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.

1:1.Chelonus.
3:3.Chrysidæ[2133].
4:2.Leucospis[2134].
5:5.Syrphus.
5:6.Halictus ♀.
6:5.Nepa.
6:6.Halictus ♂.
Belostoma.
7:5.Curculio L.
Cerambyx L.
7:6.Dytiscus ♀.
7:7.Ammophila, &c.
8:6.Dytiscus ♂.
Lucanus, &c.
8:7.Geotrupes Latr.
8:8.Pimpla.
8:10.Euchlora McL.[2135]?
8:13.Scutigera.
9:5.Carabus Latr.
9:6.Gymnopleurus Ill.
9:7.Perga Leach ♀.
9:8.Perga ♂.
10:7.Locusta Leach ♀.
10:8.—————– ♂.
10:10.Æshna.
11:7.Phasma.
11:9.Chelifer.
12:11.Thelyphonus.
Many:Myriapoda.

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 McL., 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.