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The median segment.

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.)