of the body, where

it is widest. In other insects also these legs are used for the same

purpose as soon as the insects are put in the water. But the

swimming legs of water-beetles are oars of quite peculiar

construction. _They are not turned about in the coxæ, as are other

legs, but at the foot-joint._ The coxa, namely, has grown entirely

together with the thoracic partition. The muscles we have mentioned,

exceeding in strength all the soft parts taken together, take hold

directly of the large wing-shaped tendons of the upper thigh, and

extend and retract the leg in one of the planes lying close to the

abdominal partition. The foot forms the oar, however. It is very

much lengthened and still more widened, and can be turned and bent

in by separate muscles in such a way that in the passive movement,

that is, the retraction, the narrow edge is turned to the fore, and

therefore to the medium to be dislodged; however, as soon as the

active push is to be performed and the leg is extended with greater

force, it cuts down through the water with its whole width. These

effective oar-blades are still considerably enlarged by the hairs

arising on the side of the foot, which spread out at the decisive

moment.

“Every one knows that the oar-blades of swimming beetles always go

up and down simultaneously and in regular time. On the other hand,

as soon as one puts a Dyticus on the dry land, _i.e._ on an

unyielding medium, it uses its hind legs entirely after the manner

of other land insects; that is, they are drawn in and extended again

_alternately_, as takes place clearly enough from the footsteps in

Fig. 119, _A_. We learn from this that water insects have not yet,

from want of practice, forgotten the mode of walking of land

insects.

“The forcing up of the water as a propelling power is added to the

repulsion produced by the strong strokes of the oars. If the beetle

stood up horizontally in the water, he would be lifted up.

“As the trunk, however, assumes an oblique position when the insect

wishes to swim, one can then imagine the driving up of the water as

being divided into two forces, one of which drives the body forward

in a horizontal direction, while the other, that is, the vertical

component, is supplied by the moving of the oars. The swimming

insect is thus, as it were, a snake flying in the water.

“The long streamer-like hind legs of many water-bugs, for example

Notonecta, approach more nearly our artificial oars. These legs are

turned out from the bottom.

“There is no doubt but that the legs of insects, as regards the

many-sidedness and exactitude of their locomotive actions, place the

similar contrivances of other animals far in the shade. We shall be

forced to admire these ingenious levers still more, however, when we

take into consideration their energy and strength. That the force

with which the locomotive muscles of insects is drawn together is

enormous compared with that of vertebrates, we may learn if we try

to subdue the rhythmical movements of the thorax of a large

butterfly by the pressure of our finger or to open against the

insect’s will the closed jumping leg of a grasshopper, or the

fossorial shovel of a mole-cricket.”

LITERATURE ON LEGS AND FEET

=MacLeay, W. S.= On the structure of the tarsus in the tetramerous

Coleoptera of the French entomologists. (Trans. Linn. Soc. London,

xv, 1825, pp. 63–73.)

=Speyer, O.= Untersuchung der Beine der Schmetterlinge. (Isis, 1843,

pp. 161–207, 243–264.)

=Pokorsky Joravko, A. von.= Quelques remarques sur le dernier article

du tarse des Hyménoptères. (Bull. Soc. imp. Natur. Moscou, 1844,

xvii, pp. 140–159. Ref. in Isis, 1848, v, p. 347.)

=Rossmassler, E. A.= Das Bein der Insekten. (Aus der Heimath, 1860, 3

kap., pp. 327–334, Fig.)

=West, Tuffen.= The foot of the fly; its structure and action;

elucidated by comparison with the feet of other insects, etc. Part

I. (Trans. Linn. Soc. London, xxiii, 1861, pp. 393–421, 1 Pl.)

=Sundevall, C.= On insektenas extremiteter samt deras hufoud och

munddelar. (Kongl. Vetenskaps Akad. Handlingar. iii, Nr. 9, 1861.)

=Lindemann, C.= Notizen zur Lehre vom ausseren Skelete der Insekten

(Gelenke und Muskeln der Füsse). 1 Taf. (Bull. Soc. imp. d. Natur.

Moscou, xxxvii, 1864, pp. 426–432.)

=Liebe, O.= Die Gelenke der Insekten. Chemnitz, 1873. 4º. 1 Taf.

=Canestrini, J.= Ueber ein sonderbares Organ der Hymenopteren. (Zool.

Anzeiger, 1880, pp. 421, 422.)

=Dahl, F.= Beiträge zur Kenntnis des Baues und der Funktionen der

Insektenbeine. (Archiv f. Naturgesch. 1 Jahrg., 1884, pp. 146–193, 3

Taf. Sep., 48 pp. Vorlauf. Mitteil, in Zool. Anz., 1884, pp. 38–41.)

=Langer, K.= Ueber den Gelenkbau bei den Arthrozoen. Vierter Beitrag

zur vergleichenden Anatomie und Mechanik der Gelenke. (Denkschriften

der Akad. d. Wissensch. Wien, xviii, Bd. Physikal.-mathem. Classe,

pp. 99–140. 3 Taf.)

