these thighs. This is said to be the case with _Scaphidium_, a little
tribe of beetles[486]: and one of the same order, that seems to come
between _Anobium_ and _Ptilinus_, found by our friend the Rev. R.
Sheppard, and which I have named after him _Choragus Sheppardi_,
is similarly circumstanced.--In the various tribes of frog-hoppers
(_Cercopidæ_, &c.) the posterior tibiæ appear to be principally
concerned in their leaping. These are often very long, and furnished,
on their exterior margin, with a fringe of stiff hairs, or a series
of strong spines, by pressing which against the plane of position
they are supposed to be aided in effecting this motion. On this
occasion they bend their legs like the grasshoppers, and then
unbending kick them out with violence[487]. Many of them, amongst
the rest _Cercopis spumaria_, have the extremity of the above tibiæ
armed with a coronet of spines; these are of great use in pushing
them off when the legs are unbended. This insect, when about to leap,
places its posterior thighs in a direction perpendicular to the plane
of position, keeping them close to the body; it next with great
violence pushes them out backwards, so as to stretch the leg in a
right line. These spines then lay hold of the surface, and by their
pressure enable the body to spring forwards, when, being assisted
by its wings, it will make astonishing leaps, sometimes as much as
five or six feet, which is more than 250 times its own length; or
as if a man of ordinary stature should be able at once to vault
through the air to the distance of a quarter of a mile. Upon glass,
where the spines are of no use, the insect cannot leap more than
six inches[488].--The species of another genus of the homopterous
_Hemiptera_ (_Chermes_), that jump very nimbly by pushing out their
shanks, are perhaps assisted in this motion by a remarkable horn
looking towards the anus, which arms their posterior hip.--Some bugs
that leap well, _Acanthia saltatoria_, &c. seem to have no particular
apparatus to assist them, except that their posterior tibiæ are very
long.--Several of the minute ichneumons also jump with great agility,
but by what means I am unable to say.--There is a tribe of spiders,
not spinners, that leap even sideways upon their prey. One of these
(_Salticus scenicus_), when about to do this, elevates itself upon
its legs, and lifting its head seems to survey the spot before it
jumps. When these insects spy a small gnat or fly upon a wall, they
creep very gently towards it with short steps, till they come within
a convenient distance, when they spring upon it suddenly like a
tiger.--Bartram observed one of these spiders that jumped two feet
upon a humble-bee. The most amusing account, however, of the motions
of these animals is given by the celebrated Evelyn in his Travels.
When at Rome, he often observed a spider of this kind hunting the
flies which alighted upon a rail on which was its station. It kept
crawling under the rail till it arrived at the part opposite to
the fly, when stealing up it would attempt to leap upon it. If it
discovered that it was not perfectly opposite, it would immediately
slide down again unobserved, and at the next attempt would come
directly upon the fly's back. Did the fly happen not to be within a
leap, it would move towards it so softly, that its motion seemed not
more perceptible than that of the shadow of the gnomon of a dial.
If the intended prey moved, the spider would keep pace with it as
exactly as if they were actuated by one spirit, moving backwards,
forwards, or on each side without turning. When the fly took wing,
and pitched itself behind the huntress, she turned round with the
swiftness of thought, and always kept her head towards it, though
to all appearance as immovable as one of the nails driven into the
wood on which was her station: till at last, being arrived within
due distance, swift as lightning she made the fatal leap and secured
her prey[489]. I have had an opportunity of observing very similar
proceedings in _Salticus scenicus_.
But the legs of insects are not the only organs by which they leap.
