and the legs of the middle pair rather more distant from each other
than those of the other pairs. _Ex. Scarabæus_ M^cL.
8. [Illustration: dots] Like the preceding, only the legs of the middle
pair are at a much greater distance from each other. _Ex. Copris_ M^cL.
9. [Illustration: dots] Legs of the two posterior pairs distant. _Ex.
Hister_, _Scaphidium_.
10. [Illustration: dots] Like the preceding, but the posterior legs
more distant than those of the middle pair. _Ex. Lygæus._
11. [Illustration: see previous] Like the last, but the legs of the
anterior pair also distant. _Ex. Velia._
12. [Illustration: see previous] The arms distant, intermediate legs
more distant, posterior legs close together. _Ex. Byrrhus_ L.
5. _Proportions._ In general the legs of some insects are
disproportionally _long_ and _slender_, as in _Phalangium Opilio_
and some species of _Gonyleptes_[1992]: those of others are
disproportionally _short_, as in _Elater_, &c. With regard to their
relative proportions, the most general rule is, in Hexapods, that
the anterior pair shall be the shortest and most slender, and the
posterior the longest and thickest; but there are many exceptions:
thus, in _Macropus longimanus_, _Clytra longimana_, &c., in the male
the _arms_ are the longest; again, a thing that very rarely occurs,
in the same sex of _Podalirius retusa_ the _intermediate legs_ are
the longest[1993]; but in _Rhina barbirostris_ and many weevils they
are the shortest: in _Saperda hirtipes_ Oliv.[1994] the _hind-legs_
are disproportionally long: with regard to _thickness_, they are in
general extremely slender in _Cicindela_, and in the _Scarabæidæ_ very
_thick_. In _Goliathus Cacicus_ the arms are more robust than the four
legs[1995]; in _Gyrinus_ the latter are more dilated than the former;
in many _Rutelidæ_, and particularly in the celebrated Kanguroo beetle
(_Scarabæus Macropus_ Franc.) the hind-legs are much the thickest; in
a new genus of weevils from Brazil (_Plectropus_ K.), the intermediate
pair are more slender than either the arms or the posterior pair.
6. _Clothing._ The _hairs_ on the legs of insects, though at first
sight they may seem unimportant, in many cases are of great use to
them, both in their ordinary avocations and motions: but as most
of these were sufficiently noticed when I treated of the sexes of
insects[1996], I shall not here repeat my observations, but confine
myself to cases not then adverted to. Some insects have all their
legs very hairy, as many spiders, the diamond beetle (_Entimus
imperialis_), or at least a species very near it and common in
Brazil[1997], &c.: in others they are nearly naked, as in the
stag-beetle. In the Crepuscular _Lepidoptera_ (_Sphinx_ L.) and
some of the Nocturnal ones (_Bombyx_ L.) the thighs are much more
hairy than the rest of the legs: and in _Lucanus_, _Geotrupes_,
and many other Lamellicorns, &c., the anterior ones have a yellow
or golden spot at their base, composed of decumbent hairs, which
prevent them from suffering by the violent friction to which they
are exposed in burrowing. In most Petalocerous beetles the tibiæ are
set with scattered bristles, and sometimes the thighs. The Tiger
beetles (_Cicindela_) are similarly circumstanced: but the bristles,
which are white, are generally arranged in rows. In _Dytiscus_,
_Hydrophilus_, &c., the four posterior tarsi; and in _Notonecta_
the posterior pair, and also the tibiæ--are fringed on each side
with a dense series of hairs, which structure assists them in
swimming[1998]. The tarsi, especially the anterior pair, in a certain
family of _Lamia_ F. (_L. papulosa_, &c.[1999]), are similarly
fringed, only the hairs curl inwards; and the _hand_ in _Sphex_
and _Ammophila_, but not in _Pelopæus_ and _Chlorion_, is fringed
externally with long bristles.
