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.

The fourth and last kind are what may with the utmost propriety be denominated suckers, since their use as such is clearly ascertained. These are not only to be found in a large proportion of the Diptera, in some of which there are two of them, as in the Asilidæ[2112]; and in others three, as the Tabanidæ[2113]; but also in many of the subsequent Orders: thus, in the Heteropterous Hemiptera, in Scutellera and Pentatoma, but not the Reduviadæ, and in the Neuropterous genus Nymphes Leach there is a minute one under each claw. It is discoverable between the claws in many Hymenoptera, as Apis[2114], Vespa, &c. But the genus that exhibits to the curious Entomologist the most singular and elaborate apparatus of this kind is Dytiscus Latr.; and the examination of the under side of the hand of any male of this genus will almost compel the most inattentive observer to glorify the wisdom and skill of the Allfather so conspicuously manifested in the structure of these complex organs. For this part in these, instead of two or three pedunculate cups as in the insects just mentioned, is composed of a vast number, some large and some small. If you take a male specimen of the common D. marginalis, you will find that the three first joints of the hand are very much dilated, so as to form a plate or shield nearly circular, fringed all round with stiffish hairs; if you next examine the under side of this plate with a good magnifier, you will discover at the base, where it is united to the cubit, two circular cups, the external one more than three times the size of the other, with an umbilicated centre[2115]; besides these two larger cups the rest of the shield is covered by a vast number of minute ones of a similar construction[2116]: the larger cups are nearly sessile, but the smaller are elevated upon a tubular footstalk[2117]; the three first joints of the intermediate tarsi are also dilated, but not into an orbicular shield, and thickly set with minute pedunculated suckers[2118]. The structure varies however in different species. Thus in D. limbatus the shield is triangular with the smaller suckers at the base, and two rows of larger oblong ones, concave but not umbilicated, at the apex; in another Brazilian undescribed species (D. obovatus K. MS.) the shield is oblong and quite covered with suckers like those last mentioned; in D. sulcatus (Acilius Leach) almost the whole plate is occupied by a very large sucker, above which, at some distance in the inner side, are two smaller ones, while the extremity of the shield is covered by minute ones elevated on long footstalks: the central umbilicated elevation of the large one, which nearly fills its cavity, is in this species beautifully radiated. The male of Colymbetes transversalis has also an orbicular shield, but the suckers are much less strongly marked. The use of this organ has been before sufficiently explained[2119].

A few words will be necessary upon the folding of the legs in repose. When insects walk, the thigh is usually in an ascending position, rising above the horizontal line, the tibia forming with it rather an obtuse angle, and the tarsus nearly a right one with the tibia; but in the Myriapods, as far as I can unravel their swift many-footed motions, these angles in walking do not take place; in repose however, in many insects, the coxa forms an angle with the thigh below the horizontal line and with the tibia above it, and the tibia and tarsus continue in the same line, and point downwards nearly vertically; in others, as in the Tetramerous beetles, the last-mentioned joints form an angle with each other and turn upwards, the tibia having an external oblique cavity to permit this; but the insects most remarkable for packing close their legs are those carnivorous genera Dermestes, Anthrenus, Byrrhus, &c. In the last-mentioned genus there are cavities in the under side of the trunk, in each division of the breast, and at the base of the abdomen, to receive the legs when folded; the coxæ have also a cavity to receive the base of the thigh. In the anterior legs this last part has a longitudinal one on its upper side, and in the four posterior on the under, which receives the tibiæ, which at the inner edge are straight, and at the outer curvilinear, and the tarsi are turned up and received by the concave part, on the anterior side of the first pair and the posterior side of the two last of the tibiæ, so as to lie between it and the body: when the legs are close packed, the animal looks almost as if it had none. I have observed that when Dytisci repose on the water, the posterior legs are turned up and laid over the elytra, and curved towards the head.

vi. Pectines. I must next say a few words upon a remarkable organ, which seems in some degree supplementary to the legs, by which the Creator has distinguished the genus Scorpio, called from its parallel teeth, set in a back, their pecten or comb[2120]. This back consists of two or more articulations, is attached by its anterior extremity to the sides of the posterior piece of the mesostethium, and is marked by a longitudinal furrow or channel. The teeth, which vary in number in the different species, and in the same species at different periods of its growth, are usually ovato-lanceolate, or obtusangular, furnished on their exterior edge with what appears to be a longitudinal sucker, and supported between their bases, or at the base, both within and without, by triangular, conical, or subglobose props. With regard to the use of these organs, it has not been clearly ascertained. Amouroux states that he has seen the animals use them as feet, and he conjectures that by them they may fix themselves and turn upon them as on a pivot, when they have to make a retrograde movement[2121]. M. Latreille, from their having branchial pouches immediately under them, seems to think that they are connected with respiration[2122]. This may be true; but from the suckers just described, I am inclined to think with Amouroux, that they are useful to the animal in its motions, and that like the suckers of the Gecko, flies, &c., they enable it to support itself against gravity and to climb perpendicular surfaces.

Whether the five obtriangular plates, elevated on a pedicle, which are found arranged in a series on the under side of each of the jointed coxæ of the posterior legs in Galeodes, are at all analogous to the pectens of scorpions, has not been ascertained[2123]. M. Leon Dufour watched them very attentively in one species (G. intrepidus), but he could observe no motion in them[2124].

LETTER XXXVI.

EXTERNAL ANATOMY OF INSECTS CONTINUED.

THE ABDOMEN, AND ITS PARTS.

The abdomen of insects, which we are next to consider, is the third great section of the body, and is the seat of the organs of generation, as well as of a principal part of those connected with respiration. My remarks upon it will be under the following heads: Its substance; articulation with the trunk; composition; shape and proportions; its appendages; and its clothing.

i. Substance. Under this head I may observe in general, that where the abdomen is protected by hard elytra or tegmina, as in most Coleoptera, and many Heteropterous Hemiptera, the upper side is generally of a softer and more flexible substance than the under, which from its exposure requires a greater degree of hardness and firmness to prevent its being injured. In some,—as the Dynastidæ and those beetles whose elytra are connate, or as it were soldered together, the former is scarcely more than membrane. In others of the above tribes, nearly the whole of the back of whose abdomen, as in Staphylinus; or only its anal extremity, as in Melolontha; or its sides, as in Lygæus, &c., is not covered by the elytra or tegmina, that part, as was requisite for its protection, is harder than the covered portion.

ii. Articulation with the trunk. Two distinct modes of this articulation take place:—in the first the abdomen is united to the trunk by the whole diameter of its base, without any appearance of incision; in the other only a small part of that diameter, with a very visible incision. All the Orders, except the majority of the Hymenoptera and Diptera, and the Araneidæ, belong to the first of these sections; for in all these the aperture by which the abdomen is suspended to the trunk, occupies the whole of the base; I say suspended, because, though in many cases it inosculates in the posterior cavity of the latter part, it does not in all, and the margins of the orifice are united by ligament to those of that cavity. Indeed, in the Coleoptera and others that have a somewhat prominent metaphragm[2125], the trunk may with more propriety be said to inosculate in the abdomen. With regard to the second section,—those in which the orifice is of less diameter than the base, occupying only a portion of it,—it may be further subdivided into those whose abdomen is sessile, and those in which it is united to the trunk by the intervention of a long or short pedicle or footstalk: to the first of these subdivisions belong all those Diptera that have an incision between the trunk and abdomen—for many tribes of this Order, as the Tipulidæ, Asilidæ, &c., belong rather to the first section—and the Araneidæ; the abdomen, however, in all is merely suspended, without any inosculation. To the second subdivision belong all the Hymenoptera, except the Tenthredinidæ and Siricidæ, the abdomen of which is united to the trunk by the whole diameter of its base; these may be further subdivided into those that have a very short pedicle and those that have a long one; but as the mode of articulation in both these is the same, there will be no necessity to consider them separately. M. Cuvier has included the Diptera and Araneidæ in the same tribe with such Hymenoptera as have a petiolate abdomen[2126]; but as the manner in which the latter articulates with the trunk is widely different from that of the Diptera &c., I thought it best to consider them as distinct; especially as in the Diptera there is no tendency to a pedicle, while only the above two tribes of Hymenoptera are wholly without it. This learned author thus describes the articulation where the abdomen is connected by a pedicle. "They have," says he, "a real solid articulation, a kind of hinge in which the first segment is emarginate above, and receives a saliant portion of the trunk upon which it moves; this articulation is rendered solid by elastic and powerful ligaments; muscles which have their attachment in the interior of the trunk are inserted in this first segment, and determine the extent of its movement[2126]." But this passage by no means conveys an adequate idea of the singular mechanism by which the Divine Artificer has enabled these little creatures to impart the necessary movements to an organ so bulky compared with its very diminutive point of attachment. As no author that has fallen in my way has examined the articulation of the abdomen with the trunk in these Hymenoptera with the attention which it merits[2127], I shall enlarge a little upon it. You would be surprised, and not without reason incredulous, were I seriously to assert that these insects lift their weighty posteriors by means of a rope and pulley; yet something like this really does take place, though not with all in a manner equally striking. The point of articulation in the insects in question, except in Evania, is at the base of the metathorax just above the posterior pair of legs: here you see a small orifice, either insulated or connected by a narrow opening with the larger one, when the abdomen is removed, which in many instances, as in the common wasp, is surmounted by another still smaller, through which, if you examine it attentively, you will find there is transmitted a flat and sometimes broadish ligament or rather tendon, in which the levator muscles of the abdomen, attached by their other end to the metaphragm[2128], terminate: another minute orifice above the base of the pedicle affords a point of attachment to the tendon, so as to give it prize upon the abdomen. Here the upper orifice in the trunk is the pulley (trochlea)[2129], the tendon is the rope (funiculus)[2129], and the abdomen is the weight to be lifted. When the muscles contract, the tendon, running over the edge of the aperture, is pulled in, and the part just named is elevated; and when they are relaxed the tendon is let out, and it falls. Some little variation in the structure takes place in different tribes: thus, in the Formicidæ, Scoliadæ, &c., instead of a separate orifice, the part I call the pulley is merely an upper sinus of the large orifice that receives the pedicle of the abdomen. The shape of these orifices, both of the trunk and abdomen, varies in different genera: thus, in the bee it is triangular, with the vertex reversed; and in the wasp the upper one is circular, and the lower one transversely oblong; but in all, the apertures of the trunk correspond with those of the abdomen. In Evania, in which the minute abdomen is inserted in the upper side of the metathorax, there is scarcely any trace of this structure. With regard to the articulation of the pedicle itself with the lower orifice of the trunk, it appears simply suspended, with little or no inosculation. I may observe under this head, that though the abdomen in almost all insects is wholly clear of the cavity of the trunk, yet in some Phalangidæ (Gonyleptes K.) it appears almost retracted within it[2130].

