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Ant abdomen

This region in ants is very highly specialized in all three ant phases. In some of the most primitive tribes, like the Amblyoponii and Cerapachysii, there is no sharp separation of the segments into a pedicel and gaster; the basal, though somewhat narrower and more accentuated, preserving essentially the same structure as the more distal segments. In most ants, however, there is a well­defined pedicel which may consist of either one or two segments, very movably articulated with each other and with the thorax and gaster. In the subfamilies Dolichoderinae and Camponotinae the pedicel always consists of but a single segment, the petiole, which is morphologically the second abdominal segment.

The same condition prevails in most Ponerinae, except that there is a constriction behind the following or third segment, foreshadowing the development of a postpetiole. This segment is clearly separated off in all Myrmicinae, so that in the ants of this subfamily the pedicel consists of two highly specialized, nodiform segments, and the first gastric is the fourth instead of the third abdominal segment, as in the Camponotinae, Dolichoderinae and Ponerinae. In the Dorylinx the genera Eciton and Enictus have a distinct petiole and postpetiole in the worker, but only a single segment, the petiole, in the male and female. In Doryhus and Cheliomyrmtex the base of the abdomen of the worker is more primitive and more like that bf certain Ponerine ants (Amblyopone and Cerapachys).

The base of the abdomen is the seat of an interesting sound­producing, or stidulatory, organ. Landois was the first to find this organ in a Ponerine ant. He believed that he had seen a similar structure in the Camponotine Lasius fuliginosus, and a few years later Lubbock figured what he took to be a stridulatory organ in L. flavus. Sharp and Janet have since carefully investigated these organs in several different ants. The former succeeded in finding them only in the Ponerinae and Myrmicinae (excepting the Cryptocerii) and believed them to be absent in the Dorylinae, Dolichoderinae and Camponotinae. The organ is best described as a file made of extremely fine, transverse and parallel ridges on a small area in the mid-dorsal, chitinous integument at the very base of the first gastric segment, where it is covered by the overlapping portion of the preceding segment. The edge of this segsegment is sharp and turned slightly downward or inward so that it may scrape back and forth over the file when the two segments are moved on each other and thereby produce a sound of very high pitch. The file is, in all probability, merely a local specialization of the fine, polygonal elevations or asperities which cover the adjacent portions of the segment and are so characteristic of the chitinous investment of many parts of the body. Each of these minute elevations is evidently secreted by one of the hypodermal chitinogenous cells. Sharp found great diversity in the structure of the stridulatory organ both among the different species and in the castes of the same species.

An interesting modification was found in an Australian Myrmicine ant of the genus Siryia, which has the file divided into two parts, one consisting of coarse, the other of fine, ridges, and Sharp remarks that "a stridulatory performance by this insect might produce very extraordinary effects." Janet, in his studies of Myrmica rubra, calls attention to the fact that there are accumulations of chitinous asperities at various widely separated regions of the ant's body, especially on articulations which might, by their movements, produce sounds. But the true stridulatory organs he finds to be situated where they were seen by Landois and Sharp, i.e., at the base of the first gastric segment, and also on the corresponding part of the postpetiole. These two segments certainly admit of the greatest amplitude and freedom of movement and are, therefore, the most favorable spots for the development of organs like those under discussion. In Myymica rubra there are more than 50 ridges to the postpetiolar file, but in the organ at the base of the gaster there are more than 130 and these are much finer.

The ridges, however, are twice as broad in the anterior as they are in the posterior portion of the gastric file. It appears, therefore, that the most highly developed stridulatory surfaces of the Myrmicinae and Ponerinae are not strictly homologous, since in the former subfamily the principal organ is situated on the third abdominal, whereas the only stridulatory file of the latter is on the second abdominal segment. In both cases, however, the main organ is at the base of the first gastric segment.

