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Fig. 176.—Abdomen of Termes flavipes: 1–10, the ten tergites; 1–9, the nine urites; c, cercopod.

Fig. 177.—End of abdomen of Panorpa debilis drawn out, the chitinous pieces shaded: L, lateral, D, dorsal view; c, jointed cercopoda.—Gissler del.

In the abdomen the segments are more equally developed than elsewhere, retaining the simple annular shape of embryonic life, and from their generalized nature their number can be readily distinguished (Fig. 176). The tergal and sternal pieces of each segment are of nearly the same size, the tergal often overlapping the sternal (though in the Coleoptera the sternites are larger than the tergites), while there are no pleural pieces, the lateral region being membranous when visible and bearing the stigmata (Fig. 177, L). In the terminal segments beyond the genital outlet, however, there is a reduction in and loss of segments, especially in the adults of the metabolous orders, notably the Panorpidæ (Fig. 177), Diptera, and aculeate Hymenoptera; in the Chrysididæ only three or four being usually visible, the distal segments being reduced and telescoped inward.

The typical number of abdominal segments (uromeres), i.e. that occurring in each order of insects, is ten; and in certain families of Orthoptera, eleven. In the embryos, however, of the most generalized winged orders, Orthoptera (Fig. 199), Dermaptera, and Odonata, eleven can be seen, while Heymons has recently detected twelve in blattid and Forficula embryos, and he claims that in the nymphs of certain Odonata there are twelve segments, the twelfth being represented by the anal or lateral plates. It thus appears that even in the embryo condition of the more generalized winged insects, the number of uromeres is slightly variable.

We have designated the abdomen as the urosome; the abdominal segments of insects and other Arthropods as uromeres, and the sternal sclerites as urosternites, farther condensed into urites. (See Third Report U. S. Entomological Commission, 1883, pp. 307, 324, 435, etc.)

Fig. 178.—Nymph of the pear tree Psylla, with its glandular hairs.—After Slingerland. Bull. Div. Ent. U. S. Dep. Agr.

The reduction takes place at the end of the abdomen, and is usually correlated with the presence or absence of the ovipositor. In the more generalized insects, as the cockroaches, the tenth segment is, in the female, completely aborted, the ventral plate being atrophied, while the dorsal plate is fused during embryonic life, as Cholodkowsky has shown, with the ninth tergite, thus forming the suranal plate.

In the advanced nymph of Psylla the hinder segments of the abdomen appear to be fused together, the traces of segmentation being obliterated, though the segments are free in the first stage and in the imago (Fig. 178). It thus recalls the abdomen of spiders, of Limulus, and the pygidium of trilobites.

The median segment.—There has been in the past much discussion as to the nature of the first abdominal segment, which, in those Hymenoptera exclusive of the phytophagous families, forms a part of the thorax, so that the latter in reality consists of four segments, what appearing to be the first abdominal segment being in reality the second.

Latreille and also Audouin considered it as the basal segment of the abdomen, the former calling it the “segment médiaire,” while Newman termed it the “propodeum.” This view was afterward held by Newport, Schiödte, Reinhard, and by the writer, as well as Osten-Sacken, Brauer, and others. The first author to attempt to prove this by a study of the transformations was Newport in 1839 (article “Insecta”). He states that while the body of the larva is in general composed of thirteen distinct segments, counting the head as the first, “the second, third, fourth, and, as we shall hereafter see, in part also the fifth, together form the thorax of the future imago” (p. 870). Although at first inclined to Audouin’s opinion, he does not appear to fully accept it, yet farther on (p. 921) he concludes that in the Hymenoptera the “fifth” segment (first abdominal) is not in reality a part of the true thorax, “but is sometimes connected more or less with that region, or with the abdomen, being intermediate between the two. Hence we have ventured to designate it the thoracico-abdominal segment.” Had he considered the higher Hymenoptera alone, he would undoubtedly have adopted Latreille’s view, but he saw that in the saw-flies and Lepidoptera the first abdominal segment is not entirely united with the thorax, being still connected with the abdomen as well as the thorax. Reinhard in 1865 reaffirmed Latreille’s view. In 1866 we stated from observations on the larvæ made three years earlier, that during the semipupa stage of Bombus the entire first abdominal segment is “transferred from the abdomen to the thorax with which it is intimately united in the Hymenoptera,” and we added that we deemed this to be “the most essential zoölogical character separating the Hymenoptera from all other insects.” (See Fig. 93, showing the gradual transfer and fusion of this segment with the thorax.) In the saw-flies the fusion is incomplete, as also in the Lepidoptera, while in the Diptera and all other orders the thorax consists of but three segments. (See also pp. 90–92.)

Fig. 179.—Abdomen of Machilis maritima, ♀, seen from beneath: the left half of the 8th ventral plate removed; I-IX, abdominal segments; c, cercopoda; cb, coxal glands; hs, coxal stylets; lr, ovipositor.—After Oudemans, from Lang.

The cercopoda.—We have applied this name to the pair of anal cerci appended to the tenth abdominal segment, and which are generally regarded as true abdominal legs. As is now well known, the embryos of insects of different orders have numerous temporary pairs of abdominal appendages which arise in the same manner, have the same embryonic structure, and are placed in a position homologous with those of the thorax. In the embryo of Œcanthus rudimentary legs appear, as shown by Ayers, on the first to tenth abdominal segment, the last or tenth pair becoming the cercopoda; and similar rudimentary appendages have been detected in the embryos of Coleoptera, Lepidoptera, and Hymenoptera (Apidæ). Cholodkowsky has observed eleven pairs of abdominal appendages in Phyllodromia.

They are very long and multiarticulate in the Thysanura (Fig. 179). In the Dermaptera they are not jointed and are forcep-like. It should also be observed that in the larva or Sisyra (Fig. 181) there are seven pairs of 5–jointed abdominal appendages, though these may be secondary structures or tracheal gills. In the Perlidæ and the Plectoptera (Ephemeridæ), they are very long, sometimes over twice as long as the body, and composed of upward of 55 joints; they also occur in the Panorpidæ (Fig. 177). In the dragon-flies the cerci are large, but not articulated, and serve as claspers or are leaf-like^[35] (Fig. 180). In a few Coleoptera, as the palm-weevil (Rhynchophorus phœnicis), Cerambyx, Drilus, etc., the so-called ovipositor ends in a hairy, 1–jointed, palpiform cercus. Short 25–jointed cercopoda are present in Termitidæ, and 2–jointed ones in Embiidæ.