The numerous species of the elastic beetles (_Elater_), skip-jacks as
some call them, perform this motion by means of a _pectoral process_
or mucro. These animals having very short legs, when laid upon their
backs, cannot by their means recover a prone position. To supply this
seeming defect in their structure, Providence has furnished them with
an instrument which, when they are so circumstanced, enables them
to spring into the air and recover their standing. If you examine
the breast (_pectus_) of one of these insects, you will observe
between the base of the anterior pair of legs a short and rather
blunt process, the point of which is towards the anus. Opposite to
this point, and a little before the base of the intermediate legs,
you will discover in the after-breast (_postpectus_) a rather deep
cavity, in which the point is often sheathed. This simple apparatus
is all that the insect wants to effect the above purpose. When
laid upon its back, in your hand if you please, it will first bend
back, so as to form a very obtuse angle with each other, the head
and trunk, and abdomen and metathorax, by which motion the mucro is
quite liberated from its sheath; and then bending them in a contrary
direction, the mucro enters it again, and the former attitude being
briskly and suddenly resumed, the mucro flies out with a spring, and
the insect rising, sometimes an inch or two into the air, regains
its legs and moves off. The upper part of the body, by its pressure
against the plane of position, assists this motion, during which the
legs are kept close to its underside. Cuvier, when he says that man
and birds are the only animals that can leap vertically[490], seems
to have forgotten this leap of Elaters, which is generally vertical,
the trunk being vertically above the organ that produces the leap.
Other insects again leap by means of the _abdomen_ or some organs
attached to it. An apterous species--belonging to the _Ichneumonidæ_,
and to the genus _Cryptus_--takes long leaps by first bending its
abdomen inwards, as De Geer thinks, and then pushing it with force
along the plane of position[491]. There is a tribe of minute insects
amongst the _Aptera_, found often under bark, sometimes on the water,
and in various other situations, which Linné has named _Podura_, a
term implying that they have a leg in their tail. This is literally
the fact. For the tail, or anal extremity, of these insects is
furnished with an inflexed fork[492], which, though usually bent
under the body, they have the power of unbending; during which
action, the forked spring, pushing powerfully against the plane of
position, enables the animal to leap sometimes two or three inches.
What is more remarkable, these little animals are by this organ
even empowered to leap upon water. There is a minute black species
(_P. aquatica_), which in the spring is often seen floating on that
contained in ruts, hollows, or even ditches, and in such infinite
numbers as to resemble gunpowder strewed upon the surface. When
disturbed, these black grains are seen to skip about as if ignited,
jumping with as much ease as if the fluid were a solid plane, that
resists their pressure. The insects of another genus--separated from
_Podura_ by Latreille under the name of _Sminthurus_--have also
an anal spring, which when bent under the body nearly reaches the
head. These, which are of a more globose form than _Podura_, are so
excessively agile that it is almost impossible to take them. Pressing
their spring against the surface on which they stand, and unbending
it with force, they are out of your reach before your finger can come
near them. One of them, _S. fuscus_, besides the caudal fork, has a
very singular organ, the use of which is to prevent it from falling
from a perpendicular surface, on which they are often found at a
great height from the ground. Between the ends of the fork there is
an elevated cylinder or tube, from which the animal, when necessary,
can protrude two long, filiform, flexible transparent threads covered
with a slimy secretion. By these, when it has lost its hold, it
adheres to the surface on which it is stationed[493]. Another insect
related to the common sugar-louse, and called by Latreille _Machilis
polypoda_, in some places common under stones[494], has eight pair
of springs, one on each ventral segment of the abdomen, by means of
which it leaps to a wonderful distance, and with the greatest agility.
_Climbing_ is another motion of insects that merits particular
consideration: since, as this includes their power of moving against
gravity--as we see flies and spiders do upon our ceilings, and up
perpendicular surfaces even when of glass--it affords room for much
interesting and curious inquiry. Climbing insects may be divided
into four classes.--Those that climb by means of their claws;--those
that climb by a soft cushion of dense hairs, that, more or less,
lines the underside of the joints of their tarsi, the claw-joint
excepted;--those that climb by the aid of suckers, which adhere (a
vacuum being produced between them and the plane of position) by the
pressure of the atmosphere;--and those that are enabled to climb by
means of some substance which they have the power of secreting.
The first order of climbers--those that climb by means of their
_claws_--includes a large proportion of insects, especially in the
_Coleoptera_ order--the majority of those that have five joints
in their tarsi being of this description. The predaceous tribes,
particularly the numerous and prowling ground-beetles (_Eutrechina_),
often thus ascend the plants and trees after their prey. Thus one of
them, the beautiful but ferocious _Calosoma Sycophanta_, mounts the
trunk and branches of the oak to commit fearful ravages amongst the
hordes of caterpillars that inhabit it[495]. By these the less savage
but equally destructive tree-chafers (_Melolonthæ_), and those enemies
of vegetable beauty the rose-chafers (_Cetonia aurata_), are enabled
to maintain their station on the trees and shrubs that they lay waste.