=Graber, Vitus.= Ueber die Mechanik des Insektenkörpers. (Biolog.

Centralbl., iv, 1884, pp. 560–570.)

—— Die ausseren mechanischen Werkzeuge der Tiere, ii Teil. Wirbellose

Tiere, 1886, pp. 175–182, 208–210.

=Dewitz, H.= Ueber die Fortbewegung der Tiere an senkrechten glatten

Flächen vermittelst eines Sekretes. 3 Taf. (Pflüger’s Archiv f. d.

ges. Physiologie, xxxiii, 1884, pp. 440–481.)

=Ockler, A.= Das Krallenglied am Insektenfuss. (Archiv f. Naturgesch.,

1890, pp. 221–262, 2 Taf.)

LITERATURE OF LOCOMOTION (WALKING, ETC.)

=Carlet, G.= Sur le mode de locomotion des chenilles. (Compt, rend.

Acad. Paris, 1888, cvii, pp. 131–134. Naturwiss. Rundschau, iii

Jahrg., 1888, No. 42, p. 543.)

—— De la marche d’un insecte rendu tetrapode par la suppression d’une

paire de pattes. (Ibid., pp. 565, 566.)

—— Sur la locomotion des insectes et des arachnides. (Ibid., 1879, T.

89, pp. 1124, 1125.)

—— Ueber den Gang eines vierfüssig gemachten Insekts. (Naturwiss.

Rundschau, viii Jahrg., 1888, pp. 666–667; Compt. rend. 1888, cvii.)

=Demoor, J.= Recherches sur la marche des insectes et des arachnides.

Étude experimentale d’Anatomie et de Physiologie comparées. (Archiv

de Biologie, Liège, 1880, 42 pp. 3 Pls.)

—— Ueber das Gehen der Arthropoden mit Berücksichtigung der

Schwankungen des Körpers. (Compt. rend. Acad. d. Sc. Paris, 1890,

cxi, pp. 839–840.)

=Osten-Sacken, C. R. von.= Ueber das Betragen des kalifornischen

flügellosen Bittacus (apterus McLachl.). (Wiener Ent. Zeit., 1882,

pp. 123.)

=Dixon, H. H.= Preliminary note on the walking of some of the

Arthropoda. (Proc. R. Dublin Soc. vii, pp. 574–578, 1892. Also

Nature, 1897.)

Also the works of Graber, Marey, Cheshire, etc.

LITERATURE OF WALKING ON SMOOTH SURFACES

=Blackwell, J.= Remarks on the pulvilli of insects. (Trans. Linn. Soc.

London, xvi, 1831, pp. 487–492, 767–770.)

=Lowne, B. T.= On the so-called suckers of Dytiscus and the pulvilli

of insects. (Trans. Roy. Micr. Soc., pp. 267–271, 1871, 1 Pl.)

=West, Tuffen.= On certain appendages to the feet of insects

subservient to holding or climbing. (Journ. of the Proceed. Linn.

Soc. London, Zoölogy, vi, 1862, pp. 26–88.)

=Dewitz, H.= Ueber die Fortbewegung der Tiere an senkrechten, glatten

Flächen vermittelst eines Sekrets. (Pflüger’s Archiv f. d. ges.

Physiologie, xxxiii, 1884, pp. 440–481. 3 Taf. Also Zool. Anzeiger,

1884, pp. 400–405.)

—— Wie ist es den Stubenfliegen und anderen Insekten möglich, an

senkrechten Glaswanden emporzulaufen. (Sitzungsb. Ges. naturf.

Freunde zu Berlin, 1882, pp. 5–7.)

—— Weitere Mitteilungen über den Klettern der Insekten (Ibid., 1882,

pp. 109–113).

—— Die Befestigung durch einen klebenden schleim beim springen gegen

senkrechte Flächen. (Zool. Anzeiger, 1883, pp. 273, 274.)

—— Ueber die Wirkung der Haftlappchen toter Fliegen. (Ent. Nachr., x

Jahrg., 1884, pp. 286, 287.)

—— Weitere Mitteilungen über das Klettern der Insekten an glatten

senkrechten Flächen. (Zoolog. Anzeiger, 1885. viii Jahrg., pp.

157–159.)

—— Richtigstellung der behauptungen des Herrn F. Dahl. (Archiv f.

mikroskop. Anat., 1885, xxvi, pp. 125–128.)

=Rombouts, J. E.= Ueber die Fortbewegung der Fliegen an glatten

Flächen. (Zool. Anzeiger, 1884, pp. 619–623.)

—— De la faculté qu’out les mouches de se mouvoir sur le verre et sur

les autres corps polis. (Archiv Museum Teyler (2), 4 Part, pp. 16.

Fig.)