7. _Composition._ With regard to their _composition_, both arms and
legs generally consist of _five_ pieces, which Entomologists have
denominated--the _coxa_ or _hip_--the _trochanter_--the _femur_ or
thigh--the _tibia_ or shank--and the _tarsus_ or foot. Where the
structure and use of the fore-leg is different from that of the four
hind-legs, I propose calling these pieces by names corresponding
with those which anatomists have appropriated to the _arm_ in the
higher vertebrate animals: thus, as you will see in the table, I call
the whole fore-leg the _brachium_ or arm; and the _coxa_ becomes
the _clavicula_ or collar-bone; the _trochanter_, the _scapula_ or
shoulder-blade; the _femur_, the _humerus_ or shoulder; the _tibia_,
the _cubitus_ or arm; the _tarsus_, the _manus_ or hand. But let me
not lead you to suppose that the pieces, either in the arms or legs
of insects, which are there named after certain others in vertebrate
animals, precisely correspond with them--by no means--since that
is a very doubtful point; and some of them, as the _trochanter_,
clearly do not. Many gentlemen skilled in anatomy, as I have before
observed[2000], have thought that what is regarded as the _coxa_ in
insects really represents the _femur_: but there are considerable
difficulties in the way of this supposition, several of which I then
stated. I shall not however enter further into the subject, and take
the above names; since this application of them is so general and so
well understood, except with regard to the fore-leg, under certain
circumstances, as I find them. I shall now consider them in the order
in which I have named them.
a. _Coxa_ or _Clavicula_[2001]. The _coxa_ is the joint that connects
the leg with the trunk of the insect. With regard to their _shape_,
the most general form of the four anterior is more or less that
of a truncated cone: in the _Staphylinidæ_, however, they tend
to a pyramidal or four-sided figure; as do the whole six in the
_Trichoptera_: in numbers of the weevils and capricorns they are
subglobose; in the Lamellicorns they are mostly oblong, and not
prominent: the posterior pair in the _Coleoptera_ are generally flat
and placed in a transverse position, and more or less oblong and
quadrangular: in _Elater_, &c., they are cuneiform: in _Haliplus_
Latr. they are dilated, and cover the thigh[2002]: in _Buprestis_,
_Copris_, &c., they have a cavity that partly receives it: the
corresponding part, the _clavicle_, in the _arm_ of _Gryllotalpa_,
is very large and remarkable; viewed underneath it is triangular,
and trifid where the trochanter articulates with it: in that of
_Megachile Willughbiella_ the clavicle is armed with a spine[2003].
As to their _proportions_, the most general law seems to be, that the
anterior pair shall be the shortest and smallest, and the posterior
the longest and largest. In some instances, as in _Buprestis_, the
two anterior pair are nearly equal; in others (_Mantis_, _Eurhinus_
K.), the anterior are the longest, in the former as long as the
thigh, and the four posterior the shortest: in the _Trichoptera_,
_Lepidoptera_, &c., all are nearly equal; in _Mantis_ the two
_posterior_, and in _Phengodes_ the _intermediate_ pair are the
largest; but in _Necrophorus_ they are the smallest:--though almost
universally without articulations, in _Galeodes_ the clavicle
consists of _two_ and the coxa of _three_[2004].
b. _Trochanter_ or _Scapula_[2005]. This is the _second_ joint of the
leg: and if the _coxa_ is regarded as the analogue of the _thigh_
in vertebrate animals, this should seem to represent the _patella_
or _rotula_, vulgarly called the knee-pan. Latreille and Dr. Virey
consider this articulation as merely a joint of the _coxa_[2006]; but
if closely examined, especially in Coleopterous insects, you will
find it so fixed to the thigh as scarcely to have separate motion
from it, and in many cases it seems to be merely its fulcrum; but
I am not aware that any instance occurs in which it has not motion
separate from that of the former joint.