And by these also the water-beetles (_Dytiscus_, _Hydrophilus_, &c.)
climb the aquatic plants.--But it is unnecessary further to enlarge
upon this head; I shall only observe, that in most of the insects here
enumerated, the claws appear to be aided by stiff hairs or bristles.
Other climbers ascend by means of _foot-cushions_ (_pulvilli_)
composed of hairs, as thickly set as in plush or velvet, with which
the underside of the joints of their tarsi--the claw-joint, which is
always naked, excepted--are covered. These cushions are particularly
conspicuous in the beautiful tribe of plant-beetles (_Chrysomelidæ_).
A common insect of this kind, before mentioned, called the bloody-nose
beetle (_Timarcha tenebricosa_), by the aid of these is enabled
to adhere to the trailing plants, the various species of bedstraw
(_Galium_), on which it feeds; and by these will support itself against
gravity; for both this and _Chrysomela goettingensis_ will walk upon
the hand with their back downwards, and it then requires a rather
strong pull to disengage them from their station.--The whole tribe of
weevils (_Rhynchophora_, Latr.) are also furnished with these cushions,
but not always upon all their joints, some having them only at their
apex; and the palm-weevil (_Cordylia Palmarum_) at the extremity solely
of the last joint but one.--Those brilliant beetles the _Buprestes_
have also these cushions, as have likewise the numerous tribes of
capricorn-beetles (_Longicornes_, Latr.). The larvæ of these being
timber-borers, the parent insect is probably thus enabled to adhere to
this substance whilst it deposits its eggs. Indeed in some species of
the former genus the cushions wear the appearance of suckers.--While
the linear species of _Helops_ are without them, they clothe all the
tarsi of _H. æneus_ (_Chalcites_ K. Ms.)[496]. In two other genera of
the same order, _Silpha_ and _Cicindela_, the anterior tarsi of the
males are furnished with them; in these therefore they may be regarded,
like the suckers of the larger water-beetles (_Dytisci_), as given
for sexual purposes. The three first joints of the anterior tarsi of
many of the larger rove-beetles (_Staphylinus_, L.) are dilated so as
to form, as in the last-mentioned insects, an orbicular patella, but
covered by cushions. Since in them this is not peculiar to the males,
it is probably given that they may be able to support their long bodies
when climbing.
But the most remarkable class of climbers consists of those that are
furnished with an apparatus by which they can form a _vacuum_, so as to
adhere to the plane on which they are moving by atmospheric pressure.
That flies can walk upon glass placed vertically, and in general
against gravity, has long been a source of wonder and inquiry; and
various have been the opinions of scientific men upon the subject. Some
imagined that the suckers on the feet of these animals were spunges
filled with a kind of gluten, by which they were enabled to adhere to
such surfaces. This idea, though incorrect, was not so absurd as at
first it may seem; since we have seen above in many instances, and
very lately in that of the _Sminthurus fuscus_, that insects are often
aided in their motions by a secretion of this kind. Hooke appears to
have been one of the first who remarked that the suspension of these
animals was produced by some mechanical contrivance in their feet.
Observing that the claws alone could not effect this purpose, he justly
concluded that it must be principally owing to the mechanism of the two
palms, pattens, or soles as he calls the suckers; these he describes as
beset underneath with small bristles or tenters, like the wire teeth
of a card for working wool, which having a contrary direction to the
claws, and both pulling different ways, if there be any irregularity
or yielding in the surface of a body, enable the fly to suspend itself
very firmly. That they walk upon glass, he ascribes to some ruggedness
in the surface; and principally to a smoky tarnish which adheres to
it, by means of which the fly gets footing upon it[497]. But these
tenterhooks in the suckers of flies, and this smoky tarnish upon glass,
are mere fancies, since they can walk as well upon the cleanest glass
as upon the most tarnished. Reaumur also attributes this faculty of
these animals to the hairs upon their suckers[498]. That learned and
pious naturalist, Dr. Derham, seems to have been one of the first who
gave the true solution of this enigma. "Flies," says he, "besides their
sharp hooked nails, have also skinny palms to their feet, to enable
them to stick on glass and other smooth bodies, by the _pressure of
the atmosphere_[499]." He compares these palms to the curious suckers
of male _Dytisci_, before alluded to, and illustrates their action by
a common practice of boys, who carry stones by a wet piece of leather
applied to their top. Another eminent and excellent naturalist, the
late Mr. White, adopted this solution. He observes that in the decline
of the year, when the mornings and evenings become chilly, many species
of flies retire into houses and swarm in the windows: that at first
they are very brisk and alert; but, as they grow more torpid, that they
move with difficulty, and are scarcely able to lift their legs, which
seem as if glued to the glass; and that by degrees many do actually
stick till they die in the place. Then noticing Dr. Derham's opinion
as just stated, he further remarks, that they easily overcome the
atmospheric pressure when they are brisk and alert. But, he proceeds,
in the decline of the year this resistance becomes too mighty for their
diminished strength; and we see flies labouring along, and lugging
their feet in windows as if they stuck fast to the glass[500].