=Simmermacher, G.= Untersuchungen über Haftapparate an Tarsalgliedern

von Insekten. (Zeitschr. f. wissensch. Zool. xl, 1884, pp. 481–556.

3 Taf., 2 Figs. Also Zoolog. Anzeiger, vii Jahrg., 1884, pp.

225–228.)

—— Antwort an Herrn Dr. H. Dewitz. (Ibid., pp. 513–517.)

=Dahl, F.= Die Fussdrüsen der Insekten. (Archiv f. mikroskop. Anat.,

1885, xxv, pp. 236–263. 2 Taf. See also p. 118.)

=Emery, C.= Fortbewegung von Tieren an senkrechten und überhangenden

glatten Flächen. (Biolog. Centralbl., 1884, 4 Bd., pp. 438–443.)

=Léon, N.= Disposition anatomique des organes de succion chez les

Hydrocores et les Géocores. (Bull. Soc. des Medec. et Natur, de

Jassy., 1888.)

_d._ The wings and their structure

The insects differ from all other animals except birds in possessing

wings, and as we at the outset have claimed, it is evidently owing to

them that insects are numerically so superior to any other class of

animals, since their power of flight enables them to live in the air out

of reach of many of their enemies, the greatest destruction to insect

life occurring in the wingless larval and pupal stages.

The presence of wings has exerted a profound influence on the shape and

structure of the body, and it is apparently due to their existence that

the body is so distinctly triregional, since this feature is least

marked in the synapterous insects. The wings are thin, broad leaf-like

folds of the integument, attached to the thorax and moved by powerful

muscles which occupy the greater part of the thoracic cavity. The two

pairs of wings are outgrowths of the middle and hinder part of the

thorax, the anterior pair being attached to the mesothoracic and the

hinder pair to the metathoracic segment. The larger pair is developed

from the middle segment of the thorax. The differentiation of the

tergites into scutum, scutellum, etc., is the result of the appearance

of wings, because these sclerites are more or less reduced or effaced in

wingless insects, such as apterous Orthoptera and moths, ants, etc.

The size of the hinder thoracic segments is closely related to that of

the wings they bear. In those Orthoptera which have hind wings larger

than those of the fore pair, the metathorax is larger than the

mesothorax. In such Neuroptera as have the hind wings nearly or quite as

large as the anterior pair, or in the Trichoptera and in the Hepialidæ,

the metathorax is nearly as large as the mesothorax, while in Coleoptera

the metathorax is as large and often much larger. In the Ephemeridæ,

Diptera, and Hymenoptera, which have either only rudimentary (halteres)

or small hind wings, the metathorax is correspondingly reduced in size.

The wings morphologically, as their development shows, are simple

sac-like outgrowths of the integument, _i.e._ of the free hinder edge of

the tergal plates, their place of origin being apparently above the

upper edge of the epimera or pleural sclerites. Calvert[24] however,

regards the upper lamina of the wing as tergal, and the lower, pleural.

The wings in most insects are attached to the thorax by a membrane

containing several little plates of chitin called by Audouin

articulatory epidemes.

The wings, then, are simple, very thin chitinous lamellate expansions of

the integument, which are supported and strengthened by an internal

framework of hollow chitinous tubes.

=The veins.=—The so-called “veins” or “nervures,” which are situated

between the upper and under layers of the wing are so disposed as to

give the greatest lightness and strength to the wings. Hagen has shown

that in the freshly formed wings these two layers can be separated, when

it can be seen that the veins pass through each layer.

These veins are in reality quite complex, consisting of a minute central

trachea enclosed within a larger tube which at the instant the insect

emerges from the nymph, or pupa, as the case may be, is filled with

blood (Fig. 136). Since these tubes at first contain blood, which has

been observed to circulate through them, and since the heart can be most

easily injected through them, they may more properly be called veins

than nervures. The shape and venation of the wings afford excellent

ordinal as well as family and generic characters, while they also enable

the systematist to exactly locate the spots and other markings of the

wings. The spaces enclosed by the veins and their cross-branches are

called cells, and their shape often affords valuable generic and

specific characters.

[Illustration:

FIG. 136.—Cross-section of wing of Pronuba.—After Spuler.

]

[Illustration:

FIG. 137.—Cross-section of wing of Pieris: _s_, insertions of

scales.-After Spuler.

]

The structure of a complete vein is described by Spuler. In a

cross-section of a noctuid moth (_Triphæna pronuba_, Fig. 136) the

chitinous walls are seen to consist of two layers, an outer (U) and

inner (_c_), the latter of which takes a stain and lies next to the

hypodermis (_hy_). In the cavity of the vein is the trachea (_tr_),

which shows more or less distinctly the so-called spiral thread; within

the cavity are also Semper’s “rib” (_r_) and blood-corpuscles (_bc_),

which proves that the blood circulates in the veins of the completely

formed wing, though this does not apply to all Lepidoptera with hard

mature wings. We have been able to observe the same structure in

sections of the wing of Zygæna.

