Chapter XXV: Letter XXXV (5)

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.

As to its _figure_ and _shape_--most commonly the _tibia_ grows thicker from the base to the apex, as in the majority of _Coleoptera_, _Hymenoptera_, &c.; in the _Orthoptera_, _Neuroptera_, &c., it is generally equally thick every where. Another peculiarity relating to this head observable in it, is its tendency to a trigonal figure: this, however, though very general, is not universal;--thus, in some _Orthoptera_, as _Pterophylla_ K., its horizontal section is quadrangular; in others, as _Locusta_ Leach and many other insects, it is nearly a circle; in some scorpions it is almost a hexagon. The superficial shape also of this joint in numerous instances is more or less triangular, but it sometimes recedes from this form:--thus, in _Callichroma latipes_ it is a segment of a circle; in some _Empides_ it is clavate; in _Onitis Sphinx_, dolabriform; in the _Orthoptera_, _Neuroptera_, &c., it is usually linear; in some _Lygæi_ it is angular[2034]: but the most remarkable _tibiæ_ in this respect are those of such species of this last genus as have the posterior ones winged or foliaceous, so that they resemble the leaf of some plant--the _tibia_ being the _rachis_, and the _wing_ (which in some species is veined) representing the _leaf_ itself. This structure is exemplified in _Lygæus compressipes_, _phyllopus_, _foliaceus_, &c.[2035] Under this head I must say a few words upon the _flexure_ of this joint, which in some cases merits notice. I have before mentioned its bend at the knee[2036] or base: the apex also is sometimes incurved--in the anterior one of the male of _Macropus longimanus_ so as almost to form a hook[2037]: in _Lygæus Pharaonis_ the posterior pair are flexuose[2038]; in _Bruchus Bactris_, _Leucospis_, and several species of _Chalcis_, these _tibiæ_ curve so as to adapt themselves to the bend of the thigh when folded. The notch on the inside of the anterior pair, in a large majority of _Carabus_ L., armed above by a spur[2039], a structure which probably assists them in seizing and detaining their prey, may also here be introduced: in the generality it is a little removed from the apex of the joint in question; but in _Pamborus_ it is very near to it, and in _Cychrus_, _Carabus_, &c., it becomes obsolete. I may mention here also a singular character which distinguishes the cubit of both sexes of _Gryllus campestris_, _domesticus_, &c. At the base there is an aperture which passes through the joint--anteriorly it is oval, and posteriorly elliptical and much larger, and on both sides is closed by a tense membrane.

The most striking peculiarities as to the _clothing_ of his joint have been chiefly noticed under the sexual characters of insects[2040], but some appear not to be of that description. In _Sphæridium_ Leach, while the thighs and _tarsi_ are naked, the posterior _tibiæ_ are remarkably beset with stiff bristles; in _Empis pennipes_ they are thickly fringed on both sides; in _Scarabæus_ M^cL. only externally, and in _Dytiscus serricornis_ internally; in _Necydalis barpipes_ K. this fringe is longer at the apex; and in _Saperda hirtipes_ Ol. the same _tibiæ_ at that part are adorned with a large brush, like that observable in the antennæ of some _Lamiæ_[2041].