As to its _articulation_ with the _coxa_,--in the _Coleoptera_ it
appears to be of a mixed kind; for it inosculates in that joint,
is suspended by ligament to its orifice, and its protuberances are
received by corresponding cavities in it; and its cavities receive
protuberances, which belongs to a ginglymous articulation. I have
observed two variations in this Order, in one of which the motion
of the thigh and trochanter is only in _two_ directions, and in the
other it is nearly _versatile_ or rotatory. The _Lamellicorns_ afford
an example of the first, and the _Rhyncophorous_ beetles or weevils
of the second. If you extract from the _coxa_ the thigh with the
trochanter of the larger species of _Dynastes_ M^cL., you will find
that the head of the latter is divided into two obtuse incurving
lobes or condyles: that on the inner side being the smallest and
shortest, and constricted just below its apex: and that under
this is a shallow or glenoid cavity, terminating posteriorly in a
lubricous flat curvilinear ridge. If you next examine the trochanter
in articulation with the coxa, you will perceive that the head of
the former inosculates in it, that the lower condyle is received
by a sinus of the coxa, which also has a lubricous _very_ shallow
cavity corresponding with the ridge, in which it turns; and in the
head of the coxa, on the lower side, is an external condyle, which
is received by a sinus common to both, of the head of the thigh and
of the exterior side of the trochanter[2007], in which it likewise
turns: this last condyle has also an internal protuberance, which
appears to ginglymate with a cavity of the trochanter: from this
structure the leg is limited chiefly to a motion up and down upon two
pivots, or to fold and extend itself. You will find an articulation
very near this, but on a smaller scale, in the stag-beetle. In the
other kind of articulation, which admits of freer motion, the head
of the trochanter is prolonged, and the process terminates in a
short interior condyle, which appears to work in a corresponding
cavity of the interior of the coxa; and the base of the process is
encompassed by a ridge with a cavity behind it, which is received by
another of the lower part of that piece, and admits a corresponding
ridge--a structure that allows a rotatory motion. In the hind-legs of
this tribe the motion is chiefly limited to folding and extending;
in _Carabus_, &c., also the head of the trochanter is nearly
hemispherical, and the articulation approaches ball and socket.
In most of the other Orders, the _Hymenoptera_ excepted, there is
little or no inosculation, the trochanter being simply suspended by
ligament to the _coxa_ as well as to the _thigh_; its connection
with the latter is similar in _Coleoptera_; but in _Cicindela_, &c.,
it inosculates in it. The part we are considering varies in its
position with respect to the thigh: in the hind-legs of _Carabus_,
&c., it forms a lateral fulcrum on the inner side of that part, and
does not intervene between its base and the coxa; the muscles from
the latter entering the former, not at the bottom of the base, but
at its side: but in the four anterior legs it forms their base, as
it does in all the legs in _Apion_, and in all the Orders except the
_Coleoptera_, cutting them entirely off from contact with the coxa:
in the _Lamellicorns_ they cut off part of the base obliquely, but so
as to permit their coming in contact with the condyle of the coxa, as
before mentioned. In the _Ichneumonidæ_ and some other _Hymenoptera_
the _trochanter_ appears to consist of _two_ joints particularly
visible in the posterior legs[2008].
As to _size_ in general,--the part in question is smaller than the
coxa; but in _Notonecta_ it is larger, and in the dog-tick (_Ixodes
Ricinus_) longer than that joint. It exhibits few variations in its
_shape_ or _appendages_ worthy of particular notice. In general,
in the _Coleoptera_ it is triangular or trigonal; but in _Carabus_
L., in the hind-leg it is oblong or rather kidney-shaped; in that
of _Necrophorus_[2009] it terminates in one or two teeth or spines,
varying in length in the different species: in the other Orders it is
not remarkable in this respect.