A cross-section of a vein of _Pieris brassicæ_ shows that the large

trachea is first formed, and that it extends along the track between the

protoplasmic threads connecting the two hypodermal layers.

The main tracheæ throw off on both sides a number of secondary branches

showing at their end a cell with an intracellular tracheal structure;

these accessory tracheæ afterwards branch out. The accessory or

transverse tracheæ often disappear, though in some moths they remain

permanently. Fig. 137 _tr_{2}_ represents these secondary veins in the

edge of the fore wing of _Laverna vanella_, arising from a main trachea

(_tr_) passing through vein I (_v_), two of the twigs extending to the

centre, showing that the latter has no homology with a true vein. Only

rarely and in strongly developed thick folds are the transverse tracheæ

provided with a chitinous thickening, as for example in _Cossus

ligniperda_. Since from such accessory tracheæ the transverse veins in

lepidopterous wings are developed, we can recognize in them the

homologies of the net-veins in reticulated venations. There is no

sharply defined difference between reticulated and non-reticulated

venations; no genetic difference exists between the two kinds of

venation, since there occur true Blattidæ both with and without a

reticulated venation (Spuler).

In the fore wings of Odonata, Psocina, Mantispidæ, and most Hymenoptera

is an usually opaque colored area between the costal edge and the median

vein, called the _pterostigma_.

In shape the wings are either triangular or linear oval, and at the

front edge the main veins are closer together than elsewhere, thus

strengthening the wings and affording the greatest resistance to the air

in making the downward stroke during flight. It is noticeable that when

the veins are in part aborted from partial disuse of the wings, they

disappear first from the hinder and middle edge, those on the costal

region persisting. This is seen in the wings of Embiidæ (Oligotoma),

Cynipidæ, Proctotrupidæ, Chalcids, ants, etc.

The front edge of the wing is called the costal, its termination in the

outer angle of the wing is called the apex; the outer edge (termen) is

situated between the apex and the inner or anal angle, between which and

the base of the wing is the inner or internal edge.

While in Orthoptera, dragon-flies, Termitidæ, and Neuroptera the wings

are not attached to each other, in many Lepidoptera they are loosely

connected by the loop and frenulum, or in Hymenoptera by a series of

strong hooks. These hooks are arranged, says Newport, “in a slightly

twisted or spiral direction along the margin of the wing, so as to

resemble a screw, and when the wings are expanded attach themselves to a

little fold on the posterior margin of the anterior wing, along which

they play very freely when the wings are in motion, slipping to and fro

like the rings on the rod of a window curtain.”

At the base of the hind wings of Trichoptera and in the lepidopterous

Micropteryx there is an angular fold (_jugum_) at the base of each wing

(Fig. 138); that of the anterior wings is retained in Eriocephala and

Hepialidæ.

[Illustration:

FIG. 138.—Venation of fore and hind wings of _Micropteryx purpurella_:

_j_, jugum, on each wing; _d_, discal vein; the Roman numerals

indicate veins I.-VIII. and their branches.

]

In the wings of Orthoptera as well as other insects, the fore wings,

especially, are divided into three well-marked areas, the costal,

median, and internal; of these the median area is the largest, and in

grasshoppers and crickets is more or less modified to form the musical

apparatus, consisting of the drum-like resonant area, with the file or

bow.

=The squamæ.=—In the calyptrate Muscidæ, a large scale-like membranous

broad orbicular whitish process is situated beneath the base of the

wing, above the halter; (Fig. 94, 10 _sq._) it is either small or

wanting in the acalyptrate muscids. Kirby and Spence state that when the

insect is at rest the two divisions of this double lobe are folded over

each other, but are extended during flight. Their exact use is unknown.

Kolbe, following other German authors, considers the term _squama_ as

applicable to the whole structure, restricting the term _alula_ to the

other lobe-like division.

More recently (1890 and 1897) Osten-Sacken recommends “_squamæ_; in

the plural, as a designation for both of these organs taken

together; _squama_, in the singular, would mean the posterior squama

alone, and _antisquama_ the anterior squama alone;” the strip of

membrane running in some cases between them, or connecting the

squama with the scutellum, should be called the _post-alar

membrane_. By a mistake Loew, and others following him, used the

word _tegula_ for _squama_, but this term should be restricted to

the sclerite of the mesothorax previously so designated (Fig. 90,

_A_, _t_). The squama or its two subdivisions has also by various

authors been termed alula, calypta, squamula, lobulus, axillary

lobe, aileron, cuilleron, schuppen, and scale. (Berlin Ent.

Zeitschrift, xli, 1896, pp. 285–288, 328, 338.)

=The halteres.=—In the Diptera the hind wings are modified to form the

_halteres_ or balancers, which are present in all the species, even in

Nycteribia, but are absent in Braula.

