_quiescent_ when they first assume the pupa, but become locomotive
towards the close of their existence in that state. Since they
inhabit the water when they become pupæ, Providence has furnished
them with the means of quitting that fluid without injury, when they
are to exchange it for the air; which in their winged state is their
proper sphere of action. I have before described to you the grates
which shut up their cases when they became quiescent[466]; if they
had no means of piercing these grates, they would perish in the
waters. The head of these pupæ is provided at first with a particular
instrument, which enables them to effect this purpose; its anterior
part is armed with a pair of hooks in form resembling the beak of a
bird; and with this, previously to their last change, they make an
opening in the grate which, though it once defended, now confines
them. But at this moment, perhaps, the insect has a considerable
space of water to rise through before she can reach the surface. This
is all wisely provided for; before she leaves the envelope which
covers her body, she emerges from the water, and fixes herself upon
some plant or other object, the summit of which is not overflowed.
But you will here, perhaps, ask--How can a pupa in her envelope,
with all her limbs set fast, do this? This affords another instance
of the wise provision of the beneficent Father of the universe for
the welfare of his creatures. The antennæ and legs of this tribe of
insects, when they are pupæ, are not included, as is the case with
most that are quiescent in that state, in the general envelope; but
each in a separate one, so as to allow it free motion. Thus the
insect when the time is come for its last change can use them (except
the hind-legs, which being partly covered by the wing-cases remain
without motion) with ease. It then stretches out its antennæ, and
steering with its legs makes for the surface. De Geer saw one just
escaped from its case run and swim with surprising agility over the
bottom of a saucer, in which he had put some cases of these flies;
and at last when he held a piece of stick to it, it got upon it, and
having emerged from the water, prepared to cast its envelope. It is
remarkable, that the envelope of the intermediate tarsi, like the
posterior ones of Dytisci, is fringed on one side with hairs, to
enable the insects to use them as swimming feet[467], while those
neither of the larva nor imago are so circumstanced.
I am, &c.
FOOTNOTES:
[402] _Anatom. Compar._ i. 144.
[403] _Physico-Theol._ Ed. 13. 363.
[404] _Encycl. Brit._, art. _Physiology_, 709.
[405] Cuvier, _Anat. Comp._ i. 430.
[406] De Geer, v. 210.
[407] See above, p. 7.
[408] De Geer, vi. 338.
[409] See MacLeay in _Philos. Mag. &c._ N. Ser. No. 9. 178.
[410] De Geer, vi. 65.
[411] _Hist. Ins._ 270.
[412] Vol. I. 265.
[413] Reaumur, iii. 369.
[414] Vol. I. 137. De Geer, vi. 76. Reaumur, iv. 376. Swamm. _Bibl.
Nat._ Ed. Hill, ii. 46. a. _t._ xxxix. _f._ 3, _h. h._
[415] De Geer, vi. 355.
[416] Reaum. iv. 416. _t._ xxxvi. _f._ 5. Comp. Clark _On the Bots_,
&c. 48.
[417] Mr. Clark (ibid. 62) observed only rough points on the bots of
the sheep, but these also have spines or hooks looking towards the
anus. Reaum. iv. 556. _t._ xxxv. _f._ 11, 13, 15. I also observed
them myself in the same grub.
[418] See above, p. 220.
[419] PLATE XIX. FIG. 11.
[420] De Geer, vi. _t._ xxii. f. 15, _i._ _t._ xviii. f. 8, _p._
[421] Reaum. v. _t._ vi. f. 5, _mm._
[422] De Geer, vi. 395--. PLATE XXIII. FIG. 7. Foreleg, _a._
Hind-legs, _bb._ Mr. W. S. MacLeay is of opinion that these legs are
pedunculated spiracles, (_Philos. Mag._ N. Series, No. 9. 178.) but
it is evident from De Geer's account that the animal uses them as
legs, and like legs they are armed with hooks or claws.
[423] Lesser _L._ i. 96. note †.
[424] Klemann, _Beitrage_, 324.
[425] De Geer, i. 447-- _t._ xxxi. _f._ 17.
[426] De Geer, vi. 111.
[427] Ibid. v. 233.
[428] Ibid. 228.
[429] De Geer, vi. 137. _t._ viii. _f._ 8, 9.
[430] Reaum. iii. 496. _t._ xlv. _f._ 3.
[431] Ibid. _Mem. de l'Acad. Roy. des Sciences de Paris_, An. 1714.
p. 203.