I must next call your attention to the _teeth_, _spines_, and _spurs_ with which the _tibiæ_ of insects are sometimes armed. With regard to _teeth_, you have doubtless often observed those that distinguish the _cubitus_ of the arm of most Lamellicorn beetles: these vary in number from _one_, as in _Trox suberosus_, to _seven_, as in _Geotrupes autumnalis_; but the most universal number is _three_: in some species of _Geotrupes_, as _G. stercorarius_, &c., the _third_ tooth from the apex, and those that follow it, may be called _double_. These teeth, in their cubit or anterior shank, doubtless assist these insects in burrowing. The four posterior _tibiæ_ in this tribe are also distinguished by a kind of teeth which occupy their whole diameter, and resemble so many steps. I have before noticed the remarkable cubit of the _Gryllotalpa_, and likewise that of _Scarites_, _Pasimachus_, &c., in which some of the teeth are prolonged into _spines_[2042], which are the next description of tibial arms that I mentioned. Spines are of _two_ kinds--those which are merely _processes_ of the crust of the _tibia_, and those that are _implanted_ in it, and seem to have a _gomphosis_ or perhaps an _amphiarthrosis_ articulation[2043]. An instance of the _first_ kind may be seen in the hind-legs of some grasshoppers[2044] (_Locusta_ Leach), the _Rutelidæ_, &c. though in others they are implanted:--of the _second_, in the _cubitus_ of the _Mantidæ_, and of _all_ the _tibiæ_ of the dragon-flies (_Libellulina_ M^cL.)[2045];--and of _both_ kinds in the hind-legs of _Acrida_ K., those which arm the upper angles of the tibiæ being _processes_, and those of the lower being _implanted_. The term _spine_ I think ought to be restricted to the first kind; the _second_ ought rather to be denominated _spurs_ (_calcaria_), and may perhaps be regarded as in some degree synonymous with those most important appendages of the joint in question, that are implanted in or near their apex, which have been hitherto distinguished by this last denomination, and which I am next to consider. But though I have not altered a term generally adopted, I must here express my opinion that they ought rather to be considered as minute _toes_ or _fingers_, and that the denomination best agreeing with their functions, as accessories to the main toe, would be _digituli_: this is proved particularly by a character peculiar to those of many species of the genus _Cimbex_ amongst the saw-flies, in which these organs are furnished with a _sucker_ or _pulvillus_ (as they are also in _Œnas_ a kind of blister beetle), as well as the joints of the tarsi[2046]; which makes it evident that they are applied by the animal to surfaces, and assist it in walking or climbing; and in general it may be observed that in most insects their principal use is connected with these motions, and with burrowing. This circumstance tends to prove that the generality of insects (for all have not these organs) have really a didactyle or tridactyle hand or foot; and the hypothesis so often alluded to--that the _cubitus_ or _tibia_, &c., is really analogous to the _carpus_ or _tarsus_ in vertebrate animals[2047]--seems to receive no small confirmation from it; since, if the _spurs_ be really analogous to _fingers_ or _toes_, the part they articulate with cannot be the _tibia_, &c. Though the parts in question did not escape the notice of Reaumur, Linné, De Geer, Latreille, &c., yet they have not been employed in the determination of tribes, genera, &c., except by the author last named, but perhaps adopted from Bonelli[2048], in the subgenera _Zabrus_ and _Pelorus_: in many instances, however, they afford excellent subsidiary characters, sometimes common to a whole Order, and at others distinguishing its various subdivisions. With regard to their _number_--I have noticed many variations which I will now state to you, first observing that I shall express them by _three_ figures, the _first_ representing the number of spurs on the _anterior_ leg, the _second_ that of those on the _intermediate_, and the _third_ on the _posterior_; and where there are spurs, as in the _Trichoptera_ and _Lepidoptera_, on the _middle_ as well as at the _end_ of the _tibia_, I shall express it by one figure _over_ another, the upper one representing the number of the _middle_ spurs. If you make an examination yourself, it will be proper to remind you that these little organs are extremely liable to be broken off, but the socket in which they were planted is usually very visible. The most _natural_ number is represented by 2:2:2; this you will find very prevalent in the _Coleoptera_ Order, as in the Predaceous and numerous other beetles: in the _Orthoptera_ and _Hemiptera_ Orders, however, I have not discovered an instance of it; but in all the rest it more or less occurs: next to this number--_tibiæ_ with obsolete or no spurs seem most prevalent, particularly in the _Hemiptera_; not a single instance of an insect furnished with them occurring to me in the _Heteropterous_ section; and it is doubtful whether there are any in the _Homopterous_.--Having stated the most universal structure in this respect, I will next consider the Orders seriatim. Amongst the _Coleoptera_ though the numbers 2:2:2 are most frequent in occurrence, yet there are numerous exceptions. Thus, in the Lamellicorns, 1:1:1 represents the _calcaria_ of one tribe of the _Scarabæidæ_ M^cL. formed of the genus _Scarabæus_ M^cL.; 1:2:1 represents those of another tribe of that family, including the subgenera _Ateuchus_, _Copris_, _Phanæus_, &c.; 1:2:2 again forms the character in this respect of _Aphodius_ and the great majority of the Lamellicorns; while 2:2:2 is confined in this section to _Æsalus_ F. and _Melolontha chrysomeloides_ Schranck (_Psephus_ M^cL. MS.). In the other tribes of _Coleoptera_ other numbers occur. Thus, 0:1:1 characterizes _Hylœcetus_; 0:1:2 _Mordella_; 0:2:2 _Macropus_; 1/1:2:2 _Harpalus_, and all those _Carabi_ L., except _Zabrus_, that have a _notch_ in their anterior _tibiæ_; 1/2:2:2 _Zabrus_. In the _Orthoptera_ Order it is not easy to distinguish the real _spurs_ from the implanted _spines_ that frequently arm the legs: these in _Blatta_ are extremely numerous, even at the apex of the _tibiæ_; but I cannot distinguish any that can be regarded as true analogues of the former: the most natural number of spurs in this Order is represented by 0:0:4; this you will see in all the Locusts; in _Acrida_, _Conocephala_, _Pterophylla_; and in _Truxalis_, _Pneumora_, &c.; in _Phasma_ there are none. In _Mantis_, if the terminal process of the _cubitus_ is excluded, it will be 0:2:2; in _Gryllotalpa_, admitting the terminal teeth of that part[2049] as analogues of spurs, the number is 4:4:4; in _Tridactylus_ Latr. 0:0:5[2050]; in _Gryllus_ Latr. 3:3:5; in _Gryllus monstrosus_, 4:4:6. In the whole _Hemiptera_ Order I have discovered no instance of an insect furnished with the real spurs: for though in _Tettigonia_ F., _Cercopis_, &c., there are implanted spines in the posterior _tibia_, and several at the apex, there are none of them clearly analogous to real spurs. In the _Lepidoptera_ the most general arrangement appears to be 1/0:2:2/2; and next to this, 1/0:2:2. In this Order most commonly there is no spur at the end of the cubit, but one resembling a thumb[2051] arms its middle; in _Pieris_, &c., this thumb is not present, so that the number is 0:2:2; in _Agarista_ Leach, _Erebus_, &c., you will find 1/0:2:4, the posterior _calcaria_ being all terminal; and in _Attacus Atlas_, all these organs are obsolete except the thumb. In the _Neuroptera_ the most general arrangement is 2:2:2; but in the _Libellulina_, although the legs are very spinose, there are no spurs. In the _Trichoptera_ K., in _Phryganea rhombica_ and affinities, the number of them is expressed by 1/2:1/2:1/2; and in those with long antennæ, _P. atra_, &c., by 2/2:2/2:2/2. In the _Hymenoptera_ the number 1:2:2 is most prevalent; and next to this, as in _Apis_ L., 1:1:2. In the _Ichneumones minuti_ L. the spurs are 1:1:1; in _Atta_ Latreille, a kind of ant[2052], 1:0:0. In the _Diptera_ it is often difficult to distinguish the spurs from the spines; but the number most universal is, I think, 2:2:2; in _Tipula_ it is 1:2:2; in the _Tabanidæ_ 0:2:0; and in _Culex_, _Limonia_, &c., there are none. Amongst the insects with more than six legs, most commonly the tibiæ have no spurs; but in the _Araneidæ_ each is armed with _two_, a circumstance which also distinguishes the corresponding joint of the _pedipalpi_.