Meinert finds structures in the Lepidoptera which he considers as

the homologues of the halteres of Diptera. “In the Noctuidæ,” he

remarks, “I find arising from the fourth thoracic segment (segment

médiaire), but covered by hair, an organ like the halter of

Diptera.” (Ent. Tidskrift., i, 1880, p. 168.) He gives no details.

In the Stylopidæ, on the contrary, the fore wings are reduced to little

narrow pads, while the hind wings are of great size.

The _thyridium_ is a whitish spot marking a break in the cubital vein of

the fore wing of Trichoptera; these minute thyridia occur in the fore

wings of the saw-flies; there is also an intercostal thyridium on the

costal part of the wings of Dermaptera.

The fore wings of Orthoptera are thicker than the hinder ones, and serve

to protect the hind-body when the wings are folded; they are sometimes

called _tegmina_. It is noteworthy, that, according to Scudder, in all

the paleozoic cockroaches the fore wings (tegmina) were as distinctly

veined as the hinder pair, “and could not in any sense be called

coriaceous.” (Pretertiary Insects of N. A., p. 39.) Scudder also

observes that in the paleozoic insects as a rule the fore and hind wings

were similar in shape and venation, “heterogeneity making its appearance

in mesozoic times.” In the heteropterous Hemiptera, also, the basal half

of the fore wings is thick and coriaceous or parchment-like, and also

protects the body when they are folded; these wings are called

_hemelytra_. In the Dermaptera the small short fore wings are thickened

and elytriform.

=The elytra.=—This thickening of the fore wings is carried out to its

fullest extent in the fore wings of beetles, where they form the

sheaths, shards, or _elytra_, under which the hind wings are folded. The

indexed costal edge is called the _epipleurum_, being wide in the

Tenebrionidæ. During flight “the elytra are opened so as to form an

angle with the body and admit of the free play of the wings” (Kirby and

Spence). In the running beetles (Carabidæ), also in the weevils and in

many Ptinidae, the hind wings are wanting, through disuse, and often the

elytra are firmly united, forming a single hard shell or case. The

firmness of the elytra is due both to the thickness of the chitinous

deposit and to the presence of minute chitinous rods or pillars

connecting the upper and lower chitinous surfaces.

[Illustration:

FIG. 139.—Longitudinal section through the edge of the elytrum of

_Lina ænea_: _gl_, glands; _r_, reservoir; _fb_, fat-body; _m_,

matrix; _u_, upper,—_l_, lower, lamella.—After Hoffbauer.

]

Hoffbauer finds that in the elytra of beetles of different families the

venation characteristic of the hind wings is wanting, the main tracheæ

being irregular or arranged in closely parallel longitudinal lines, and

nerve-fibres pass along near them, sense-organs being also present. The

fat-bodies in the cavity of the elytra, which is lined with a matrix

layer, besides nerves, tracheæ, and blood, contain secretory vesicles

filled with uric-acid concretions such as occur in the fat-body of

Lampyris. There are also a great many glands varying much in structure

and position, such occurring also in the pronotum (Fig. 139).

Meinert considers the elytra of Coleoptera to be the homologues of

the tegulæ of Lepidoptera and of Hymenoptera. He also calls

attention to the alula observed in Dyticus, situated at the base of

the elytra, but which is totally covered by the latter. The alulæ of

these beetles he regards as the homologues of the anterior wings of

Hymenoptera and Diptera. No details are given in support of these

views. (Ent. Tidskrift, i, 1880, p. 168.)

Hoffbauer (1892) also has suggested that the elytra are not the

homologues of the fore wings of other insects, but of the tegulæ.

Kolbe describes the alula of Dyticus as a delicate, membranous lobe

at the base of the elytra, but not visible when they are closed: its

fringed edge in Dyticus is bordered by a thickening forming a tube

which contains a fluid. The alula is united with the inner basal

portion and articulation of the wing-cover, forming a continuation

of them. Dufour considered that the humming noise made by these

beetles is produced by the alulets.

Hoffbauer finds no structural resemblances in the alulæ of Dyticus

to the elytra. He does not find “the least trace of veins.” They are

more like appendages of the elytra. Lacordaire considered that their

function is to prevent the disarticulation of the elytra, but

Hoffbauer thinks that they serve as contrivances to retain the air

which the beetle carries down with it under the surface, since he

almost always found a bubble of air concealed under it; besides,

their folded and fringed edge seems especially fitted for taking in

and retaining air. Hoffbauer then describes the tegulæ of the hornet

and finds them to be, not as Cholodkowsky states, hard, solid,

chitinous plates, but hollow. They are inserted immediately over the

base or insertion of the fore wings, being articulated by a

hinge-joint, the upper lamella extending into a cavity of the side

of the mesothorax, and connected by a hinge-like, articulating

membrane with the lower projection of the bag or cavity. The lower

lamella becomes thinner towards the place of insertion, is slightly

folded, and merges without any articulation into the thin, thoracic

wall at a point situated over the insertion of the fore wing. The

tegulæ also differ from the wings in having no muscles to move them,

the actual movements being of a passive nature, and due to the

upward and downward strokes of the wings.

