Fig. 93.—Transformation of the bumble bee, Bombus, showing the transfer of the 1st abdominal larval segment (c) to the thorax, forming the propodeum of the pupa (D) and imago; n, spiracle of the propodeum. A, larva; a, head; b, 1st thoracic; c, 1st abdominal segment. B, semipupa; g, antenna; h, maxillæ; i, 1st; j, 2d leg; k, mesoscutum; l, mesoscutellum; m, metathorax; d, urite (sternite of abdomen); e, pleurite; f, tergite; o, ovipositor; r, lingua; q, maxilla.
In the higher or aculeate Hymenoptera, besides the three segments normally composing the thorax, the basal abdominal segment is during the change from the larva to the pupa transferred to this region, making four segments. This first abdominal is called “the median segment” (Figs. 93–95). In such a case the term alitrunk has been applied to this region, i.e. the thorax, as thus constituted. Latreille wrongly stated that in the Diptera the first abdominal segment also entered into the composition of the thorax; but Brauer has fully disproved that view, as may be seen by an examination of his sketches which we have copied (Fig. 94).
Fig. 94.—7, 8, thorax of Tipula gigantea; 9, of Leptis; 10, thorax of Tabanus bromius after the removal of the abdomen, in order to bring into view the inner mesophragma (f), and to show the extension of the metathorax g and g′; tr, trochanter; 11, hind end of the mesothorax, the entire metathorax, and the 1st and 2d abdominal segments of Volucella zonaria, seen from the side. The internal mesophragma (f), and the position of the muscle inserted in it, are indicated by the two lines M. p, Callus postalaris; pr (pz in 8), callus præalaris Osten Sacken (= “patagium” of some authors); g, metanotum; g′, metepimerum, “segment médiaire” of Latreille (wrongly considered by him to be the 1st abdominal segment); 4, metasternum (hypopleura of Osten Sacken); 5 (? “episternum of metathorax” (Brauer) = metapleura of Osten Sacken); 6, and also H, halter; st1, mesothoracic stigma; st2, metathoracic stigma; st3, first abdominal stigma; γ, dorsopleural; δ, sternopleural; ε, mesopleural sutures; h, 1st, i, 2d, abdominal segment; al, wing; alul, alula. 12, the head and the three thoracic rings, and the 1st abdominal segment of Ephemera vulgata, the connecting membranes are in white: a, prothorax; b, præscutum; c, scutum; d, scutellum; e, postscutellum; ps, postscutellum of mesothorax.—After Brauer.
Fig. 95.—Alitrunk of Sphex chrysis: A, dorsal aspect; a, pronotum; b, mesonotum; c, tegula; d, base of fore,—e, of hind, wing; f, g, divisions of metanotum; h, median (true first abdominal) segment; i, its spiracle; k, second abdominal segment, usually called the petiole or first abdominal segment. B, posterior aspect of the median segment; a, upper part; b, superior,—c, inferior, abdominal foramen; d, ventral plate of median segment; e, coxa.—After Sharp.
The sternum is in rare cases subdivided into two halves, as in the meso- and metathorax of the cockroach; in Forficula the prosternum is divided into four pieces besides the sternum proper (Fig. 96); and in Embia, also, the sternites, according to Sharp, are complex.
Fig. 96.—Sternal view of pro-, meso-, and metathorax of Forficula tæniata: pst, præsternum, divided into 4 pieces; st, pro-, st′, meso-, st″, metasternum; cx, coxa; not, notum.