What seem to be incipient stages in the development of the organ from ordinary polygonal asperities of the chitinous integument, are seen in the Dorylinae. Of the first gastric segment in one genus of this subfamily Sharp says: "I have examined workers of several species of Eciton, and find that they have no stridulatory organ, the sculpture being uniform all over the dorsum of the neck of the segment." My own observations on the workers of several species of Eciton, AEnictus, Dorylus and Cheliomyrimex confirm this statement. In all these genera the neck of the postpetiole and that of the first gastric segment are covered with polygonal asperities, but these are much more conspicuous than on other portions of the segments, and in one species (Eciton opacithorax) they are transversely lengthened in the mid-dorsal region so that they foreshadow the file ridges of the Ponerinae and Myrmicinae.

Although the number of segments in the gaster is morphologically eight, when the pedicel consists of a single segment, and seven when it consists of two, only four segments are externally visible in the worker and female and five in the male. The remaining segments are very small and telescoped into the larger ones in front of them. Tracheal stigmata are present on the eight basal abdominal segments, i.e., on the epinotum, pedicel and the five or six basal gastric segments.

The terminal segments in the female and worker may bear a sting, which is of considerable interest, because it can be traced back to its primitive homologue, the ovipositor. In many Orthoptera, like the katydids and crickets, this organ consists of three pairs of appendages, which, as I have shown, are the modified embryonic legs of the eighth, ninth and tenth abdominal segments. Owing to a very early embryonic fusion of their corresponding segments the pair belonging to the tenth segment moves up and comes to lie between the ninth pair, so that the ninth segment appears to bear two pairs of appendages.

In the Hymenoptera the ovipositor is still retained with its Orthopteroid function in certain families like the ichneumons and gall-flies, which oviposit in the tissues of insects and plants. In the bees, wasps and ants, however, the organ has lost this primitive function and has become an organ of defence.

Its embryological origin in these forms, however, is the same as in the Orthoptera. Dissections of the sting of the pupal and adult ant show that the pairs of appendages become closely applied to one another so that they appear as a single organ. The appendages of the tenth segment actually fuse to form a single, pointed, grooved piece, the gorgeret (Stachelrinne) which encloses the pair of appendages belonging to the eighth segment. These are very slender and pointed and are known as the stylets, or prickles (Stech­borsten). The appendages of the ninth segment become somewhat lamelliform and, without fusing with each other, enclose the gorgeret as the sting-sheath (Stachelschiede). In stinging, the pointed gorgeret is thrust into the skin and then the stylets are alternately pushed deeper into the wound beyond the tip of the gorgeret which they do not surpass when the sting is at rest. The duct of the gland that supplies the poison, which produces the burning sensation, enters the base of the gorgeret. The stylets are smooth and not barbed on their sides as they are in the bee; hence the ant is able to withdraw its sting from the wound. While the sting is very large and well-developed in the Ponerinae, Dorylinae and, most Myrmicinae, it is vestigial or absent in the other subfamilies.

At the tip of the male gaster there are three pairs of rather complicated appendages forming the genital armature. They are developed on the ninth abdominal segment, i.e., the segment which in the female gives rise to the sting sheath. The sternal plate of this segment, which in the male lies in front of the appendages, is known as the annular lamina. The three pairs of appendages enclose one another, so that we may distinguish an outermost, a median and an innermost pair. The outermost pair has been called the stipites. The median pair is sometimes more or less completely divided into two pairs, known as the volsellae and laciniae respectively; and the whole group of appendages comprising the stipites, volsellae and laciniae are known as the external paramera.

The innermost pair alone is known as the internal paramera. They are closely applied to each other in the median sagittal plane of the body all and function as a penis. During copulation the stipites, which are large, robust and often covered with hairs, function as claspers. The volselle and lacinix, which are smaller and less heavily chitinized and furnished with numerous tactile sense organs, in all probability also have a clasping function. The inner paramera are very delicate. In some ants they have serrated edges which probably serve to hold them in place in the vagina of the female ant. In addition to the genital valves there is a pair of small, hairy appendages, the penicilli, attached to the tergite, or dorsal plate of the tenth abdominal segment. There can be little doubt that these represent the cerci of Blattoid and other primitive insects and must therefore belong to the anal or eleventh abdominal segment. The presence or absence of the penicilli and the conformation, permanent retraction or protrusion of the different paramera are used in classification as valuable diagnostic characters.