Comstock adopts Meinert’s view that the elytra are not true fore

wings, but gives no reasons. (Manual, p. 495.)

Dr. Sharp,[25] however, after examining Dyticus and Cybister,

affirms that this structure is only a part of the elytron, to which

it is extensively attached, and that it corresponds with the angle

at the base of the wing seen in so many insects that fold their

front wings against the body. He does not think that the alula

affords any support to the view that the elytra of beetles

correspond with the tegulæ of Hymenoptera rather than with the fore

wings.

That the elytra are modified paraptera (tegulæ) is negatived by the

fact that the latter have no muscles, and that the elytra contain

tracheæ whose irregular arrangement may be part of the modified

degenerate structure of the elytra. Kolbe finds evidences of veins.

The question may also be settled by an examination of the structure

of the pupal wings. A study of a series of sections of both pairs of

wings of the pupa of Doryphora and of a Clytus convinces us that the

elytra are the homologues of the fore wings of other insects.

_e._ Development and mode of origin of the wings

=Embryonic development of the wings.=—The wings of insects are

essentially simple dorsal outgrowths of the integument, being

evaginations of the hypodermis. They begin to form in the embryo before

hatching, first appearing as folds, buds, or evaginations, of the

hypodermis, which lie in pouches, called peripodal cavities. They are

not visible externally until rather late in larval life, after the

insect, such as a grasshopper, has moulted twice or more times; while in

holometabolous insects they are not seen externally until the pupa state

is attained.

The subject of their origin is in a less satisfactory state than

desirable from the fact that at the outset the development of the wings

of the most generalized insects, such as Orthoptera, Termes, etc., was

not first examined, that of the most highly modified of any insects,

_i.e._ the Muscidæ, having actually been first studied.

In the course of his embryological studies on the Muscidæ (_Musca

comitoria_ and _Sarcophaga carnaria_) Weismann (1864) in examining the

larvæ of these flies just before pupation, found that the wings, as well

as the legs and mouth-appendages, developed from microscopic masses of

indifferent cells, which he called “imaginal discs.” From the six

imaginal discs or buds in the lower part of the thorax arise the legs,

while from four dorsal discs, two in the meso- and two in the

metathoracic segment, arise the fore and hind wings (Fig. 141.) These

imaginal buds, as we prefer to call these germs, usually appear at the

close of embryonic life, being found in freshly hatched larvæ.

[Illustration:

FIG. 140.—Imaginal buds in Musca,—_A_, in Corethra,—_B_, in

Melophagus,—_C_, in embryo of Melophagus; dorsal view of the head;

_b_, bud; _p_, peripodal membrane; _c_, cord; _hy_, hypodermis;

_cl_, cuticula; _st_, stomodæum; _v_, ventral cephalic, behind are

the two dorsal cephalic buds.—After Pratt.

]

As first observed by Weismann, the buds are, like those of the

appendages, simply attached to tracheæ and sometimes to nerves, in the

former case appearing as minute folds or swellings of the peritoneal

membrane of certain of the tracheæ. In Volucella the imaginal buds were,

however, found by Künckel d’Herculais to be in union with the

hypodermis. Dewitz detected a delicate thread-like stalk connecting the

peripodal membrane with the hypodermis, and Van Rees has since proved in

Musca, and Pratt in Melophagus, the connection of the imaginal buds with

the hypodermis (Fig. 140). These tracheal enlargements increase in size,

and become differentiated into a solid mass which corresponds to the

upper part of the mesothorax, while a tongue-shaped continuation becomes

the rudiment of the wing. During larval life the rudiments of the wings

crumple, thus forming a cavity. While the larva is transforming into the

pupa, the sheath or peripodal membranes of the rudimentary wings are

drawn back, the blood presses in, and thus the wings are everted out of

the peripodal cavities.

Due credit, however, should be given to Herold, as the pioneer in

these studies, who first described in his excellent work on the

development of _Pieris brassicæ_ (1815) the wing-germs in the

caterpillar after the third moult. This discovery has been

overlooked by recent writers, with the exception of Gonin, whose

statement of Herold’s views we have verified. Herold states that the

germs of the wings appear on the inside of the second and third

thoracic segments, and are recognized by their attachment to the

“protoplasmic network” (_schleimnetz_), which we take to be the

hypodermis, the net-like appearance of this structure being due to

the cell-walls of the elements of the hypodermal membrane. These

germs are, says Herold, also distinguished from the flakes of the

fat-body by their regular symmetrical form. Fine tracheæ are

attached to the wing-germs, in the same way as to the flakes of the

fat-body. It thus appears that Herold in a vague way attributes the

origin of these wing-germs, and also the germs of the leg, to the

hypodermis, since his schleimnetz is the membrane which builds up

the new skin. Herold also studied the later development of the

wings, and discovered the mode of origin of the veins, and in a

vague way traced the origin of the scales and hairs of the body, as

well as that of the colors of the butterfly.