[432] De Geer, vi. 380-- _t._ xxiv. _f._ 1-9.
[433] Swamm. _Bibl. Nat._ Ed. Hill, ii. 64. b.
[434] De Geer, vi. 389--.
[435] VOL. I. 431.
[436] Reaum. iv. _t._ 43. _f._ 3. _nn._
[437] De Geer, vi. 375. _t._ xxiii. _f._ 4, 5.
[438] Swamm. _Bibl. Nat._ Ed. Hill, ii. 44. b. 47. a.
[439] For examples of larvæ having these joints, see De Geer, iv.
289. _t._ xiii. _f._ 20. _t._ xv. _f._ 14. ii. _t._ xii. _f._ 3. _t._
xvi. _f._ 5, 6. _t._ xix. _f._ 4, &c.
[440] Ibid. v. _t._ xi. _f._ 11. _t._ ix. _f._ 9. o.
[441] Lyonet, _Tr. Anat._ _t._ iii. _f._ 8.
[442] Mr. W. S. MacLeay, where quoted above, objects to this term;
but as the organs in question are generally given to the animal to
assist in its motions, and have been universally regarded as a kind
of legs, it was judged best for the sake of distinction to give them
a different name from perfect legs, and at the same time one that
showed some affinity to them.
[443] Lyonet, 82-- _t._ iii. _f._ 10-16.
[444] Ibid. _t._ i. _f._ 4.
[445] De Geer, i. 379. _t._ xxv. _f._ 1. 3.
[446] VOL. I. 192--.
[447] De Geer, i. 12. 40. _t._ i. _f._ 27. _q. t._ vi. _f._ 11. _e._
[448] De Geer, i. 424.
[449] Kirby in _Linn. Trans._ v. 258.
[450] _Anatom. Comp._ i. 430.
[451] Rösel, I. iv. 112. vi. 14.
[452] Reaum. ii. 375--.
[453] Miger, _Ann. du Mus._ xiv. 441.
[454] De Geer, ii. 621.
[455] Ibid. 725--.
[456] De Geer, ii. 675-- Compare Reaum. vi. 393.
[457] VOL. I. 66.
[458] See above, p. 255.
[459] De Geer, iii. 284.
[460] Ibid. vi. 308.
[461] Ibid. iv. 43.
[462] Dumeril, _Trait. Element._ ii. 49. n. 603.
[463] VOL. I. 475; and above, p. 23.
[464] Reaum. ii. 450.
[465] Lyonet. _Trait. Anat._ 15--.
[466] See above, p. 264.
[467] De Geer, ii. 518--.
LETTER XXIII.
_MOTIONS OF INSECTS._ (_Imago._)
III. The motions of insects in their perfect or _imago_ state are
various, and for various purposes; and the provision of organs
by which they are enabled to effect them is equally diversified
and wonderful. It will be convenient to divide this multifarious
subject; I shall therefore consider their motions under two principal
heads:--motions of insects _reposing_--and motions of insects _in
action_;--and this last head I shall further subdivide into motions
whose object is change of place, and sportive motions.
* * * * *
The first of these, motions of insects _reposing_, will not detain
us long. The most remarkable is that of the long-legged gnats or
crane-flies (_Tipulæ_).--When at rest upon any wall or ceiling,
sometimes standing upon four legs, and sometimes upon five, you
may observe them elevate and depress their body alternately. This
oscillating movement is produced by the weight of their body and the
elasticity of their legs, and is constant and uninterrupted during
their repose. Unless it be connected with the respiration of the
animal, it is not easy to say what is the object of it. Moths, when
feeling the stimulus of desire, or under alarm, set their whole
body into a tremor[468]. A living specimen of the hawk-moth of the
willow being once brought me, upon placing it upon my hand, after
ejecting a milky fluid from its anus, it put its wings and body into
a most rapid vibration, which continued more than a minute, when it
flew away. A butterfly, called by Aurelians "The large skipper,"
(_Hesperia Sylvanus_,) when it alights, which it does very often,
for they are never long on the wing, always turns half-way round; so
that, if it settles with its head from you, it turns it towards you.
Others of the motions in question are merely those of parts.
Butterflies, when standing still in the sun, as you have doubtless
often observed,
"Their golden pinions ope and close;"
thus, it should seem, unless this motion be connected with their
respiration, alternately warming and cooling their bodies. You have
probably noticed a very common little fly, of a shining black, with
a black spot at the end of its wings (_Seioptera vibrans_[469]).