c. _Femur_ or _Humerus_[2010]. The _femur_ or thigh is the _third_,
and usually the largest and most conspicuous joint of the leg. In
the hypothesis before alluded to[2011] it is considered as the
analogue of the _tibia_ of vertebrate animals. With regard to
the _articulation_ of this part with the trochanter, it has been
sufficiently explained under that head, and that with the _tibia_
I shall treat of when I come to that joint. As to the _size_ of
the thighs, and their relative _proportions_ to each other and to
the remaining joints of the leg, the most general law is, that
the _anterior_ pair shall be the _shortest_ and _smallest_, and
the _posterior_ the _longest_ and _largest_. With respect to the
remaining articulations, most commonly the _thigh_ is longer and
larger than the _tibia_, and the _tibia_ than the _tarsus_. But there
are numerous exceptions to both these rules. With respect to the
_first_, we may begin by observing that the increase of the magnitude
of the thigh, from the anterior to the posterior pair, is usually
_gradual_: but in many jumping insects, and likewise many that do
not jump, the posterior pair are _suddenly_ and disproportionally
thicker than the rest[2012]. Again, in many insects the _anterior_
pair are the _longest_ and _thickest_, as in _Macropus longimanus_,
_Bibio_, _Nabis_, &c.: in others, the _intermediate_ exceed the rest
in _magnitude_, as in _Onitis Aygulus_, _cupreus_; _Sicus flavipes_,
&c.; in many Lamellicorns _all_ the thighs are incrassated and nearly
equal in size: but in some, as _Ryssonotus nebulosus_ M^cL.[2013],
the _intermediate_ pair are rather smaller than the rest. With
respect to the _second_ rule--in some, as in the male of _Macropus
longimanus_, the _anterior tibia_, though more slender, is longer
than the _thigh_; in _Hololepta maxillosa_ it is longer and more
dilated; in _Lamia marmorata_, or one related to it from Brazil, the
_intermediate_ pair are longer; in _Ateuchus gibbus_ and others of
that tribe the _posterior thighs_ are smaller than the _tibiæ_: and,
to mention no more; in _Callichroma latipes_ the _posterior_ tibia
is wider than the part last named. Again, the _tarsi_ are as long
as either _tibia_ or _thigh_ in many of the larger _Dynastidæ_, as
_Megasoma Actæon_, &c.; longer than either in _Melolontha subspinosa_
F.; and in _Tiphia_, _Scolia_ and affinities, often as long, or
longer than both together.
As to _shape_,--the _thigh_, especially in the fore-leg, varies
considerably: most generally it is flat, linear, and a little thicker
where it is united to the _tibia_, on the outer side convex, and
concave next the body; but in many it is gradually thicker from the
base to the apex: in some _Cerambyces_ (_C. thoracicus_) it is clavate;
in others of this genus and _Molorchus_ they may be called capitate;
in _Pterostichus_ they are rather lanceolate; in _Onitis Sphinx_
the humerus is triangular, and the intermediate thigh rhomboidal;
in _Bruchus Bactris_ it is bent like a bow; and in some Brazilian
_Halticæ_ it is nearly semicircular. The _humerus_ in _Phasma_ is
attenuated at the base; in _Empusa gongyloides_ it is at first
ovato-lanceolate, and terminates below in a kind of footstalk[2014];
in _Phasma flabelliforme_ it is dolabriform[2015]; in _Mantis_ often
semioval or semielliptical, and thickest at the inner edge, which
affords space for two rows of spines with which it is planted. In
_Phyllium siccifolium_ all the thighs are furnished on both sides
with a foliaceous appendage nearly from base to apex[2016]: in a
species of _Empusa_ (_E. macroptera_), the four posterior ones are so
distinguished only on their posterior side[2017]: others of this last
genus, as _E. gongyloides_, have an alary appendage on _both_ sides at
the apex of these thighs[2018]; and another family, as _E. pauperata_,
have only one on the _posterior_ side[2019]. The thighs of no insect
are more remarkable for their elegant shape,--tapering gradually from
the base to the apex, where they swell again into a kind of knee,--than
the posterior ones of the locusts (_Locusta_ Leach); each side of
these thighs is strengthened with three longitudinal nearly parallel
ridges, and the upper and under sides are adorned by a double series,
in some coalescing as they approach the tibia, of oblique quadrangular
elevations resembling scales[2020].
I shall next say a few words upon the _spines_ and other _processes_
which arm the thigh. Those moveable ones of _Mantis_ which help to
form a fearful instrument of destruction, have just been mentioned,
and similar ones, but less conspicuous, arm the intermediate thighs
of _Sicus flavipes_: other appendages of this kind are for a less
destructive purpose--to keep the tibia when folded in its place. This
seems to be the use of the serratures and spine that arm the thigh of
_Bruchus Bactris_, or the Hymenopterous genera _Leucospis_, _Chalcis_,
&c.; in _Onitis Aygulus_ a short filiform horn arms the humerus, and
a longer crooked one that of many species of _Scaurus_[2021]. In many
_Stenocori_ the thighs terminate in two spines, and in _Gonyleptes_
K. the posterior ones are armed internally with very strong ones;
with which, as the legs converge at their knee[2022], they may
probably detain their prey. The knee-pan (_Gonytheca_) of the thigh,
or the cavity at its end, which receives the head of the _tibia_,
is very conspicuous in the weevils; but in no insects more than in
_Locusta_[2023], in which tribe it deserves your particular attention.