Herold also says that as the caterpillar grows larger, and also the

wing-germs, “the larval skin in the region under which they lie

hidden is spotted and swollen,” and he adds in a footnote: “This is

the case with all smooth caterpillars marked with bright colors. In

dark and hairy caterpillars the swelling of the skin through the

growth of the underlying wing-germs is less distinct or not visible

at all” (pp. 29, 30).

It should be added that Malpighi, Swammerdam, and also Réaumur had

detected the rudiments of the wings in the caterpillar just before

pupation under the old larval skin. Lyonet (1760) also describes and

figures the four wing-germs situated in the second and third

thoracic segments, but was uncertain as to their nature. Each of

these masses, he says, is “situated in the fatty body without being

united to it, and is attached to the skin in a deep fold which it

makes there.” He could throw no certain light on their nature, but

says: “their number and situation leads to the supposition that they

may be the rudiments of the wings of the moth” (pp. 449, 450).

During the transformation into the pupa the imaginal buds unite and grow

out or extend along their edges, while the enveloping membrane

disappears. The rudimentary wings are now like little sacs, and soon

show a fusion of the two wing-membranes or laminæ with the veins, while

the tracheæ disappear, the places occupied by the tracheæ becoming the

veins. “Very early, as soon as the scales are indicated, begin in a very

peculiar way the fusion of the wing-laminæ. There occur openings in the

hypodermis into which the cells extend longitudinally and then laterally

give way to each other. Hence no complete opening is found, but the

epithelium appears by sections through a straight line sharply bordered

along the wingcavity. It is a continuous membrane formed of plasma which

I will call the ground membrane of the epithelium. Through this ground

membrane pass blood-corpuscles as well as blood-lymph.” (Schaeffer.)

[Illustration:

FIG. 141.—Anterior part of young larva of _Simulium sericea_, showing

the thoracic imaginal buds: _p_, prothoracic bud (only one not

embryonic); _w_, _w′_, fore and hind wing-buds; _l_, _l′_, _l″_,

leg-buds; _n_, nervous system; _br_, brain; _e_, eye; _sd_, salivary

duct; _p_, prothoracic foot.—After Weismann.

]

Afterwards (1866) Weismann studied the development of the wings in

_Corethra plumicornis_, which is a much more primitive and generalized

form than Musca, and in which the process of development of the wings is

much simpler, and, as since discovered, more as in other holometabolous

insects. He also examined those of Simulium (Fig. 141).

In Corethra, after the fourth and last larval moulting, there arises

at first by evagination and afterwards by invagination a cup-shaped

depression on each side in the upper part of the mesothoracic

segment within which the rudiment of the wings lies like a plug. The

wings without other change simply increase in size until, in the

transformation into the pupa by the withdrawal of the hypodermis,

the wings project out and become filled with blood, the tracheæ now

being wholly wanting, and other tissues being sparingly present.

[Illustration:

FIG. 142.—Section through thorax of a Tineid larva on sycamore,

passing through the 1st pair of wings (_w_): _ht_, heart; _i_,

œsophagus; _s_, salivary gland: _ut_, urinary tube; _nc_, nervous

cord; _m_, recti muscles; a part of the fat body overlies the heart.

_A_, right wing-germ enlarged.

]

These observations on two widely separate groups of Diptera were

confirmed by Landois, and afterwards by Pancritius, for the

Lepidoptera, by Ganin for the Hymenoptera, by Dewitz for Hymenoptera

(ants) and Trichoptera; also for the Neuroptera by Pancritius. In

the ant-lion (_Myrmeleon formicarius_) Pancritius found no rudiments

of the wings in larvæ a year old, but they were detected in the

second year of larval life, and do not differ much histologically or

in shape from those of Lepidoptera. In the Coleoptera and

Hymenoptera the imaginal buds appear rather late in larval life, yet

their structure is like that of Lepidoptera. In Cimbex the rudiments

of the wings are not found in the young larva, but are seen in the

semipupa, which stage lasts over six weeks.

[Illustration:

FIG. 143.—Section of the same specimen as in Fig. 142, but cut through

the second pair of wings (_w_): _i_, mid-intestine; _h_, heart;

_fb_, fat-body; _l_, leg; _n_, nervous cord.

]

The general relation of the rudiments (imaginal buds) of the wings of a

tineid moth to the rest of the body near the end of larval life may be

seen in Figs. 142, 143 (Tinea?), the sections not, however, showing

their connection with the hypodermis, which has been torn away during

the process of cutting. That the wing is but a fold of the hypodermis is

well seen in Fig. 144, of Datana, which represents a much later stage of

development than in Figs. 142 and 143, the larva just entering on the

semipupa stage.