It has received its trivial name (_vibrans_) from the constant
vibration which, when reposing, it imparts to its wings. This
motion also, I have reason to think, assists its respiration.--Some
insects when awake are very active with their antennæ, though their
bodies are at rest. I remember one evening attending for some time
to the proceedings of one of those caseworm-flies (_Leptocerus_),
that are remarkable, like certain moths, for their long antennæ. It
was perched upon a blade of grass, and kept moving these organs,
which were twice as long as itself, in all directions, as if by
means of them it was exploring every thing that occurred in its
vicinity.--Many Tipulæ, and likewise some mites (_Acarus vibrans_
and _Gamasus motatorius_), distinguished by long anterior legs, from
this circumstance denominated _pedes motatorii_ by Linné, holding
them up in the air impart to them a vibratory motion, resembling that
of the antennæ of some insects[470].--I scarcely need mention, what
must often have attracted your attention, the actions of flies when
they clean themselves; how busily they rub and wipe their head and
thorax with their fore legs, and their wings and abdomen with their
hind ones.--Perhaps you are not equally aware of the use to which the
rove-beetles (_Staphylinus_, L.) put their long abdomen. They turn it
over their back not only to put themselves in a threatening attitude,
as I lately related[471], but also to fold up their wings with it,
and pack them under their short elytra.
* * * * *
With respect to the motions of insects in _action_, they may be
subdivided, as was just observed, into motions whose object is change
of place--and sportive motions.
The _locomotions_ of these animals are walking, running, jumping,
climbing, flying, swimming, and burrowing. I begin with the _walkers_.
The mode of their _walking_ depends upon the number and kind of their
legs. With regard to these, insects may be divided into four natural
classes; viz. _Hexapods_, or those that have only _six_ legs: such
are those of every order except the _Aptera_ of Linné, of which only
three or four genera belong to this class.--_Octopods_, or those
that have _eight_ legs, including the tribes of mites (_Acarina_);
spiders (_Araneidæ_); long-legged spiders (_Phalangidæ_); and
scorpions (_Scorpionidæ_):--_Polypods_, or those that have _fourteen_
legs, consisting of the woodlouse tribe (_Oniscidæ_);--and
_Myriapods_, or those that have more than fourteen legs--often
more than a hundred--composed of the two tribes of centipedes
(_Scolopendridæ_) and millepedes (_Julidæ_). The first of these
classes may be denominated _proper_, and the rest _improper_ insects.
The legs of all seem to consist of the same general parts; the hip,
trochanter, thigh, shank, and foot; the four first being usually
without joints (though in the _Araneidæ_, &c. the shank has two), and
the foot having from one to above forty[472].
In _walking_ and _running_, the hexapods, like the larvæ that have
perfect legs, move the anterior and posterior leg of one side and the
intermediate of the other alternately, as I have often witnessed.
De Geer, however, affirms that they advance each pair of legs at
the same time[473]; but this is contrary to fact, and indeed would
make their ordinary motions, instead of walking and running, a kind
of canter and gallop. Whether those that have more than six feet
move in this way--which is not improbable--from the difficulty of
attending at the same time to the movements of so many members, is
not easily ascertained.
The dog-tick (_Ixodes Ricinus_), if when young and active it moves in
the same way that it does when swoln to an enormous size with blood,
seems to afford an exception to the mode of walking just described.
It first uses, says Ray, its two anterior legs as antennæ to feel
out its way, and then fixing them, brings the next pair beyond them,
which being also fixed, it takes a second step with the anterior,
and so drags its bloated carcase along[474]. Redi observes, that
when scorpions walk they use those remarkable comb-like processes at
the base of their posterior legs to assist them in their motions,
extending them and setting them out from the body, as if they were
wings: and his observation is confirmed by Amoreux, who calls them
ventral swimmers[475]. I have often noticed a millepede (_Julus
terrestris_), frequently found under the bark of trees, and where
there is not a free circulation of air, the motions of which are
worthy of attention. Observed at a little distance, it seems to
glide over the surface, like a serpent, without legs; but a nearer
inspection shows how its movement is accomplished. Alternate portions
of its numerous legs are extended beyond the line of the body, so
as to form an obtuse angle with it, while those in the intervals
preserve a vertical direction. So that, as long as it keeps moving,
little bunches of the legs are alternately in and out from one end
to the other of its long body; and an amusing sight it is to see
the undulating line of motion successively beginning at the head and
passing off at the tail.--The motion of centipedes (_Scolopendra_),
as well as that of this insect and its congeners, is retrogressive as
well as progressive. Put your finger to the common one (_Lithobius
forficatus_), and it will immediately retrograde, and with the same
facility as if it was going forwards. This difference, however, is
then observable--it uses its four hind legs, which, when it moves in
the usual way, are dragged after it. Almost all the other apterous
insects, as well as many of those in the other orders, can move
in all directions; backwards, and towards both sides, as well as
forwards. Bonnet mentions a spider (not a spinner) that always walked
backwards when it attacked a large insect of its own tribe; but when
it had succeeded in driving it from a captive fly, which however it
did not eat, it walked forwards in the ordinary way[476].