These little organs inosculate each in an appropriate socket of the end, or in many cases of the middle of the _tibia_; and that part of their head or base that is received by it, is often constricted for the purpose: from hence it follows that they are capable of some degree of motion, but in some insects, as those on the four posterior legs of _Scarabæus sacer_ and its more immediate affinities, and those at the end of the cubit of _Gryllotalpa_, they are immoveable, and appear almost processes of the joint to which they belong. They are commonly sharp, of a subtriquetrous figure, with the lower side flat: where there are two, the outer one is usually the longest; and in general the spurs on the hind legs are longer than those on the four anterior: but there are exceptions--thus, in _Acanthopus_ Latr. the _intermediate_ spurs are the longest; and in _Cicindela_ the _anterior_ are longer than the former; in _Blaps mortisaga_ those on all the legs are nearly equal in length. They vary sometimes in _shape_--those on the middle of the cubit of many _Lepidoptera_, which may be regarded as a kind of thumb[2053], are of a lanceolate shape; in _Meloe_ the external posterior one is flat and obtuse; in _Œnas_ Latr. it is obconical, concave at the extremity, and apparently furnished with a sucker; in _Ateuchus smaragdulus_ the _anterior_, and in _Copris Carolina_ the _posterior_ is forked and emarginate; in _Sirex_ the former is hooked and winged; in _Lamprima_ it is triangular and dilated; in _Aphodius analis_ it is dolabriform; in _Dynastes retusus_ and _Juvencus_ the spurs are bent like a bow. In many _Hymenoptera_, as the _Sphecidæ_, they are pectinated[2054], with a series of minute parallel spines--a structure which assists the animal in burrowing[2055]; in _Acanthopus_ Latr. they are armed with little teeth or spines[2056]; in the hive bee the spur of the cubit is furnished with a membranous appendage which I have called the velum[2057]; and in a subgenus related to _Saropoda_ Latr. (_Ctenoplectra_ K. MS.), the interior spur of the posterior leg is crescent-shaped, fixed transversely, and fitted on the inner side with a membrane, the edge of which is finally pectinated.