d. _Tibia_ or _Cubitus_[2024]. The _tibia_ or shank is the _fourth_
joint of the leg, which according to the hypothesis lately alluded
to is the analogue, in the _anterior_ leg of the _carpus_ or carpal
bones, and in the _four posterior_ ones of the _tarsus_ or tarsal
bones of vertebrate animals. This may be called the most conspicuous
of the articulations of the leg; for though it is generally more
slender and often shorter than the thigh, it falls more under the eye
of the observer, that joint being more or less concealed by the body:
it consists in general of a single joint; but in the _Araneidæ_ and
_Phalangidæ_ it has an accessory one, often incrassated at its base,
which I have named the _Epicnemis_[2025].
With respect to the _articulation_ of the _tibia_ with the thigh--we
may observe that in general it is by means of _three_ processes or
condyles, two lateral and one intermediate, of the head of the former
joint[2026]: the lateral ones are usually received by a cavity or
sinus of the _gonytheca_ of the thigh[2027]; and upon these the
_tibia_ turns, with a semirotatory motion up and down as upon a
pair of pivots: at the same time the _mola_ or head of the latter
joint, which has often a flexure so as to form an elbow with the
rest of it, inosculates in the _gonytheca_, and is also suspended
by ligament to the orifice through which the muscles, nerves, and
bronchiæ are transmitted: so that in fact the articulation, strictly
speaking, belongs exclusively to none of the kinds observable in
vertebrate animals, but partakes of several, and may properly be
denominated a _mixed_ articulation,--a term applicable in numerous
instances also to the other articulations of the legs of insects.
In the different Orders some variations in this respect take
place,--I will notice some of the most remarkable. In no Coleopterous
insects is the structure more distinctly visible than in the larger
Lamellicorns. In _Copris bucephalus_, for instance, if you divide
the thigh longitudinally, you will find on each side, at the head,
that it is furnished with a nearly hemispherical protuberance,
perforated in the centre for the transmission of muscles, and
surrounded externally by a ridge, leaving a semicircular cavity
between them[2028]: if you next examine the _tibia_, after having
extracted it, you will find on each side, at the base, a cavity
corresponding with the protuberance of the thigh which it receives,
having likewise a central orifice, and surrounded by a semicircular
ridge corresponding with the cavity in the thigh in which it acts:
below this ridge another cavity, forming a small segment of a
circle, receives the ridge of the thigh[2029]. You will observe that
the ridge of the _tibia_ represents the lateral condyle lately
noticed: in the _Dynastidæ_ this is more prominent, and often forms
a smaller segment of a circle. In these also the protuberance of
the thigh is more minute, and its ridge is received by a cavity of
the _tibia_ nearly semicircular[2030]; in _Geotrupes_ Latr. the
articulation is not very different, though on a reduced scale; in
_Calandra Palmarum_ the lateral condyles of the _tibiæ_ are flatter
and broader[2031]; and the articulation not being quite so complex,
this joint is kept steady by an intermediate process observable in
the _gonytheca_[2032]. From the above description it appears that
the dislocation of the _tibia_ is effectually prevented in the
Lamellicorns by the protuberance and ridge of the thigh working
in their corresponding cavities, while the condyle of that part
turns with a rotatory motion in the cavity of the thigh. In the
_Orthoptera_ Order the _tibia_ is suspended by a ligament, in the
_gonytheca_ the lateral condyles, which are very prominent, working
in a sinus of that part[2033]. The subsequent Orders exhibit no
very striking variations from these types of articulation, I shall
therefore not detain you longer upon this head.
With regard to the _proportions_ and _magnitude_ of the joint we
are considering,--the most general law is, that the _anterior_ pair
should be shorter and more slender than the _intermediate_; and
the _intermediate_ than the _posterior_; and that all the _tibiæ_
should be shorter and more slender than the _thighs_, and longer
and thicker than the _tarsi_. Various exceptions, however, to this
rule in all these cases might be produced; but I shall only observe
that in all those insects in which the fore-legs are calculated for
digging or seizing their prey, as in the _Petalocerous_ beetles, the
_Gryllotalpa_, _Mantis_, &c., this joint of the leg is usually much
enlarged and more conspicuous than the others.