Sir Joseph Banks, to whom every branch of Natural History becomes
daily more indebted, has lately excited an inquiry, the results
of which have confirmed Derham's system concerning this motion of
animals against gravity. When abroad, he had noticed that a lizard,
on account of the sound that it emits before rain, named the
Gecko[501] (_Lacerta Gecko_) could walk against gravity up the walls
of houses; and comparing this with the parallel motions of flies, he
was desirous of having the subject more scientifically illustrated
than it had been. This inquiry was put into the able hands of Sir
Everard Home, so justly celebrated as a comparative anatomist, who
was assisted in it by the incomparable pencil of Mr. Bauer: and
it has been proved most satisfactorily, that it is by producing a
vacuum between certain organs destined for that purpose and the plane
of position, sufficient to cause atmospheric pressure upon their
exterior surface, that the animals in question are enabled to walk
up a polished perpendicular, like the glass in our windows and the
chunam walls in India, or with their backs downward on a ceiling,
without being brought to the ground by the weight of their bodies.
The instruments by which a fly effects this purpose are two suckers
connected with the last joint of the tarsus by a narrow infundibular
neck, which has power of motion in all directions, immediately under
the root of each claw. These suckers consist of a membrane capable
of extension and contraction: they are concavo-convex with serrated
edges, the concave surface being downy, and the convex granulated.
When in action they are separated from each other, and the membrane
expanded so as to increase the surface: by applying this closely to
the plane of position, the air is sufficiently expelled to produce the
pressure necessary to keep the animal from falling. When the suckers
are disengaged, they are brought together again so as to be confined
within the space between the two claws. This may be seen by looking at
the movements of a fly in the inside of a glass tumbler with a common
microscope[502]. Thus the fly you see does no more than the leech has
been long known to do, when moving in a glass vessel. Furnished with a
sucker at each extremity, by means of these organs it marches up and
down at its pleasure, or as the state of the atmosphere inclines it.
Dipterous insects, which in general have these organs, and some three
on each foot[503], are not exclusively gifted with them; for various
others in different orders have them, and some in greater numbers. As
I lately observed, the foot-cushions of the Buprestes are something
very like them, particularly those of _B. fascicularis_.--A Brazilian
beetle in my cabinet, belonging to the family of the _Cleridæ_,
but not arranging well under any of Latreille's genera, which I
have named _Priocera variegata_, has curious involuted suckers on
its feet.--The strepsipterous genera _Stylops_ and _Xenos_, are
remarkable for the vesicles of membrane that cover the underside of
their tarsi, which, though flaccid in old specimens, appear to be
inflated in the living animal or those that are recent[504]. It is
not improbable that these vesicles, which are large and hairy, may
act in some degree as suckers, and assist it in climbing.
