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Show 56 ON CLASSIFICATION. posed essentially as in this and the two preceding classes. But the total number of somites of the body never exceeds twenty. Fig. 28. Fig. ~!!. / 1 1 0 2 I i 3 - 4: ( 5 --8 --g 10 X .Fig. 29.-Longitudinal and vertical section of the abdomen of a male Cockroach (Btatta).-1, 2, 3, 4, &c., terga and sterna of the abdomen ; t, testis ; v~ aperture of the vas deferens ; A, anus. THE INSJ!JU'l'A. [57 Of these five certainly, and six probably, constitute the head, which possesses a pair of antennro, a pair of man<hbles, and two pairs of Inaxillm ; the hi'nder pair of whieh arc coalescent, and forrn the organ called the " labiun1." Three, or perhaps, ~in some cases, rnore, somites unite and become specially modified to fonn the thorax, to which the three pairs of locomotive limbs, characteristic of perfect insects,* are attached. rrwo additional pairs of locomotive organs-the wings-are developed, in most insects, from the tergal walls of the second and third thoraci(~ somites. No locomotive limbs are ever developed from the abdomen of the adult insect, but the ventral portions of the abdominal somites, from the eighth backwards, are often metmnorphosed into apparatuses ancillary to the generative function (Figs. 28 and 29). * The female Stylops is stated to possess no thoracic limbs. |