Insects vary much in their walking paces: some crawling along; others
walking slowly; and others moving with a very quick step. The field
cricket (_Gryllus campestris_) creeps very slowly--the bloody-nose
beetle (_Timarcha tenebricosa_) and the oil-beetle (_Meloe
Proscarabæus_) march very leisurely; the spider-wasps (_Pompilus_)
walk by starts, as it were, vibrating their wings, at the same time,
without expanding them; while flies, ichneumons, wasps, &c., and many
beetles, walk as fast as they can. One insect, a kind of snake-fly
(_Mantispa pagana_), is said to walk upon its knees. The crane-flies
(_Tipula oleracea_) and shepherd-spiders (_Phalangium_) have legs so
disproportionately long, that they seem to walk upon stilts; but
when we consider that they have to walk over and amongst grass,--the
former laying its eggs in meadows,--we shall see the reason of this
conformation. Insects do not always walk in a right line; for I have
often observed the little midges (_Psychoda_, Latr.), when walking up
glass, moving alternately from right to left and from left to right,
as humble-bees fly, so as to describe small zigzags.
Numerous are the insects that _run_. Almost all the predaceous
tribes, the black dors, clocks, or ground-beetles (_Eutrechina_), and
their fellow destroyers the _Cicindelæ_, and other _Eupterina_--which
Linné, with much propriety, has denominated the tigers of the
insect world,--are gifted with uncommon powers of motion, and run
with great rapidity. The velocity, in this respect, of ants is also
very great.--Mr. Delisle observed a fly--so minute as to be almost
invisible--which ran nearly three inches in a demi-second, and in
that space made 540 steps. Consequently it could take a thousand
steps during one pulsation of the blood of a man in health[477].
Which is as if a man, whose steps measured two feet, should run
at the incredible rate of more than twenty miles in a minute! How
astonishing then are the powers with which these little beings
are gifted!--The forest-fly (_Hippobosca_), and its kindred genus
_Ornithomyia_ parasitic upon birds, are extremely difficult to take,
as I have more than once experienced, from their extreme agility.
I lost one from this circumstance two years ago that I found upon
the sea-lark (_Charadrius Hiaticula_) and which appeared to be
non-descript. Another most singular insect, which though apterous is
nearly related to these--I mean the louse of the bat (_Nycteribia
Vespertilionis_), is still more remarkable for its swiftness. Its
legs, as appears from the observations of Colonel Montague, are fixed
in an unusual position on the upper side of the trunk. "It transports
itself," to use the words of the gentleman just mentioned, "with such
celerity, from one part of the animal it inhabits to the opposite
and most distant, although obstructed by the extreme thickness of
the fur, that it is not readily taken."----"When two or three were
put into a small phial, their agility appeared inconceivably great;
for, as their feet are incapable of fixing upon so smooth a body,
their whole exertion was employed in laying hold of each other;
and in this most curious struggle they appeared actually flying in
circles: and when the bottle was reclined, they would frequently pass
from one end to the other with astonishing velocity, accompanied by
the same gyrations: if by accident they escaped each other, they
very soon became motionless: and as quickly were the whole put in
motion again by the least touch of the bottle, or the movement of an
individual[478]."--Incredibly great also is the rapidity with which
a little reddish mite, with two black dots on the anterior part of
its back (_Gamasus Baccarum_), common upon strawberries, moves along.
Such is the velocity with which it runs, that it appears rather to
glide or fly than to use its legs.