e. _Tarsus_ or _Manus_[2058]. This is the _last_ portion of the leg, usually supposed to be analogous to the hand or foot of vertebrate animals; but, according to the hypothesis so often alluded to, rather the representative of their jointed finger or toe. In treating of this part I shall consider its _articulation_ with the _tibia_, and of its joints _inter se_; the _number_ of those joints; their _proportion_ and _shape_; their _parts_ and _appendages_.

I seem to have observed _three_ kinds of tarsal _articulation_. The _first_ is a species of _enarthrosis_ or ball and socket, the joints terminating in a globular head, perforated indeed for the transmission of muscles, &c., and which is received by a corresponding cavity of the _tibia_ or preceding joint, as may be seen in many weevils (_Curculio_ L.[2059]). This admits of some rotatory motion.--The _second_ is a _mixed_ articulation between _enarthrosis_ and _ginglymus_, when at the base of the ball a deep transverse channel receives a corresponding ridge of the _tibiæ_ or preceding joint: this may be found in _Rutela_ and probably many other Lamellicorn beetles; and something very similar in the Predaceous ones.--The _third_ kind is where there is little or no inosculation, and the joints are scarcely more than _suspended_: this takes place in the _Orthoptera_, _Neuroptera_, &c.; but in _Blatta_ and the hind legs of _Mantis_ there is some approach to the foregoing kinds.

We are now to consider the _number_ of joints of the _tarsus_, which varies considerably in the different Orders, and in one has been assumed as a clue for a subdivision of it into sections[2060], which, though not perfectly natural, is very convenient, and has been adopted by most modern Entomologists. In treating of this head, I shall use those denominations that have been employed by M. Latreille and others to express the variations of the number of the tarsal joints in the _Coleoptera_, but shall apply them to insects in general. Insects in this view, therefore, may be called _pentamerous_; _heteromerous_; _tetramerous_; _trimerous_; _dimerous_; or _monomerous_.

_Pentamerous_ insects are those which have _five_ joints in _all_ their _tarsi_. This is the most universal, and may be called the _natural_ number of these joints. More than half the _Coleoptera_ belong to this section; in the _Orthoptera_--the _Blattidæ_, _Mantidæ_, and _Phasmidæ_; all the _Lepidoptera_ except those butterflies called _tetrapi_ (_Vanessa_, &c.); all the _Trichoptera_, _Hymenoptera_, and _Diptera_; in the _Neuroptera_--_Ascalaphus_, _Myrmeleon_, _Hemerobius_, _Corydalis_, &c.; and in the _Aptera_--_Pulex_[2061].

