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].