In caterpillars of stage I, 3 to 4 mm. in length, Gonin found the

wing-germs as in Fig. 145, _A_ being a thickening of the hypodermis,

with the embryonic cells, _i.e._ of Verson, on the convex border. The

two leaves, or sides of the wing, begin to differentiate in stage II

(_C_, _D_), and in stage III the envelope is formed (_E_), while the

tracheæ begin to proliferate, and the capillary tracheæ or tracheoles at

this time arise (Fig. 145, _tc_). The wall of the principal trachea

appears to be resolved into filaments, and all the secondary branches

assume the appearance of bundles of twine. Landois regarded them as the

product of a transformation of the nuclei, but Gonin thinks they arise

from the entire cells, stating that from each cell arises a ball

(peloton) of small twisted tubes.

[Illustration:

FIG. 144.—Section through mesothoracic segment of _Datana ministra_,

passing through the wings (_w_): _c_, cuticula; _hyp_, hypodermis:

_ap_, apodeme; _dm_, dorsal longitudinal.—_vm_, ventral

longitudinal. muscles; _dmt_, depressor muscle of tergum; _t_,

trachea; _n_, nerve cords; _i_, intestine; _u_, urinary tubes; _l_,

insertion of legs.

]

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.”

[Illustration:

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.

[Illustration:

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.)

[Illustration:

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.

[Illustration:

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.

]

[Illustration:

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.”

[Illustration:

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.

]

[Illustration:

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.

[Illustration:

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.

[Illustration:

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.

[Illustration:

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.

]

[Illustration:

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.”

[Illustration:

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.

[Illustration:

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.”

[Illustration:

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.

[Illustration:

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.

[Illustration:

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

]

[Illustration:

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.”

[Illustration:

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.

[Illustration:

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.

[Illustration:

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.

[Illustration:

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.”

[Illustration:

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.

[Illustration:

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.)

[Illustration:

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.

[Illustration:

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.

[Illustration:

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.

[Illustration:

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.

[Illustration:

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.

]

[Illustration:

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.

]

On the other hand, we have been entirely mistaken in reference to

the mechanism which lowers the wings. The muscle concerned, that is

_kd_, is not at all the antagonist of the elevator muscle of the

wing, since it is placed close by this latter, but nearer to the

thoracic wall. But then, how does it come to be the counterpart of

its neighbor? In fact, the lever of the wing is situated in the

projecting piece alone. The extensor muscle of the wing does not

pull on the power arm, but on the resistant arm on the other side of

the fulcrum (_c_). The illustration shows, however, how such a case

is possible. The membrane of the joint fastening the wing-stalk to

the thorax is turned up outwards below the stalk like a pouch. The

tendon of the flexor of the wing passes through this pouch to its

point of attachment (_c_) lying on the other side of the fulcrum

(_d_). Thus it is very simply explained how two muscles which act in

the same direction can nevertheless have an entirely contrary

working power.

This is in a way the bare physical scheme of the flying machine by

the help of which we shall more easily become acquainted with its

further details.

Dragon-flies are unquestionably the most suitable objects for the

study of the muscles pulling directly on the wing itself. If the

lateral thoracic wall (Fig. 173) be removed or the thorax opened

lengthwise there appears a whole storehouse of muscular cords which

are spread out in an oblique direction between the base of the wing

and the side of the thoracic plate. There is first to be

ascertained, by the experiment of pulling the individual muscles in

the line with a pincers, which ones serve for the lifting and which

for the lowering of the wings. In dragon-flies the muscles are

arranged in two rows and in such a way that the flexors or

depressors (_s_, 1 bis) cling directly to the thoracic wall (compare

also the muscle _dk_ in Fig. 172 and _se_ in Fig. 174), while the

raiser or extensor (_h_ 1, to _h_ 2, Fig. 172, _hi_ and Fig. 174

_he_) lie farther in. The form of the wing-muscles is sometimes

cylindrical, sometimes like a prism, or even ribbon-like. However,

the contracted bundles of fibres do not come directly upon the

joint-process we have described, but pass over often indeed at a

very considerable distance from them, into peculiar chitinous

tendons. These have the form of a cap-like plate, often serrate on

the edge, which is prolonged into a thread, which should be

considered as the direct continuation of the base of the wings. The

wings, therefore, sink down into the thoracic cavity as if they were

a row of cords ending in handles where the strain of the muscles is

applied.

[Illustration:

FIG. 174.—Transverse section through the thorax of a locust

(Stenobothrus): _b_{1}_, leg; _h_, heart; _ga_, ventral cord;

_se_, depressor,—_he_, elevator, of the wing (_fl_); _b-r_,

lateral muscles which expand the thoracic walls;—_lm_,

longitudinal muscles which contract them; _shm_, _uhm_, muscles to

the legs; _bg_, apodemes.—After Graber.

]

[Illustration: