i. Tergum (the Tergum). The upper or supine surface of the abdomen[1109]. It includes the Segmenta Dorsalia, and Pulmonaria.
1. Segmenta Dorsalia (the Dorsal Segments). Transverse segments of the back, the sides of which often lap over and cover those of the ventral segments[1110].
2. Pulmonaria (the Pulmonary Space). Two longitudinal soft spaces, capable of tension and relaxation, one on each side of the back of the abdomen, in which, where they exist, the dorsal spiracles are planted[1111]. They include the Spiracula Dorsalia.
a Spiracula Dorsalia (the Dorsal Spiracles). Lateral breathing-pores observable in the dorsal segments, often covered by the preceding segment[1112].
ii. Venter (the Belly). The lower or prone part of the abdomen[1113]. It includes the Hypochondria, Epigastrium[1114], Segmenta Ventralia, and Elastes.
1. Hypochondria (the Hypochondria). Two portions of segments, one on each side; which in some genera[1115] (Carabus L., &c.) intervene between the first intire ventral segment and the posterior part of the Postpectus.
2. Epigastrium (the Epigastrium). The first intire ventral segment[1116]. It includes the Mucro and Tympana.
A Mucro (the Mucro). The central posterior point of the Epigastrium observable in many of the Orders, which reposes between the posterior legs; and, according to M. Chabrier, is useful to the insect during flight[1117].
B Tympana (the Drums). Two deep cavities, containing a complex machinery on each side of the Epigastrium in male Tettigoniæ, which are the instruments of sound[1118].
3. Segmenta Ventralia (the Ventral Segments). Transverse sections of the belly[1119]. In Elytrophorous insects they are usually of a firmer consistence than those of the back. They include the Spiracula Ventralia.
A Spiracula Ventralia (the Ventral Spiracles).—Breathing-pores observable in some genera in the intermediate ventral segments, one on each side[1120]. Ex. Dynastes Aloeus, &c.
4. Elastes (the Elastes). The elastic organs on the ventral segments of Machilis polypoda which assist this insect in leaping.
iii. Petiolus (the Footstalk). A slender part by which the abdomen of many Hymenoptera is united to the trunk, in some genera very long, in others very short, and in others wanting[1121]. It includes the Funiculus, Foramen, Squama, and Nodus.
1. Funiculus (the Funiculus). A small cartilaginous cord, passing through a minute orifice of the Postfrænum, just above the point where the footstalk is fixed, to an opposite hole above it, which enables the animal the better to elevate or drop the abdomen[1122].
2. Foramen (the Foramen). The orifice in the abdomen, through which the above cord passes[1123].
3. Squama (the Scale). A vertical flat scale, observable on the footstalk of the genus Formica, &c.[1124]
4. Nodi (the Knots). One or more subrotund protuberances of the footstalk in the genus Myrmica[1125].
iv. Cauda (the Tail). Where the abdomen grows suddenly slenderer, and terminates in a long jointed tail, as in Scorpio and Panorpa[1126]. It includes the Centris.
1. Centris (the Centris). The last inflated joint of the tail, terminating in the Sting.
v. Anus (the Anus). The termination of the abdomen, consisting of the two last segments. It includes the Podex, Hypopygium, Culus, Ovipositor, and Appendices.
1. Podex (the Podex). The last dorsal segment of the abdomen[1127].
2. Hypopygium (the Hypopygium). The last ventral segment of the abdomen[1128].
3. Culus (the Culus). The orifice at the end of the anus.
4. Ovipositor (the Ovipositor). The instrument of oviposition, by which the insect conducts the eggs to their appropriate nidus, and often bores a way to it; the same instrument is by some genera used as a weapon of offence, when it is called the Aculeus[1129]. It includes the Unci, Tubulus, Valvæ, Vaginula, and Terebellæ.
A Unci (the Unci). Two pair of robust organs, the upper incurved and the lower recurved, with which the anus of Locusta Leach is furnished[1130].
B Tubulus (the Tubulus). A tubular ovipositor, consisting of several pieces often retractile within each other, like the tubes of a telescope[1131].
C Valvæ (the Valves). Two lateral laminæ, often coriaceous, by which the ovipositor when unemployed is covered[1132].
D Vaginula (the Sheath). A corneous case, with two grooves, in which the Terebella or Spicula play[1133].
E Terebellæ (the Terebellæ). Instruments by which the insect saws or bores a passage for its eggs to the place in which her instinct directs her to deposit them[1134].
5. Aculeus (the Sting). The above instrument, when fitted for an offensive weapon[1135]. It includes, besides the Valvæ and Vaginula before defined, the Spicula.
A Spicula (the Darts). The proper stings which inflict the wound: retractile within the sheath, externally serrulate at the apex[1136]. They include the Retinaculum.
a Retinaculum (the Retinaculum). A minute horny moveable scale or plate with which the darts are furnished, which prevents their dislocation by being shot forth too far[1137].
vi. Appendices (the Appendages). Other instruments and organs, with which the anus of various insects is furnished. They include—the Forceps, Forfex, Furca, Styli, Foliola, Flosculus, Caudulæ, Fila, Mammulæ, Papillæ, and Siphonuli.
1. Forceps (the Forceps). A pair of anal organs that open and shut transversely, and meet at their inner margin, or at the apex. Ex. Forficula.
2. Forfex (the Forfex). A pair of anal organs, which open or shut transversely, and cross each other[1138]. Ex. Male of Raphidia Ophiopsis.