_Heteromerous_ insects are those in which the number of these joints varies in the different pairs of legs[2062]. These variations, like the spurs, may be expressed by three figures, the first representing the _anterior tarsus_, the second the _intermediate_, and the third the _posterior_. I begin with 5:5:4. This number represents those beetles that have been _exclusively_ regarded as _heteromerous_ by modern Entomologists--of this description is the Linnean _Tenebrio_, _Meloe_, &c., now subdivided into numerous genera; they have _five_ joints in the two anterior pair, and _four_ in the posterior. The tarsal joints of the aquatic genus _Hydroporus_ (a singular anomaly in the Order to which they belong) are expressed by 4:4:5, thus reversing the number in the preceding tribe: other Heteromerous genera are to be found amongst the _Hemiptera_. Thus, in _Ranatra_ the numbers are 2:1:1; in _Sigara_ and _Nauceris_ 1:2:2; in a new subgenus between _Belostoma_ and _Naucoris_ (_Xiphostoma_ K. MS.), brought by Dr. Bigsby from Canada, 3:2:2; in the _Lepidoptera_ the butterflies called _tetrapi_ (_Vanessa_, &c.) may be expressed by 1:5:5. Amongst the _Aptera_ and _Arachnida_ there are three remarkable genera, which if their pedipalps are included may be deemed Heteromerous. I mean _Phrynus_, _Thelyphena_, and _Galeodes_;--in the former the numbers will be *:4:4:4, the asterisk denoting more than ten; in the second, 8:4:4:4.; and in _Galeodes_ (in which the first pair of pedipalps are not chelate, the mandibles performing their office) the numbers are 1:1:3:3:3.[2063]

_Tetramerous_ insects are those in which all the _tarsi_ consist of _four_ joints; these in the _Coleoptera_ are next in number to the _pentamerous_--indeed a very large proportion of them strictly speaking are really of the latter description, since in Linné's four great genera, _Curculio_, _Cerambyx_, _Chrysomela_, and _Cassida_ and some others, the _claw-joint_ (_ungula_) consists of _two_ articulations, one very short, forming merely the ball at its base[2064], which inosculates in the socket of the preceding joint, and the other constituting the remainder: if you carefully separate these two pieces, you will find that the last inosculates in the summit of the ball, and is moved by appropriate muscles[2065]. This structure probably permits the readier elevation and depression of this joint. In the _Orthoptera_ the tetramerous genera are those which Linné called _Tettigonia_ amongst his _Grylli_ (_Locusta_ F.); _Acheta monstrosa_ also, and in the _Neuroptera_, _Raphidia_ belong to this section.

_Trimerous_ insects are those whose _tarsi_ consist of only _three_ joints. Amongst beetles the Lady-birds (_Coccinella_ L.) are remarkable for this structure, but in them the claw-joint is also _biarticulate_, so that strictly speaking they are _tetramerous_; in the _Orthopterous_ Order the migratory locusts (_Locusta_ Leach) belong to this section, as likewise _Gryllus_ Latr. and _Gryllotalpa_ Latr.: in the first of these genera is an appearance of there being more joints in the _tarsus_, because there is more than one cushion below the first[2066]. To this section also belong the great majority of the _Hemiptera_, excluding only those tribes that connect the two sections of the Order constituting the two Linnean genera _Nepa_ and _Notonecta_; the _Libellulina_ likewise belong here, as do also the _Scorpionidæ_ and _Scolopendridæ_.

_Dimerous_ insects are those that have _two_ joints in all their _tarsi_. Such are the _Pselaphidæ_ in the Coleoptera Order[2067]; in the _Hemiptera_--_Belostoma_ and _Notonecta_; in the hexapod _Aptera_--_Pediculus_; in the octopod--the _Acari_ of Linné; in the myriapod--_Iulus_; and in the _Arachnida_--the _Araneidæ_.

