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