3. Furca (the Fork). An inflected elastic anal organ, ending in a fork, by which the animal is enabled to leap[1139]. Ex. Podura.
4. Styli (the Styles). Rigid, exarticulate, long and narrow anal organs[1140]. Ex. Staphylinus.
5. Foliola (the Leaflets). Rigid, exarticulate, dilated, leaf-like anal organs[1141]. Ex. Libellulina.
6. Flosculus (the Floret). A small, tubular, lunulate anal organ, with a central style[1142]. Fulgora candelaria, &c.
7. Cerci (the Cerci). Two short, flattish, sublanceolate, jointed, lateral anal organs[1143]. Ex. Blatta.—N.B. Analogous organs are observable in the Gryllina, but usually conical and without joints[1144]. In Gryllus Latr. they are setiform[1145].
8. Caudulæ (the Caudulæ). Two or more slender, filiform or setaceous, jointed, flexile anal organs[1146]. Ex. Lepisma, Machilis, Ephemera.
9. Fila (the Threads). Two exarticulate, slender, filiform anal organs[1147]. Ex. Machilis.
10. Mammulæ (the Mammulæ). Anal protuberances, containing instruments for spinning web[1148]. Ex. Araneidæ. They include the Fusi.
11. Fusi (the Spinners). Organs, consisting of two retractile pieces, issuing from the Mammulæ, and rendering the threads[1149].
12. Siphonuli (the Siphonets). Truncated, fistular, setiform anal organs, emitting a saccharine fluid[1150]. Ex. Aphis.
You will observe, that when the whole upper-side of the Truncus is spoken of, it is called the Thorax; and as in Coleoptera, and some other Orders, the whole of the Mesothorax except the Scutellum is covered by the Thorax, and the whole of the Metathorax by the Mesothorax and Elytra—the Thoracic shield, may without danger of mistake be denominated the Thorax, as it has always been. When the whole under-side of the Trunk is spoken of, it is called the Pectus. When the three Sternums are spoken of together, they may be called the Sternum; and the whole interior elevation of the Pectus may be called the Endosternum.
[LETTER XXXIV.]
EXTERNAL ANATOMY OF INSECTS.
THE HEAD, AND ITS PARTS.
Before I confine my observations to the head of insects, which I propose to consider separately in the present letter, I must premise a few words upon their body in general, or rather its crust, or external integument. In this we may notice its substance, general form, sculpture, pubescence, and composition.
i. I have already noticed the substance of this integument in the preparatory states of insects[1151]; I shall not, therefore, here repeat what I then said, but restrict myself chiefly to the consideration of it as it is found in their last state, in which it is usually firmer than in their previous stages of existence. In this respect, however, it varies much in the different Orders, and even in the different genera of the same Order. In some Coleopterous insects, for instance, it is very hard, and difficult to perforate; while in others it is soft, flexible, and a pin easily passes through it[1152]. And in general, from a substance in hardness resembling horn or shell, it passes through the intermediate degrees of that of leather and parchment, almost to a thin membrane. Yet in all cases there is enough of rigidity and hardness to answer the principal uses of a skeleton—to afford, namely, a sufficient point of attachment for the muscles, and to support and defend the interior organization; so that the play and action of the vital and secretory systems may not be interrupted or impeded.
With respect to the principles which enter into the composition of this integument, very little seems to be known at present; but few insects having been submitted to a chemical analysis. The blister-beetle (Cantharis vesicatoria), from its importance in medicine, has, however, been more than once analysed; and though the products have not been very precisely stated, yet we find amongst them phosphate of lime, albumen, and some other usual components of the substance of vertebrate animals[1153]. But which of these products belong to the integument, and which to its contents, cannot be ascertained, without a separate process for each; which would not, I conceive, be very feasible. The substance, however, of the integument of insects, though we know not its precise contents, which probably vary in different genera, &c., appears not to be exactly of the nature of any of those substances after which it has usually been denominated: it is not properly analogous either to real horn, shell, skin, or leather, &c. This seems to result from the following circumstance:—Most of the excretions of vertebrate animals, as horn, skin (at least when tanned), feathers, wool, hair, &c. when exposed to the action of fire liquify, more or less, before they incinerate; emitting at the same time a peculiar and disagreeable scent: but upon applying this test to the parts of insects of the different Orders, I found, in every instance, that incineration took place without liquefaction, and was unaccompanied by that peculiar scent which distinguishes the others. Even the claws, which to the eye appear, as to their substance, exactly like those of Mammalia, birds, &c. burn without melting, and retain their form after red heat. That the insect integument is not calcareous like that of the Crustacea, and the shells of Molluscæ, you may easily satisfy yourself, by immersing them in an acid test. I made this experiment upon portions of insects of several of the Orders, in an equal mixture of muriatic acid and water, and the result was, not only that all hexapods, but octopods, Arachnida, and even Scolopendridæ, upon immersion only emitted a few air-bubbles; while, when the other myriapods, Polydesmus, Iulus, Glomeris, &c. and the Oniscidæ, were immersed, a violent effervescence took place; proving the different nature of their substance. It is remarkable that the two great branches of the Myriapods, the Scolopendridæ and Iulidæ (Chilopoda and Chilognatha Latr.), should in this respect be so differently circumstanced—the latter having a calcareous integument, and the former not.—A further difference distinguishes these two tribes: old specimens of the Iulidæ usually lose their colour and turn white, like Oniscidæ; while those of the Scolopendridæ retain it.