iii. Composition. I shall next consider the segments into which the abdomen is usually divided, their number, and other circumstances connected with them. In the Hippoboscidæ, Acaridæ, Phalangidæ, and Araneidæ, the part we are considering is not divided into segments, though in some instances, as in Gonyleptes and the cancriform Epeiræ[2131], they are represented by folds; but in the great majority of insects it consists of several dorsal and ventral pieces or segments, forming by their union the annuli or rings into which it appears divided. The number of these abdominal segments varies in different insects; I have noticed more than twenty such variations, and probably there are many more. Before I give you them in detail, I must first observe that the dorsal and ventral segments, though sometimes they correspond in number, yet very often do not, the dorsal most commonly exceeding the ventral by a segment; in a few cases however the reverse takes place. In the sexes also there is frequently a difference in the number of segments, as has been before observed[2132]. I shall express the variations in question by two figures, the first representing the number of dorsal segments, and the second the ventral—they usually only express the apparent segments: perhaps a very general examination and dissection might bring many of them nearer to a common type.

I shall next explain the articulation of the segments with each other, both that of the rings formed by the union of the dorsal and ventral pieces, and that of those pieces themselves. In general it may be stated with respect to the former, that each ring is suspended by ligament to that which precedes it; but this takes place in three ways—in some the margins of the suspended rings touch each other only, with little or no inosculation; in others the dorsal segments only touch, and the base of each ventral is covered more or less by the apex of the preceding one; and in others again the base of the whole ring, both above and below, is so covered, or inosculates. The first kind here mentioned you will find exemplified in Melolontha, Geotrupes, Musca, &c.; the second in Scorpio; and the third in Staphylinus, the Hymenoptera, and many others. In the Coleoptera, says M. Cuvier, speaking of the movements of the abdomen, the rings only touch each other at the margin, and the movement is very limited; whilst in the Hymenoptera they are so many little hoops, which inosculate in each other as the tubes of a telescope, one third only of their extent often appearing uncovered[2136]. We see the reason of this structure when we consider the calls they have for greater powers of movement in this part in laying their eggs, and annoying their enemies and assailants; and also in the Staphylinidæ to enable them to turn up their abdomen like a scorpion, both as a posture of attack, and to fold their wings: in all cases, however, as far as my observation goes, these animals, when they want to lengthen this part, can disengage the rings from almost all inosculation, so that no impediment remains to any movement.