_Monomerous_ insects are those which have only a single tarsal joint. Only one Coleopterous and also one Hemipterous genus is so distinguished: the first is _Dermestes Armadillus_ De Geer[2068], and the second the common water-scorpion, _Nepa_ Latr. Among the _Aptera_ we find _Nirmus_, _Podura_, _Sminthurus_, &c., that belong to this section.

To the above sections another may be added for those insects whose _tarsi_ have more than _five_ joints, which may be denominated _Polymerous_. Here belong the genera _Gonyleptes_ K., _Phalangium_ and _Scutigera_ Latr. In the first the number of joints varies from _six_ to _eleven_, and in the two last they far exceed that number, amounting in some species of _Phalangium_ to more than _fifty_, and becoming convolute like the antennæ of Ichneumons[2069].

I am next to notice the _proportions_ and _shape_ of the _tarsus_ and its joints. The most general law is, that it shall be shorter and more slender than the _tibia_; but it admits of several exceptions--thus, in _Megasoma_ K.[2070], in _all_ the legs; in _Agrostiphila_ M^cL. MS.[2071] in the _intermediate_, and in _Amphicoma lineata_ in the _posterior_ pair the _tarsi_ are the longest; in _Trichius Delta_ these last are longer than the thigh and _tibia_ together. In some insects the _tarsi_ are disproportionally short, as in _Cassida_, the _Pselaphidæ_, _Locusta_ Leach, &c. Though generally more slender than the _tibia_, in several instances they are as thick or thicker, or more dilated, as in most of the tetramerous beetles, which being climbers require a dilated _tarsus_. Again, comparing the three pairs of this joint with each other, the most general rule is, that the _anterior_ should be the _shortest_, and the _posterior_ the _longest_: but in some, as the Capricorn beetles, &c., they are nearly equal in length; in others, as _Lytta marginata_, the _anterior_ pair, and in _Rhipiphorus_ the _intermediate_, are the longest; in _Trichius Delta_ these last are the shortest. With respect to _thickness_, the anterior _tarsi_, except in many males[2072], are not very strikingly different from the rest.

With regard to the proportion of the joints of the _tarsus_ to each other,--according to the most general law, the first is the longest, the last next in length, then the second and third, and the fourth is the shortest. In _Gonyleptes_ K. and other _Phalangidæ_ the first is almost thrice the length of all the rest taken together; but there are numerous exceptions to the rule. In the female _Carabi_ the first joint is not longer than the last, and in the males not so long; and in _Hydrophilus_, &c., it is the shortest of all. Again, the second joint is longer than the three following ones in _Dasytes ater_[2073]; and than the last in _Cicindela sylvatica_: the _third_ joint is shorter than the fourth in _Lampyris ignita_: it is longer than the first in _Donacia_, many _Melolonthidæ_, &c. Once more, the _fourth_ joint, usually the shortest of all, is longer than the second and third in _Anthia_, &c. Lastly, the claw-joint, usually the second in length, in the _Eproboscidea_ Latr. (_Hippobosca_ L.) is very long and large, while the four first joints are so extremely short as to be scarcely distinguishable from each other: it is the shortest of all in _Colymbetes_, &c.; it is of the length of the _third_ in _Cicindela sylvatica_, of the _fourth_ in _C. sexguttata_. Though commonly the slenderest joint of all, particularly so in _Raphidia_, in many Heteromerous and Lamellicorn beetles it is the largest, conspicuously so in _Mellinus tricinctus_. Sometimes, as in _Buprestis chrysis_, &c., all the tarsal joints are nearly equal in length and thickness.

