Chapter IX: Part I: Morphology and Physiology (5)

“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.

FIG. 122.—The same by the left fore (_l_{1}_), middle (_l_{2}_), and
hind, leg (_l_{3}_) of a Carabus. Natural size.
]

FIG. 123.-Tracks of a _Blaps mortisaga_ marked by the differently
painted tibial points: ●, tracks of fore, —○, middle, —/, hind
leg. Natural size.
]

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.

FIG. 125.—Tracks of Trichodes; the middle sinuous line is made by
the tip of the abdomen. Natural size.
]

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.
]

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.

FIG. 128.—The walk of an orthopterous insect: series to be followed
from right to left.—After Marey.
]

FIG. 129.—Beetle walking: series to be followed from left to
right.—After Marey.
]

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.

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.

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.)

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_, ♂.
]

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.

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.

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

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.

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.

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.

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.)

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.

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.

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.

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.
]