The articulation of the dorsal and ventral segments with each other is next to be considered. In Iulus and some Centroti the ring appears to be formed of a single piece, with scarcely any trace of the existence of any such division; it is however almost universal, and is of three descriptions; in the first the dorsal segments are united to the ventral at the lateral margin or edge of the abdomen; in the second it is above this margin, and in the third below it. You will find that in Fulgora and many other Homopterous Hemiptera these segments unite at the margin, as they do likewise in Cimex lectularius belonging to the other Hemipterous section; but in the rest of the Heteropterous tribes, the ventral segments turn upwards, and their union with the dorsal is in the back of the abdomen; in these the Hemelytra and wings only cover the dorsal segments, leaving the edge, formed of the ends of the ventral, uncovered. The Lamellicorn beetles also, and many other Coleoptera, exhibit the same structure. To the last description, in which the dorsal segments turn down to meet the ventral, belong the Lepidoptera, Locusta Leach; likewise Sirex, Chrysis, and many other Hymenoptera. The articulation between these segments is by means of an elastic membranous ligament, which usually is not externally visible; but in many instances, in which the connecting ligament is of a firmer substance, as in Scorpio, Thelyphonus, and Phrynus, it is very conspicuous, and in the latter genus exhibits many longitudinal folds, as it does likewise in Gryllotalpa, which must permit a vast extension of the abdomen. In this membrane, in some cases, as in Dynastes M^cL., Melolontha, &c., the two or three first spiracles are fixed[2137]. In the Hymenoptera and many other insects the dorsal segments do not unite by their margin with the ventral, but the end of each dorsal laps over that of the corresponding ventral.

Dorsal segments[2138]. I shall next notice the segments seriatim, in the order of their occurrence, beginning with the dorsal ones. The most remarkable circumstance with respect to these that occurs to my recollection takes place in the Cancroid spiders (Epeira cancriformis, aculeata, &c.), in which the back of the abdomen is formed by a plate, in some extended in a transverse direction (E. cancriformis), in others in a longitudinal one (E. aculeata), of a much harder substance than the under side and quite flat, set with strong sharp spines, in the former species apparently moveable, and terminating behind in a piece resembling in some measure the scutellum of the Stratyomidæ and similarly armed with a pair of spines[2139]: in E. aculeata the sides of the abdomen, under the plate, have a number of longitudinal folds like those of Phrynus. In Cryptocerus, a genus of ants peculiar to South America, the first segment, not reckoning the pedicle, forms almost the whole back of the abdomen, and the three last are so minute as scarcely to be distinguishable. Nothing very remarkable is exhibited by the other segments, except that in Trichius the penultimate is the largest; in some Staphylinidæ (S. splendens) and Brachini (B. melanocephalus) it is emarginate, and in the former tribe also often terminating in a white membrane. The dorsal segment most worthy of notice is the last, which is called the podex; for though in general it is a minute piece, often retracted within the abdomen and invisible, as in many Diptera, yet sometimes it is the most conspicuous of the dorsal segments. It is most commonly triangular, and usually deflexed and forming an angle with a horizontal line; but in Clytra, Chlamys, and Oryctes, it is inflexed; in many Lamellicorns it is nearly vertical. In Tettigonia F., many other Homopterous Hemiptera, and some Hymenoptera (Cimbex), its sides turn down and become ventral; on its lower side it has in these a longitudinal cavity which receives the ovipositor in repose[2140]. In many other insects it unites with the last ventral segment, the hypopygium, to form a tube for that organ, as you will find in Callidium violaceum[2141], many Muscidæ, and Thelyphonus. As to its termination the podex is sometimes bifid, Blatta; bipartite, Ranatra; mucronate, Sirex; acuminate, Melolontha vulgaris, Trichius hemipterus. Generally this part is flat; but the disk is elevated or gibbous in Oryctes and some other Lamellicorns. In the majority of the Coleoptera Order it is quite covered by the wings and elytra; but in many of the last-mentioned tribe, and sometimes the penultimate segment also, it is not covered by them[2142]. In some insects the piece we are considering appears to consist of two segments; in the male of Locusta morbillosa the whole podex is rhomboidal, but it is formed by two triangular pieces which articulate with each other; this structure permits the more easy elevation of the terminal one for the extrusion of the feces.

