Although the prosternum of a Hymenopterous Insect is not usually visible owing to its being overwrapped by the side-pieces, it is really, as shown in Fig. 335, B, of complicated form. In Cimbex and some other sawflies the side-pieces are not so large as usual, but the prosternum is larger and is exposed. The prothoracic spiracle is rarely visible externally, but its position is remarkably constant, and is usually indicated by a peculiar lobe or angle of the pronotum projecting backwards just below the insertion of the front wing. This stigmatic lobe is frequently fringed with short hairs.
The mesothorax is the largest of the three divisions of the thorax proper; its notum is large, and usually divided into two parts by a transverse suture. The side-pieces are so placed that the epimeron is rather behind than below the episternum. The mesosternum forms the larger part of the under-surface of the alitrunk. A very large phragma projects from the mesothorax into the interior of the body. The mesothoracic spiracle is usually not visible; its existence was unknown to the older entomotomists, who were in consequence led to consider the spiracle of the median segment as belonging to the thorax. The mesothoracic spiracle is, however, easily seen in Cimbex, placed in the suture between the mesothoracic epimeron and the metathoracic episternum, a little below the insertion of the front wing; close to this spot the mesophragma, just spoken of, comes, in Cimbex, to the surface. The mesothoracic spiracle is generally conspicuous in the worker ant. The parts of the metathorax are usually small, but so much variety prevails in this respect that no general description can be given.
Fig. 335.—Articulation of front legs of the hornet (Vespa crabro, ♀). A: a, side-piece of prothorax overlying the prosternum; b, coxa; c, trochanter. B, prosternum proper, as seen from front when extracted.
The structure of the posterior part of the alitrunk has given rise to an anatomical discussion that has extended over three-quarters of a century,[[410]] with the result that it is now clear that the posterior part of what appears to be thorax in Hymenoptera is composed of an abdominal segment. This part has been called "Latreille's segment," the "median segment," and the "propodeum." The latter term was proposed by Newman, under the form of propodeon,[[411]] and appears to be on the whole the most suitable term for this part, which is of great importance in systematic entomology, owing to the extreme variety of characters it affords. Although it is clear that the propodeum is, in large part, an abdominal segment, yet its morphology is still uncertain; what parts are pleural, what tergal, and what may be chitinised spiracular area, or portions of the metathorax, being undetermined. The ventral portion of the propodeum affords a strong contrast to the dorsal part, being so small that it has frequently been described as absent; it is, however, not difficult to detect it in the position shown at d, Fig. 333, B.
Although the true first segment of the abdomen is detached from its normal position and added to the thorax, yet the term abdomen is conventionally restricted to the part that commences with the true second segment, which, in counting the number of abdominal segments, is reckoned as being the first. There are two modes of articulation of the Hymenopterous abdomen with the alitrunk; in one (Fig. 336, A) the base of the abdomen remains of the calibre usual in Insects, while in the other (Fig. 336, B) it is greatly contracted, so that the two parts are connected only by a slender stalk, the petiole. The petiole, besides articulating in a very perfect manner with the propodeum by means of certain prominences and notches, is also connected therewith by means of a slender ligament placed on its dorsal aspect and called the funiculus, shown in Fig. 333, A, just at the extremity of the pointing line k. This mode of articulation gives great freedom of motion, so that in some Petiolata (Ampulex) the abdomen can be doubled under the body and the sting brought to the head. It is worthy of note that even in the Sessiliventres—as the sub-Order with broad-based abdomen is called—some amount of movement exists at the corresponding spot; while, as shown in Fig. 336, A, between a and b, there exists an exposed membrane, the homologue of the funiculus.
Fig. 336.—Articulation of abdomen and alitrunk of, A, Cimbex, B, Vespa. a, Propodeum or median segment; b, dorsal plate of first (second true) abdominal segment or petiole; c, spiracle of the propodeum; d, hind coxa; e, ventral plate of first (second true) abdominal segment.
The number of abdominal segments that can be seen in the perfect Insect varies greatly. In Tenthredinidae nine can be distinguished, while in some of the Chrysididae it is difficult to detect more than three behind the petiole. These distinctions are, however, superficial or secondary, being due to changes in the later life in connexion with the stings or borers; in the larvae that have been examined thirteen segments behind the head have usually been detected.
Nothing is more remarkable in the Hymenoptera than the great differences that exist in the form of the petiole. This may be very short, as in the bees, so that the abdomen when not deflexed does not appear to be separated from the thorax (Fig. 331, C); in this condition it is said to be sessile, a term which it would be well to replace by that of pseudosessile. In many of the solitary wasps the petiole is very long. In ants it is replaced by one or two curiously-shaped small segments called nodes (Fig. 60, B, 2, 3), and in many ants these are provided with structures for the production of sound. The abdomen is formed by a system of double imbrications; each dorsal plate overlaps each ventral plate, and the hind margin of each segment embraces the front part of the one following; thus this part of the body has not only great mobility, but is also capable of much distension and extension. The pleura are apparently absent, but each one has really become a part of the dorsal plate of the segment to which it belongs. This is shown to be the case by Cimbex, where the division between pleuron and dorsal plate exists on each segment except the basal one. Owing to this arrangement, the abdominal stigmata in Hymenoptera appear to be placed on the dorsal plates.