When insects walk or run, their legs are not the only members that
are put in motion. They will not, or rather cannot, stir a step till
their antennæ are removed from their station of repose and set in
action. When the chafers or petalocerous beetles are about to move,
these organs, before concealed, instantly appear, and the laminæ which
terminate them being separated from each other as widely as possible,
they begin their march. They employ their antennæ, however, not as
feelers to explore surrounding objects,--their palpi being rather used
for that purpose,--but, it should seem, merely to receive vibrations,
or impressions from the atmosphere, to which these laminæ, especially
in the male cockchafers, or rather tree-chafers (_Melolonthæ_) present
a considerable surface. Yet insects that have filiform or setaceous
antennæ appear often to use them for exploring. When the turnip-flea
(_Haltica oleracea_) walks, its antennæ are alternately elevated and
depressed.--The same thing takes place with some woodlice (_Oniscidæ_),
which use them as tactors, touching the surface on each side with them,
as they go along. This is not however constantly the use of this kind
of antennæ; for I have observed that _Telephorus lividus_,--a narrow
beetle with soft elytra, common in flowers,--when it walks vibrates
its setaceous antennæ very briskly, but does not explore the surface
with them. The parasitic tribes of _Hymenoptera_, especially the minute
ones, when they move vibrate these organs most intensely, and probably
by them discover the insect to which the law of their nature ordains
that they should commit their eggs; some even using them to explore
the deep holes in which a grub, the appropriate food of their larva,
lurks[479]. But upon this subject I shall have occasion to enlarge when
I treat of the senses of insects.--Antennæ are sometimes used as legs.
A gnat-like kind of bug (_Ploiera vagabunda_) has very short anterior
legs, or rather arms; while the two posterior pair are very long. Its
antennæ also are long. When it walks, which it does very slowly, with
a solemn measured step, its fore legs, which perhaps are useful only
in climbing, or to seize its prey, are applied to the body, and the
antennæ being bent, their extremity, which is rather thick, is made
to rest upon the surface on which the animal moves, and so supply the
place of fore-legs[480].--Mr. Curtis suspects that _Xyela pusilla_, a
hymenopterous insect related to _Xiphydria_, uses its maxillary palpi
as legs[481]. I have observed that mites often use the long hairs with
which the tail of some species is furnished, to assist them in walking.
Another mode of motion with which many insects are endowed is
_jumping_. This is generally the result of the sudden unbending of the
articulations of the posterior legs and other organs, which before
had received more than their natural bend. This unbending impresses a
violent rotatory motion upon these parts, the impulse of which being
communicated to the centre of gravity, causes the animal to spring
into the air with a determinate velocity, opposed to its weight more
or less directly[482]. Various are the organs by which these creatures
are enabled to effect this motion. The majority do it by a peculiar
conformation of the hind legs; others, by a pectoral process; and
others, again, by means of certain elastic appendages to the abdomen.
The _hind legs_ of many beetles are furnished with remarkably large
and thick thighs. Of this description are several species of weevils;
for instance, _Orchestes_ and _Ramphus_; the whole tribe of skippers
(_Haltica_), and the splendid Asiatic tribe of _Sagra_[483], &c. The
object of these disproportioned and clumsy thighs is to allow space
for more powerful muscles, by which the tibiæ, when the legs are
unbent, are impelled with greater force. In the _Orthoptera_ order
all the grasshoppers, including the genera _Gryllotalpa_; _Gryllus_;
_Tridactylus_; _Locusta_; _Acrida_; _Pterophylla_; _Pneumora_;
_Truxalis_; _Acrydium_; _Tetrix_, &c.--are distinguished by incrassated
posterior thighs; which however are much longer, more tapering and
shapely, (they are indeed somewhat clumsy in the two first genera, the
crickets,) than those of most of the _Coleoptera_ that are furnished
with them. When disposed to leap, these insects bend their hind leg
so as to bring the shank into close contact with the thigh--which has
often a longitudinal furrow armed with a row of spines on each side
to receive it. The leg being thus bent, they suddenly unbend it with
a jerk, when pushing against the plane of position, they spring into
the air often to a considerable height and distance. A locust, which
however is aided by its wings, it is said will leap two hundred times
its own length[484].--Aristophanes, in order to make the great and
good Athenian philosopher, Socrates, appear ridiculous, represents
him as having measured the leap of a flea[485]. In our better times
scientific men have done this without being laughed at for it, and
have ascertained that, comparatively, it equalled that of the locust,
being also two hundred times its length. Being effected by muscular
force, without the aid of wings, this is an astonishing leap.--There
are several insects, however, which, although they are furnished
with incrassated posterior thighs, do not jump. Of this description
are some beetles belonging to the genus _Necydalis_, (_Oedemera_,
Oliv.) in which this seems a peculiarity of the male: and amongst the
_Hymenoptera_, not to mention others, several species of _Chalcis_, and
all that are known of that singular genus _Leucospis_.