Ventral Segments[2143]. We are now to turn our attention to the ventral segments of the abdomen. The first of them is what is called the epigastrium[2144] in the table. This part, according to M. Chabrier, is of considerable importance to the animal in flight, as, by its pressure against the trunk, not only regulating the movements of the abdomen, but as, in his opinion, contributing to push forward the trunk[2145] in the descent of the animal. It is remarkable only in the Coleoptera and Heteropterous Hemiptera, to which my observations upon it will be confined. It may be stated as usually consisting of two articulations, that nearest the trunk being narrow, and in the Predaceous beetles[2146], as also in Scutellera, Pentatoma, &c., interrupted in the middle[2147]. In many Lamellicorns this joint is concealed under the posterior coxæ, and with the anterior part of the second forms a hollow cavity for their reception; this last joint is what is properly the Epigastrium, the former, especially when distinct, being called in the table the Hypochondria. In Sagra and Brentus the epigastrium is particularly conspicuous for its size, in the former occupying half, and in the latter nearly two-thirds of the under side of the abdomen; but in general it is distinguished from the remaining segments only by the central mucro or point that terminates it towards the trunk[2148], and which is received by a sinus of the metasternum; this point is generally minute and triangular, but in Sagra it is large and rounded at the extremity, and in Calandra it terminates nearly in a transverse line somewhat waving. It is most remarkable, however, in some species of the Heteropterous genus Edessa F.; for in E. nigripes and affinities it is a sharp sterniform conical horn, which passing between the four posterior legs covers the end of the promuscis. In fact, this part appears a kind of abdominal sternum. In the Cetoniadæ, &c., the Hypochondria unite before this mucro, and form a ridge which articulates with it, and dips towards the abdominal cavity; in Scolytus the epigastrium is much elevated from the rest of the ventral segments, so that the under side of the abdomen appears as if it were suddenly cut off, whence Herbst's awkward though not inexpressive name, Ekkoptogaster; this part in this genus has something of a posterior mucro.

The intermediate ventral segments exhibiting no very remarkable peculiarities, I shall pass them without further notice, and call your attention to the last, which is opposed to the podex, and which I have named the hypopygium[2149]. Though usually a single small piece, in Edessa and many Pentatomæ it consists of several plates; and in Trichius it is very large: it is mostly intire, but in the male Dytisci it is cleft; in Lamia ocellata trilobed; in Edessa tripartite; in Centrotus Taurus it is boat-shaped and hollowed out to receive the stalk of the ovipositor. It is also generally in the same line with the body, but in Xenos it is turned up and bent inwards[2150].

iv. Shape. With regard to shape, in some Orders the abdomen varies considerably; but the most general form is one that approaches to trigonal, so that a transverse section will be a triangle, with the vertex more or less obtuse, and the base more or less convex; some tendency to this form will often be found even in those insects whose abdomen appears almost as flat as a leaf, as in many Aradi. In the hive-bee the transverse section is almost an equilateral triangle; in Belostoma grandis the disk of the under side of the part in question is longitudinally elevated into a trigonal ridge, the section of which is an equilateral triangle, the sides being quite flat. In general, in the vertical section of an abdomen, the vertex of the triangle points downwards, but in Libellula F. it points upwards. In Blatta this section is nearly lanceolate; in Staphylinus olens it is a segment of a circle with the convex side downwards; in Æshna F. with that side upwards; and in Agrion the section is circular. In Copris, Ateuchus, &c., the abdomen is very short and thick; in Staphylinus slender and long; in Aradus, Nepa, &c., depressed and flat; compressed in Ophion and Evania; conical in Cœlyoxis; rhomboidal in many Mantes; boat-shaped in many Lygæi; fusiform in various Papilionidæ; lanceolate in some Ichneumonidæ, falcate in others; nearly round in Diapria pupurascens; ovate in Lyrops; elliptical in Andrena; oblong in many Xylocopæ; heart-shaped in the naked Euglossæ; triangular in Dytiscus; gibbous in Flata; and vaulted in Chrysis. At its base it is truncated in Sirex; retuse in most bees; forming the segment of a circle in Andrena; in general sessile, but in the majority of Hymenoptera, as has been already observed, terminating in a pedicle. The pedicle is very short in the Andrenidæ and Apidæ; long in the Sphecidæ; thick in the Formicidæ; slender in Evania; fusiform in Pelecinus; clavate in Ammophila; campanulate in many Vespidæ; nodose in Myrmica[2151]; squamigerous in Formica[2152]: it sometimes also consists of two joints, as in Ammophila and many Vespidæ. As to margin, some have none, as Centrotus; in others, as Dytiscus, it is very narrow; in others again, wide and flat, as in the Nepidæ; in Staphylinus, &c., it is distinguishable only on the upper side of the abdomen; in Locusta Leach only on the under side, though mostly intire; it is serrated in Blatta, sinuated in Acanthia paradoxa, and crenated in Cerceris.