The insects of the _Orthoptera_ order are, many of them, remarkable
for two kinds of appendages connected with my present subject,
being furnished both with suckers and cushions. The former
are concavo-convex processes, varying in shape in different
species--being sometimes orbicular, sometimes ovate or oblong, and
often wedge-shaped--which terminate the tarsus between the claw,
one on each foot. They are of a hard substance, and seem capable
of free motion. In some instances[505], another minute cavity is
discoverable at the base of the concave part, similar to that in
_Cimbex lutea_[506]. The latter, the foot-cushions, are usually
convex appendages, of an oblong form, and often, though not always,
divided in the middle by a very deep longitudinal furrow, attached
to the underside of the tarsal joints. Sir E. Home is of opinion
that the object of these foot-cushions is to take off the jar, when
the body of the animal is suddenly brought from a state of motion
to a state of rest[507]. This may very likely be one of their uses,
but there are several circumstances which militate against its being
the only one. By their elasticity they probably assist the insects
that have them in their leaps; and when they climb they may in some
degree act as suckers, and prevent them from falling. But their use
will be best ascertained by a review of the principal genera of the
order. Of these the cock-roaches (_Blatta_), the spectres (_Phasma_),
and the praying insects (_Mantis_), are distinguished by tarsi of
five joints[508]. The grasshoppers with setaceous antennæ (_Acrida_)
have four tarsal joints. Those with filiform antennæ (_Locusta_ and
_Acrydium_), those with ensiform (_Truxalis_[509]), and the crickets
(_Gryllus_), have only three. In _Blatta_, the variations with
respect to the suckers and cushions (for many species are furnished
with both) are remarkable. The former in some (_Blatta gigantea_)
are altogether wanting; in others (_B. Petiveriana_) they are mere
rudiments; and in others (_B. Maderæ_) they are more conspicuous,
and resemble those of the _Gryllidæ_. The foot-cushions also in some
are nearly obsolete, and occupy the mere extremity of the four first
tarsal joints (_B. orientalis_, _americana_, _capensis_, &c.). In
_B. Petiveriana_ there is none upon the first joint; but upon the
extremity of the four last, not excepting the claw-joint, there is
a minute orbicular concave one, resembling a sucker. In others (_B.
gigantea_, &c.) they extend the length of the four first joints, and
are very conspicuous. In some (_B. Mouffeti_, K.[510]), which have
no claw-sucker, there appears to be a cavity in the extremity of the
claw-joint, which may serve the purpose of one. These foot-cushions
are usually of a pale colour; but in one specimen of a hairy female
which I have, from Brazil, they are black. The spectre genus
(_Phasma_) exhibits no particular varieties in this respect. The
tarsal joints of the legs have cushions at their apex, which appear
to be bifid. They have a large orbicular sucker between the claws.
In _Mantis_ the fore feet have neither of the parts in question, and
the others have no suckers. They have cushions on the four first
tarsal joints of the two last pair of legs, which, though smaller,
are shaped much like those in _Phasma_. In _Acrida_ the feet have no
suckers between the claws, but they are distinguished by two oval,
soft, concave, and moveable processes attached to the base of the
first joint of the tarsus, which probably act as suckers[511]. In
this genus there are two foot-cushions on the first joint of the
tarsi, and one on each of the two following ones[512].--The species
of the genus _Locusta_ come next. This genus is called _Acrydium_ by
Latreille after Geoffroy; but, since it includes the true _locust_,
it ought to retain the name _Locusta_ given by Linné to the tribe
to which it belongs[513]. All these insects have the terminal
sucker between the claws, three foot-cushions on the first joint of
the tarsus, and one on the second[514]; and the same conformation
also distinguishes the feet of _Truxalis_[515]. In the species of
_Acrydium_, F. (_Tetrix_, Latr.), the foot-cushions, I believe--for
in the dead insect they are the reverse of conspicuous--are arranged
nearly as in the two preceding genera, but these insects are without
the claw-sucker. And lastly, _Gryllus_ has neither suckers nor
cushions. From this statement it seems to follow--since _Blatta_,
_Phasma_, and _Mantis_, that do not leap, are provided with cushions;
and _Gryllus_, a heavy tribe of insects that does, are without
them--that their object cannot be exclusively to break the fall of
the insects that have them. And for the same reason we may conclude,
that they must have some further use than augmenting their elasticity
when they jump. When we consider that the _Blattæ_--many of which
have no suckers, or very small ones--are climbing insects (I have
seen _B. Germanica_ run up and down the walls of an apartment with
great agility), and that the long and gigantic apterous spectres, &c.
(_Phasma_) require considerable means to enable them to climb the
trees in which they feed, and to maintain their station upon them,
we may conclude that these cushions, by acting in some degree as
suckers, may promote these ends.