We are next to say something upon the _shape_ of the _tarsi_ and their joints. In general we may first observe that their upper surface is commonly more or less convex, and the lower flat or concave: in insects that are swift _runners_, as the terrestrial Predaceous beetles, they are usually slender and filiform[2074]; in those that swim, as _Dytiscus_, the two posterior pair taper nearly to a point from the base to the apex[2075]; in some that climb, as _Buprestis_, they are rather flat and linear; and in others (the Weevils, _Curculio_ L.) they grow gradually wider towards the claw-joint[2076]; sometimes, as in _Mordella_ Latr., the four anterior _tarsi_ are of this shape, and the posterior pair setaceous. In _Gyrinus_ the four posterior are flat and triangular; and in that extraordinary insect _Gryllus monstrosus_ the _tarsi_ are foliaceous and lobed[2077]. In many males and some others the _anterior_ pair or _hands_ are of a different shape from the two _posterior_: thus, in several _Carabi_ they are lanceolate; in _Staphylinus_, _Creophilus_, &c. in both sexes they are often nearly circular, like those of male _Dytisci_[2078]. With regard to the _shape_ of individual joints it may be said in general that they are rather triangular, with an anterior sinus for the reception of the succeeding joint: the first joint usually departs most from this form; in the bees it is commonly much larger than the rest, especially in the last pair of legs, and nearly forming a parallelogram[2079]; in _Euglossa_ it is trapezoidal; in the majority nearly linear or filiform. With regard to their _termination_--in _Brachycerus_ and some ants (_Ponera_, _Myrmica_, &c., Latr.) the _three first_ joints; in _Dascillus_, _Lycus reticulatus_ and affinities, the _third_ and _fourth_; and in the great majority of the Tetramerous insects the _penultimate_ joint is bilobed; although in most Predaceous beetles this joint is entire or simply emarginate, yet in _Colliuris_ it terminates in a single oblique lobe; and in _Lebia_, _Drypta_, &c., it is nearly bipartite. I must now advert to the _Ungula_ or claw-joint: it is usually clavate or thickest at the end and curved; but in the _Asilidæ_ it is shaped like a vase or cup; in _Phanæus_, in the four posterior _tarsi_, in which the claws are obsolete, it is thickest at the base and sharpest at the extremity[2080]; it usually forms an angle with the rest of the _tarsus_, rising upwards, which enables the insect to move more easily without hindrance from the claws, and also more readily to lay hold of any object it meets with; but in the Lamellicorn beetles and many other insects it is in the same line with it. As in the beetles last mentioned this joint is often inserted in the extremity of the preceding one; but in _Œdemera_ it articulates with the middle of its upper surface; and in _Lycus_ and a numerous host of _Tetramerous_ beetles it springs from its base, just behind where it diverges into two lobes.