v. Proportions. These vary greatly in the different tribes; in some the abdomen is long and slender, as in Locusta, and Staphylinus; disproportionably so in a remarkable degree in some Agrionidæ from South America, as A. lineare, &c.[2153]; in others it is extremely short and thick, as in Copris, &c.; a mere appendage in Evania; it is shorter than the elytra in Trox; of the same length in most beetles; longer in Melolontha, Hister, &c.; disproportionably so in Staphylinus: though usually of the same width with the trunk, in many Mantidæ it is much wider[2154]; and more slender in the Libellulina, Myrmeleon, &c.

vi. Arms and Appendages[2155]. These are various; and may be considered under the following heads: processes; organs of respiration, motion and prehension; weapons; and other anal appendages the use of which is unknown.

1. Processes. Under this term I include all prominences of whatever kind, whether tubercles, teeth, spines, or horns, that arm any part of the abdomen. Many of these are sexual characters, and have been sufficiently described in a former letter[2156]; I need not therefore detain you long on this head. Of the first kind is a remarkable elevation that distinguishes the second ventral segment of Scolytus Destructor (Ips Scolytus Marsh.) or of a species allied to it[2157]; in S. pygmæus (I. multistriatus Marsh.) the same segment is armed by a flat horizontal tooth or horn; in an Aradus from Brazil, before alluded to[2158] (A. laminatus K. MS.), the margin of the abdomen is surrounded by eight flat subquadrangular laminæ; in another species figured by Stoll[2159], it is cut out into bays by a number of denticulated teeth; and in Acanthia paradoxa by long spinose lobes[2160]. In Edessa F., another genus of bugs, the abdomen usually terminates in four strong sharp dentiform spines, the intermediate ones being the shortest, and in some the margin is also armed with spines[2161]; occasionally the anal spines are very long[2162]. In addition to the ventral horns before mentioned that distinguish the sexes of some insects[2163]; the males of the genus Conops, a two-winged fly, have, on the antepenultimate ventral segment, a singular process, varying in length and shape in the different species, standing nearly at right angles with the belly, convex towards the trunk, and concave towards the anus. De Geer supposes that with the anal extremity this forms a forceps with which this fly seizes the other sex[2164].

2. Organs of respiration[2165]. I shall defer my account of the spiracles, and other external respiratory organs, till I come to treat of the system of respiration in insects, when every thing connected with that subject will be most properly discussed; but there are certain appearances in some insects, which at first sight seem to partake of the same character, but which being really independent of that vital function, may here have their place. If you examine the abdomen of the mole-cricket (Gryllotalpa vulgaris), you will easily discover the true spiracles in the folds of the pulmonarium, which separates the back of that part from the belly; if you next inspect the five intermediate segments of the latter, you will discover on each nearer the base a pair of oblique little channels, which precisely resemble closed spiracles. These may be denominated false or blind spiracles. Again, if you examine the pupa of any Scutellera or Pentatoma, in which tribe the true spiracles are ventral, you will discover, placed in a square on the two or three intermediate dorsal segments, four or six elevated points resembling spiracles, but not perforated, connected often by corrugations in the skin or crust[2166]; in the larvæ also of some Reduvii the first minute dorsal segment, at each lateral extremity, has a similar elevation with a central umbilicus precisely resembling a spiracle, but still not perforated: another instance of false spiracles in this section of the Hemiptera, is furnished by Aradus laminatus before mentioned, in the perfect insect; between the spiracle and the margin of each ventral segment is a white round callus, with a dark point resembling a perforation on its exterior side, and terminating internally in a channel covered by membrane leading to the disk of the segment, so that the whole in shape resembles a tobacco-pipe[2167]. A number of similar callosities with a central impression, but without any channel, variously disposed, are also to be found in another bug, Rhinuchus compressipes K.[2168] In the Homopterous section of this Order, a series of impressed points, which may be easily mistaken for spiracles, are to be discovered on both sides of the abdomen, at the margin in Centrotus, in which the real spiracles are quite concealed.