I shall next call your attention to the different kinds of _appendages_ with which the _tarsi_ are furnished. They are seldom armed, like the _tibiæ_, with teeth, or spines, or horns; but something of the kind occasionally distinguishes them. In _Phileurus_, _Oryctes_, and several other _Dynastidæ_, the first joint is armed at the apex externally with a considerable mucro; in the fore-leg of _Dasytes ater_ a similar process is prolonged into a crooked horn[2081]. But the most important appendages of the _tarsi_ are the _claws_ which almost universally arm their extremity, and which appear clearly analogous to those of _birds_, _quadrupeds_, &c., though probably differing as to their substance[2082]. Some few, however, are without them; this, as I lately observed, is the case with _Phanæus_ with respect to the four _posterior_ legs; the _anterior_ ones of _Vanessa_ amongst the _Lepidoptera_, and all those of _Stylops_ and many _Acari_ L., are also without them: this is likewise the case with the first pair of legs, or the second of the pedipalps of _Galeodes_. In this genus these organs consist of two joints[2083]. With respect to _number_ they vary in different tribes, but not so much as the _calcaria_: these variations may likewise be represented by three numbers. The most natural is _two_ in all the _tarsi_, exhibited by the Predaceous beetles and the great majority; 2.2.1. are to be found in _Hoplia_, _Anisonyx_, &c.[2084]; 1.2.2. in _Belostoma_; three in all the legs in the _Araneidæ_[2085]; in _Meloe_[2086], _Elater_, &c., each claw is double or consisting of two, which makes _four_ in each leg; and in many _Hippoboscidæ_ there are _six_[2087]; in _Nepa_ and the Myriapods there is only _one_. In most insects, perhaps, the claws are simple or undivided[2088]; but in _Galeruca_, _Melolontha subspinosa_[2089], &c., they are bifid at the apex; as is the exterior claw of the four posterior legs in _Chasmodia_ and _Macraspis_[2090] M^cL., and of _all_ in _Melolontha horticola_; in _Serica brunnea_ M^cL. the claws are all cleft at the extremity, but the internal tooth is broad, flat, and obtuse[2091]; in _Melolontha vulgaris_ and _Pelidnota punctata_ M^cL.[2092], the claws are armed with an internal tooth near the base[2093]. In the _Araneidæ_, which have three claws, the two external ones are furnished with several parallel teeth, which the animal uses to keep separate the threads of its web, and probably for other purposes[2094]; and some Predaceous beetles, as _Lebia_ and _Cymindis_, have both their claws similarly furnished[2095]. These organs vary in their relative _proportions_: thus, in _Anoplognathus_ the inner claw is much smaller than the other[2096]; and in _Elater sulcatus_, _fuscipes_, &c., it is represented by a mere bristle; in _Hoplia_, in the anterior _tarsus_ it is not half the length of the outer one[2097]; in _Areoda_ and _Pelidnota_ M^cL. this last is the smallest. They vary also in length--in _Rynchænus_, _Ascalaphus_, &c., they are very short; in the Lamellicorns, _Galeodes_, &c., very long; and in _Myrmeleon_ longer than the claw-joint. With regard to their _curvature_ they generally form the segment of a circle; in many _Asilidæ_ they are crooked like the claws of the eagle[2098], and the posterior one of the _Hopliæ_ is bent like a hook[2099]; they most commonly diverge from each other; but in the _Rutelidæ_, _Anoplognathidæ_, &c., they are perfectly parallel, and in the former often inflexed[2100]. With regard to _other_ appendages of the part we are treating of, if you examine the stag-beetle and many other Lamellicorns, you will find between the claws a minute but conspicuous joint terminated by two bristles which seem to mimic the _ungula_ and its claws; these parts are what are denominated in the table the _palmula_, _plantula_, and _pseudonychia_: in the stag-beetle these are long[2101]; in the _Melolonthidæ_ short[2102]; and in many _Cetoniadæ_ they resemble an intermediate claw.

The most remarkable of the _appendages_ of the _tarsi_ are to be looked for on their under side or sole (_solea_), and are the means by which numbers of insects can overcome atmospheric pressure and walk against gravity. Many of these have been fully described in a former letter[2103]; but much that relates to them was there omitted, which I shall now detail to you. _Four_ kinds of _pulvilli_, as I would call these appendages, are found in the sole of insects, upon each of which I shall make a few remarks.

The _first_ is a _cushion_ or brush composed of very thickly set hairs or short bristles: examples of this you will find in the majority of _Tetramerous_ and _Trimerous_ beetles. In _Chrysomela_, _Timarcha_, &c., there is _one_ of these cushions on each of the _three_ first joints; in _Prionus_, _Liparus_, &c., there is a pair; and in _Coccinella_ on the _two_ first; in others (_Balaninus Nucum_, &c.) a pair only on the penultimate joint; in _Calandra Palmarum_, _Rhina barbirostris_, &c., that joint has an intire cushion; in _Eurynotus muricatus_ K.[2104] the three first joints of the four anterior tarsi are similarly circumstanced, but the cushions resemble sponge[2105].

The _second_ kind of cushion is a vesicular _membrane_ capable of being inflated. This distinguishes the tarsi of _Thrips_[2106], and many _Acari_ L.[2107]; likewise those of _Xenos_[2108]; and also of many _Orthoptera_ fully described on a former occasion[2109], though the fact of their capacity of inflation has not been ascertained, belong to this section.

The _third_ kind of covering of the sole is when the three or four first joints of the _tarsus_ each terminate in _one_ or _two_ membranous lobes or appendages: of the first description is _Priocera_ K., in which the lobes are involute[2110]; and of the second _Rhipicera_ Latr.[2111], in which there is a pair on each joint, in the Brazil species set with very fine hairs.